Loop Antenna at ~60 kHz

[rickman]

I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.

I spent some time simulating antennas in spice and was able to get a bit
of a feel for the circuit, but I'm not convinced it would work the way I
want. Just before I set the project aside I was told I needed to model
the radiation resistance. That has the potential of wrecking the Q of
the circuit. I am counting on the high Q to boost the output voltage.
If the radiation resistance is at all appreciable I would lose the high
Q and need to start over.

Anyone have an idea of how to estimate the radiation resistance of a
tuned, shielded loop antenna?

The other factor I don't understand how to factor in is the distributed
capacitance of the coax. Is that a significant influence on an antenna
or is it in the noise compared to the tuning capacitor. The coax is
RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The
specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or
would the resistance be a round trip measurement of both inner conductor
and shield? I assume the shield has a much lower resistance than the
inner conductor but I don't know that for sure.

--

Rick

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2oukc$ss2$1...@dont-email.me:

> I have a project in mind that would need a very good antenna in the
> frequency range of 60 kHz. Originally I looked at loop antennas and
> liked the idea of a large shielded loop made of coax tuned with a
> capacitor. My goal is to get as large a signal as possible from the
> antenna and matching circuit to allow the use of a receiver with very
> low sensitivity... in fact an all digital receiver.
>

MSF time signals? Just a thought... If you're interfacing an analog signal to
digital, one trick I used (for audio but it ought to help here too) is a
CA3140 with a bit of positive feedback through a few Mohms for hysteresis to
clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or
6V, could be sensitive to take a lot of strain off your antenna. Whether this
alone gives you enough gain I don't know, but it is cheap to try.

[Ralph Mowery]

"rickman" <gnu...@gmail.com> wrote in message
news:m2oukc$ss2$1...@dont-email.me...

>I have a project in mind that would need a very good antenna in the
>frequency range of 60 kHz. Originally I looked at loop antennas and liked
>the idea of a large shielded loop made of coax tuned with a capacitor. My
>goal is to get as large a signal as possible from the antenna and matching
>circuit to allow the use of a receiver with very low sensitivity... in fact
>an all digital receiver.
>
> I spent some time simulating antennas in spice and was able to get a bit
> of a feel for the circuit, but I'm not convinced it would work the way I
> want. Just before I set the project aside I was told I needed to model
> the radiation resistance. That has the potential of wrecking the Q of the
> circuit. I am counting on the high Q to boost the output voltage. If the
> radiation resistance is at all appreciable I would lose the high Q and
> need to start over.
>

I don't think I would try and reinvent that type of antenna. There are
several designs on the web that use a loop about 3 feet in diameter and
several turns of wire inside the shield. In most cases a low noise preamp
is needed, but that shold be simpleand inexpensive to build.

Go to this page and go toward the bottom for some loop antenna ideas.
http://www.w4dex.com/lf.htm

I have known Dexter for around 40 years.




---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

[ji...@specsol.spam.sux.com]

Google DIY WWVB antenna

16,900 results.

As for the output voltage, you do know FET input opamps work quite
well at 60 Khz and are dirt cheap?

FYI for those on the other side of the pond, WWVB is a US 60 kHz time
and frequency station.


--
Jim Pennino

[Lostgallifreyan]

ji...@specsol.spam.sux.com wrote in news:vrg5ib-...@mail.specsol.com:

> As for the output voltage, you do know FET input opamps work quite
> well at 60 Khz and are dirt cheap?
>

Which is one reason I mentioned the CA3140. :0 But there is a huge voltage
gain too, with the hyeteresis (positive feedback) and the ability to get a
single rail supply working well for direct interfacing to a digital input.
I'm not sure if that is what rickman means when he says ;digital receiver',
but if I'm right in assuming he's after MSF time signals, this direct input
to a digital IC is a technique often used, I was talking about it to someone
just last week, but we didn't discuss anything to do with antennas. One thing
I forgot to mention in the earlier post is to AC couple the input. An earlier
gain stage might be needed too, but nothing you can't do with a single
CA3240E. Next step uo might be LT1215, but not needed, the speed of CA3240 is
plenty. Easily good for 200 KHz with fairly good pulse shapes.

[rickman]

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive, the question is whether I need mV
level signals or maybe just uV signals which might not require an amp.
By using positive feedback the threshold would be shifting and the
amount of level shift would set the floor for the signal level from the
antenna I think.

--

Rick

[Spike]

On 28/10/14 20:33, rickman wrote:

> I have a project in mind that would need a very good antenna in the
> frequency range of 60 kHz. Originally I looked at loop antennas and
> liked the idea of a large shielded loop made of coax tuned with a
> capacitor. My goal is to get as large a signal as possible from the
> antenna and matching circuit to allow the use of a receiver with very
> low sensitivity... in fact an all digital receiver.

To my mind you seem to be over-thinking, and perhaps over-engineering,
this project.

I'm a string-and-sealing-wax UK-based Amateur, and my solution to a
similar problem was to take a simple approach: I put a one-turn loop
round the outside of a wardrobe and linked that straight into the
600-ohm balanced input to my receiver. That was enough to drop the local
noise levels by a dramatic amount, and was easily sufficient for my
purposes. Using an electric aerial, the signal was unreadable.

My suggestion is to start simple and find out if that is enough, and
make improvements one at a time. There could well be no real need to
have a computer-generated solution requiring high-grade components to
function.

Whatever route you choose, good luck!

--
Spike

"The greatest dangers to liberty lurk in the insidious encroachment by
men of zeal, well meaning but without understanding" Louis D. Brandeis

[rickman]

Yes, I am familiar with op amps.

--

Rick

[rickman]

I am not sure what you mean by "reinvent" that type of antenna. Every
antenna can be optimized for a given design. My requirements are very
unique. I need as much voltage from the antenna as possible. My
receiver input impedance can be very high (~1 Mohm) which is very
different from a typical receiver.

I have already gone down the road of looking extensively at loop antenna
designs. I have not found a significant difference other than the ease
of construction. That is one reason why I chose to use coax rather than
wire within a shield like pipe or a bicycle rim (as I found in one
project).

My current design is 100 feet (the 50 feet I said originally was due to
my poor recollection) wound on a 2 foot diameter spoke arrangement of
wood which turned out pretty well for a first pass. I have yet to
characterize the antenna which may be the easier path than trying to
construct a good model from theory and the known details.

Several people have suggested that a preamp will be required. That may
be possible. But this is not an analog receiver and don't need a lot of
SNR for it to work. The time code signal is modulated at 1 bps using
both phase and amplitude modulation and pulse width bit encoding. I
will need a resolution of no worse than 100 milliseconds to decode the
bits. So I figure a bandwidth of 10 Hz should be plenty enough. This
means I can vastly over sample the signal and get lots of gain digitally.

So the tricky part is to overcome the poor analog characteristics of the
differential digital input. I only need it to turn the input signal
into a one or a zero, but it needs to be sensitive to a very small
signal. With the various imperfections of input offset, hysteresis,
etc., I will be lucky if it works with very low voltage signals at all.
I could rig up a test circuit and see just what signal levels are
needed.

The other part is that the purpose of this design is to receive the
signal digitally on as low a power level as possible. The entire power
budget is a couple hundred microwatts. I have yet to find an amplifier
that will fit this power budget. Oddly enough some folks in s.e.d told
me that transistors don't work well with low bias currents, but that may
only apply to bipolar amps. They make time code receiver chips to do
this on a few hundred microwatts and have an internal amplifier. So
obviously it can be done. I just can't find a low enough power opamp
for a 60 kHz signal.

Also this a learning exercise for me. So reinventing something would be
ideal!

--

Rick

[ji...@specsol.spam.sux.com]

How about WWVB and the many existing examples?


--
Jim Pennino

[rickman]

Not sure what you are saying. There are tons of examples. But when I
did my search there was very little info on the design of loop antennas.
At least not much in depth enough to let me figure out how much signal
I might get from a given circuit.

I am asking about specific details of loop antenna design. I'm not sure
why people keep suggesting I look at "examples".

--

Rick

[ji...@specsol.spam.sux.com]

rickman <gnu...@gmail.com> wrote:
> On 10/28/2014 8:18 PM, ji...@specsol.spam.sux.com wrote:

<snip>

> Not sure what you are saying. There are tons of examples. But when I
> did my search there was very little info on the design of loop antennas.
> At least not much in depth enough to let me figure out how much signal
> I might get from a given circuit.
>
> I am asking about specific details of loop antenna design. I'm not sure
> why people keep suggesting I look at "examples".

WWVB is a US time and frequency standard station and the Internet is
full of articles on DIY antennas and receivers for WWVB.

Many of those articles go into great detail about their design.

As you asked about 60 kHz, it would seem to me to be the place to start
to look for words of wisdom on the subject, no matter what particular
detail you are looking for.


--
Jim Pennino

[Paul]

For commercial designs, I keep seeing references to a
ferrite core with a winding on it, as an antenna.

The article here, describes two kinds of receivers. One
is sensitive to AC pickup, so would only be a candidate
in special physical circumstances. The other uses the
high impedance input.

http://home.pon.net/785/equipment/build_your_own.htm

It suggests to me at least, you want plenty of gain
on the input stage, plus enough filtering to reject
louder noise sources. Your digital processing section
can provide the selectivity. But if spurious out of
band signals saturate your gain stage, you might not
get the desired result.

It would all depend on the tradeoffs you want to make.
You'll always require a gain stage.

Perhaps the antenna of your choice (not your final design)
and a spectrum analyser that works in that range of
frequencies, you can do a survey to see what is possible.
What noise sources are immediately evident, and so on.

No big antenna here. The antenna is one of these.

http://www.maplin.co.uk/p/ferrite-rod-aerial-lb12n

http://www.burningimage.net/clock/2007/10/23/sensitive-60khz-receiver/

I think by "sensitive" what they meant was "it picked
up the signal I wanted". The circuit diagram would
have been labeled "insensitive" if no signal was
found. Or if it didn't oscillate at 60KHz on its
own (like a couple amplifiers to drive speakers
have done here) :-) I think some audio circuit
I built, checking with a scope later on, indicated
a nice fat signal at 500KHz. Great.

Perhaps using your big loop of wire, you get to
remove one of the op-amps.

*******

The circuit above uses TL-081, with gain bandwidth product
of 3MHz. So I guess that's why there is still a bit of gain
at 72KHz.

In school, were were shown an example of a filter that
used only resistors. An example is seen on Fig 2.27(c)
on PDF page 70. The neat thing about this topology, is it
was working at 50KHz on a pair of $0.25 opamps. It uses the pole
of the output stage of the opamp, as a filter element. We
had some afternoon lab to do, with this circuit as part
of the work.

http://www.springer.com/cda/content/document/cda_downloaddocument/9780817683573-c1.pdf?SGWID=0-0-45-1354022-p174507347

9780817683573-c1.pdf 3,791,230 bytes

The book table of contents is here. It's by Mohan, P.V.A.
With ISBN 978-0-8176-8357-3. I was hoping the topology
had a name, but I don't see one.

http://www.springer.com/cda/content/document/cda_downloaddocument/9780817683573-t1.pdf?SGWID=0-0-45-1354069-p174507347

So the circuit could be in range of some opamps. And then
you might not need a huge antenna.

HTH,
Paul

[rickman]

I believe I said I have read much of that info. I did this a couple of
years ago and picked an approach. I was not able to convince myself it
would work properly. So now, before I build anything more, I would like
to fill in some of the details.

Of all the design info I found, not one discussed optimizing the antenna
for maximum voltage. When I was discussing this in another group,
specifically about a spice simulation of the circuit, someone pointed
out that I needed to include the effect of the radiation resistance.
Again, I have not found any other discussions of the radiation
resistance of a receiving antenna, specifically a tuned, shielded loop
antenna. Is this a red herring? When designing an antenna with a very
high Q, can the radiation resistance of a shielded loop antenna be ignored?

You say I should "start" with the many words of wisdom on the subject.
I am not "starting" and I have found many words of wisdom on loop
antennas in general, but not much on the specific questions I am asking.

It's a little bit funny, but when the one who shall not be named asked
about short antennas the discussions were full of info on radiation
resistance and details. Now that I am asking about my design, no one
wants to discuss the technical issues and just recommend some site where
they tell you how to build the antenna that suited their purpose.

--

Rick

[ji...@specsol.spam.sux.com]

rickman <gnu...@gmail.com> wrote:
> On 10/28/2014 9:27 PM, ji...@specsol.spam.sux.com wrote:
>> rickman <gnu...@gmail.com> wrote:
>>> On 10/28/2014 8:18 PM, ji...@specsol.spam.sux.com wrote:
>>
>> <snip>
>>
>>> Not sure what you are saying. There are tons of examples. But when I
>>> did my search there was very little info on the design of loop antennas.
>>> At least not much in depth enough to let me figure out how much signal
>>> I might get from a given circuit.
>>>
>>> I am asking about specific details of loop antenna design. I'm not sure
>>> why people keep suggesting I look at "examples".
>>
>> WWVB is a US time and frequency standard station and the Internet is
>> full of articles on DIY antennas and receivers for WWVB.
>>
>> Many of those articles go into great detail about their design.
>>
>> As you asked about 60 kHz, it would seem to me to be the place to start
>> to look for words of wisdom on the subject, no matter what particular
>> detail you are looking for.
>
> I believe I said I have read much of that info. I did this a couple of

Yes you did, after you made the post I responded to.

> years ago and picked an approach. I was not able to convince myself it
> would work properly. So now, before I build anything more, I would like
> to fill in some of the details.
>
> Of all the design info I found, not one discussed optimizing the antenna
> for maximum voltage. When I was discussing this in another group,
> specifically about a spice simulation of the circuit, someone pointed
> out that I needed to include the effect of the radiation resistance.
> Again, I have not found any other discussions of the radiation
> resistance of a receiving antenna, specifically a tuned, shielded loop
> antenna. Is this a red herring? When designing an antenna with a very
> high Q, can the radiation resistance of a shielded loop antenna be ignored?

The radiation resistance is a reciprocal property, i.e. it is the same
for transmitting and receiving.

I will assume you already know the relationship of resistance to Q.

> You say I should "start" with the many words of wisdom on the subject.
> I am not "starting" and I have found many words of wisdom on loop
> antennas in general, but not much on the specific questions I am asking.

That's all well and good but not evident until way into the postings.

> It's a little bit funny, but when the one who shall not be named asked
> about short antennas the discussions were full of info on radiation
> resistance and details. Now that I am asking about my design, no one
> wants to discuss the technical issues and just recommend some site where
> they tell you how to build the antenna that suited their purpose.

Again, radiation resistance is a reciprocal property.

To determine such things, you need to use an antenna analysis tool
and plug the resultant numbers into Spice which will tell you whether
or not it can be ignored.



--
Jim Pennino

[rickman]

Yes, a ferrite antenna is commonly used because of it's small size. But
when I crunched the numbers a larger loop produces a larger output
voltage than did the small loop of a ferrite antenna. The ferrite only
increases the output by the relative permeability, a constant of the
ferrite material that is relatively small compared to the gain of a
larger loop which goes by the the area of the loop proportional to the
square of the radius/circumference or for a constant length of wire is
inversely proportional to the number of turns. In other words you can
do more by making your loop larger than you can by using a ferrite
core... assuming you are not restricted to your loop size.

The length of the antenna wire is important because it determines much
of your losses and so the Q. The Q of the antenna is the ratio of the
total loss resistance to the inductive reactance. Since the Q depends
on the inductance things get complex.

L ∝ N^2 * A where N is the number of turns and A is the loop area

The output voltage of the tuned antenna circuit is the product of the
effective height, Q and field strength or

V ∝ he * Q

since the field strength is constant.

Effective height is the number of turns times the area divided by the
wavelength.

he ∝ N * A

since the wavelength is constant. This gives

V ∝ N^3 * A^2 / Rloss

Looses are from wire resistance with skin effect and radiation
resistance. Assuming Rloss is mostly from the resistance of the wire
with skin effect which will be related to the wire length we can hold
that constant and look at V as a function of the tradeoff between A and N.

N ∝ 1/r and A ∝ r^2. So replacing both N and A we have

V ∝ (1/r)^3 * r^4 or r, so a larger radius gives the strongest signal
everything else being equal. While the permittivity may affect the
signal from the antenna, the typical ferrite antenna is many small if
not tiny loops while fewer, larger loops without a ferrite should give a
stronger signal.

I think this is the first time I have done this all as one line of
thought, so I may have made a mistake somewhere. But I'm pretty sure
the result is correct. It may be mitigated by the small gauge of the
wire normally used for ferrite coils allowing more turns to be used.
But again, that same wire can be used with a larger loop size even if it
does lower the Q.

More interesting is the impact of wire diameter on the whole thing. The
RG-6 wire I chose is about optimal regarding the conductor diameter with
the skin affect making anything larger not of much value. Of course the
fact that it is coax makes it a lot larger when using lots of turns.

This page has a very good drawing of the circuit showing all the
elements about a quarter of the way down the page.

http://sidstation.loudet.org/antenna-theory-en.xhtml

> The article here, describes two kinds of receivers. One
> is sensitive to AC pickup, so would only be a candidate
> in special physical circumstances. The other uses the
> high impedance input.
>
> http://home.pon.net/785/equipment/build_your_own.htm
>
> It suggests to me at least, you want plenty of gain
> on the input stage, plus enough filtering to reject
> louder noise sources. Your digital processing section
> can provide the selectivity. But if spurious out of
> band signals saturate your gain stage, you might not
> get the desired result.
>
> It would all depend on the tradeoffs you want to make.
> You'll always require a gain stage.

I'm not sure what you mean by AC pickup, I guess you mean stray power
line signal? The E field receiver is pretty much what I don't want.
The antenna picks up very little signal because of the small physical
size while being very large. The E field is allegedly the source of a
lot of near field interference from appliances. The (again alleged)
advantage of the magnetic antenna is that the shield blocks the E field
and reduces many interference sources. I say alleged because I have not
seen much verifiable info on this and at least one source I found (and
have since lost) disputed the claim of reduced interference by the shield.

The only thing I found of value from this link was the emphasis on low
pass filters, which in my case will be band pass filters, first in the
antenna itself and then in the receiver.

Thanks for your suggestions. My purpose in building this is not to
receive the WWVB signal. If it were I would just buy one of the small
kits that do it with two chips and a ferrite antenna. My purpose is to
receive the WWVB signal with a digital receiver that is close to the
power consumption of the analog receiver.

--

Rick

[rickman]

Yes, but I've never calculated it for either. I have been working with
effective height. That is one thing I'm not clear on, how the two
effects can be separated. I guess the radiation resistance has
different impact depending on the circuit used. It will be more
apparent in a high Q circuit than a low Q one.

> I will assume you already know the relationship of resistance to Q.
>
>> You say I should "start" with the many words of wisdom on the subject.
>> I am not "starting" and I have found many words of wisdom on loop
>> antennas in general, but not much on the specific questions I am asking.
>
> That's all well and good but not evident until way into the postings.
>
>> It's a little bit funny, but when the one who shall not be named asked
>> about short antennas the discussions were full of info on radiation
>> resistance and details. Now that I am asking about my design, no one
>> wants to discuss the technical issues and just recommend some site where
>> they tell you how to build the antenna that suited their purpose.
>
> Again, radiation resistance is a reciprocal property.
>
> To determine such things, you need to use an antenna analysis tool
> and plug the resultant numbers into Spice which will tell you whether
> or not it can be ignored.

Actually, I was researching to verify my conclusions made the last time
I took a stab at this and found a page that gives a formula for
radiation resistance proportional to (μr N A/λ^2)^2. I hope the Greek
letters show properly. Holding μr and λ constant that makes the
radiation resistance proportional to r^2. I will need to check this to
see if it is significant compared to the resistive losses.

--

Rick

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2p6ln$seh$1...@dont-email.me:

>> MSF time signals? Just a thought... If you're interfacing an analog
>> signal to digital, one trick I used (for audio but it ought to help
>> here too) is a CA3140 with a bit of positive feedback through a few
>> Mohms for hysteresis to clean the signal a bit. The resulting Schmitt
>> trigger, powered by about 5 or 6V, could be sensitive to take a lot of
>> strain off your antenna. Whether this alone gives you enough gain I
>> don't know, but it is cheap to try.
>
> Thanks for the suggestion. I'm not sure this would be any better than
> feeding it directly into my digital input. That is a differential input
> and I expect to use feedback to overcome the residual input offset. So
> the input will be pretty sensitive

Well, try it. :) If it works then inputs are better these days. Or at least,
more sensitive to small changes. As far as I know, digital inputs are usually
specified with a wide dead band for levels, amounting to HUGE hysteresis and
a need for a lot of gain first sp you already ned an op-amp stage no matter
what unless your digital inputs have hair triggers at exactly the threshold
you wanr.

The thing about the CA3140 is that with just three passive parts: M-ohmage of
positive feedback, input series capacitance, and input ground resistor after
the cap, you can empirically set some very nice signal preconditioning as
well as raw gain, all on a very convenient single rail supply at 5V.

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2p6ln$seh$1...@dont-email.me:

> By using positive feedback the threshold would be shifting and the
> amount of level shift would set the floor for the signal level from the
> antenna I think.
>

Yes, basically like a noise gate. The op-amp trick is nice though, it gives
you fine control of it.

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2p85q$1ui$1...@dont-email.me:

> I only need it to turn the input signal
> into a one or a zero, but it needs to be sensitive to a very small
> signal.

My widget was aimed at exactly this need. :) That's why I recognised a new
need. My original need was for an electret mic. but it had to be so sensitive
I could whistle gently with barely pitched sound on the other side of a
quiet room and have it track like a fighter jet's navigation. It was a lovely
combination of sensitivity and clean reliability too, intended as the front
end control of an electronic musical instrument. I'd used a bit of gain and
bandpassing before the CA3140 Schmitt trigger, but in the case of time
signals I doubt it would need this extra preprocessing. Just add a cap and
resistor on output to integrate the 60KHz into clean slow pulses.

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2p85q$1ui$1...@dont-email.me:

> The entire power
> budget is a couple hundred microwatts.

There's a tiny Texas Instruments one that might do it, very cheap too.
TLV2341, uses as little as 17µA single rail supply at up to 8V. I didn't use
it because it wasn't fast enough for what I bought it for, but it might be
worth trying for MSF signals.

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2p85q$1ui$1...@dont-email.me:

> The entire power
> budget is a couple hundred microwatts. I have yet to find an amplifier
> that will fit this power budget.

That TLV2341 will stretch to do this drawing just 17渙, UGB is only 27KHz,
but if you set it for medium bias, consuming 250渙, you'll get 300KHz. Not
sure how much gain it will let you have for 60KHz, but I think it's one to
try.

[Wimpie]

El 28-10-14 21:33, rickman escribió:

To get some idea of the output voltage of a loop you need to know:

The fieldstrength of the desired signal at your area. This is probably
given in V/m (dBuV/m, etc). As a first guess use E/H = 377 Ohms to
convert this to H-field [A/m].

EMF = n*A*u0*w*H gives you the EMF for a loop with area A and n number
of turns, w = radian frequency, u0 = magn. permeability for air.

The EMF is boosted with the Q-factor of your tuned loop. Guessing the
Q is the difficult part. You can't just use resistive loss (even
when corrected for skin effect). As you have a multi-turn loop there
is an eddy current loss due to proximity of the turns (the so-called
proximity loss). At these frequencies loss due to radiation is
negligible, unless you make very large coils.

Practically spoken you can't model the proximity loss in spice. In my
opinion you should measure the Q of your loop, or do some search on
Q-factor of VLF/MF coils for your coil geometry. That result you can
put into spice together with the induced EMF.

At these frequencies, external (induced) noise is the dominant factor,
think of man made noise. Only the resistive loss part of the capacitor
generates thermal noise. Using a coaxial cable as tuning capacitance
will not give the highest Q as you have a long/thin conductor. A
parallel plate capacitor has less resistive loss.

Are you able to use good quality RG58? As far as I know RG6 for
consumer CATV has low copper content and may have a CCS center conductor.



--
Wim
PA3DJS
Please remove abc first in case of PM

[John S]

It is good to hear from you again, Wim. I have missed your very
knowledgeable posts.

John KD5YI

[rickman]

On 10/29/2014 6:53 AM, Lostgallifreyan wrote:
> rickman <gnu...@gmail.com> wrote in news:m2p6ln$seh$1...@dont-email.me:
>
>>> MSF time signals? Just a thought... If you're interfacing an analog
>>> signal to digital, one trick I used (for audio but it ought to help
>>> here too) is a CA3140 with a bit of positive feedback through a few
>>> Mohms for hysteresis to clean the signal a bit. The resulting Schmitt
>>> trigger, powered by about 5 or 6V, could be sensitive to take a lot of
>>> strain off your antenna. Whether this alone gives you enough gain I
>>> don't know, but it is cheap to try.
>>
>> Thanks for the suggestion. I'm not sure this would be any better than
>> feeding it directly into my digital input. That is a differential input
>> and I expect to use feedback to overcome the residual input offset. So
>> the input will be pretty sensitive
>
> Well, try it. :)

Yes, easier said than done. The receiver isn't built yet, I am
currently looking at the antenna design again and wish to improve my
simulation by adding the radiation resistance. If the antenna will only
put out microvolts even after tuning I will need to figure out how to
add the amp without having to double or quadruple the power budget.

> If it works then inputs are better these days. Or at least,
> more sensitive to small changes. As far as I know, digital inputs are usually
> specified with a wide dead band for levels, amounting to HUGE hysteresis and
> a need for a lot of gain first sp you already ned an op-amp stage no matter
> what unless your digital inputs have hair triggers at exactly the threshold
> you wanr.

This is a differential input which is not far from an analog input.
Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that. But there is always some because of
the parasitic capacitance between the input and output of the buffer.

> The thing about the CA3140 is that with just three passive parts: M-ohmage of
> positive feedback, input series capacitance, and input ground resistor after
> the cap, you can empirically set some very nice signal preconditioning as
> well as raw gain, all on a very convenient single rail supply at 5V.

This design won't have a 5 volt rail. Most of the design will run on
1.2~1.8 volts with some I/O at 3.3 volts to drive an LCD. It's very low
power, remember?

--

Rick

[rickman]

GBW is only 0.79 MHz @ 3V Vdd, so I could only get a gain of... well not
much at 60 kHz. For an opamp to work as an opamp it needs to have
significant gain over the BW in use. I suppose I could use it open
loop, but then it would act as a low pass filter with a high gain and a
very low corner frequency.

--

Rick

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2rcp6$nng$1...@dont-email.me:

> Actually even single ended digital inputs don't have much hysteresis
> unless they are designed for that.

Well, as a proportion if they only go high above soem fairly close approach
to V+, then low when close to 0V, then the dead band could be wide, the aim
was to eliminate false states so they ARE usually designed for it. :) I take
your point on very low volt systems, if the actual difference is small even
though proportionally it may not be.

Anyway, now I know that the supply is so small, your suggestion of discrete
transistors is almost certainly the way to go, unless there is enough similar
demand out there to have cause an off-shelf part to be made.

Normally I'd just look at how others are solving similar problems, so I guess
the question I can ask is: what is the signficant difference in this case
that prevents the nearest off-shelf answer from working?

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2rd7l$p0t$1...@dont-email.me:

> GBW is only 0.79 MHz @ 3V Vdd, so I could only get a gain of... well not
> much at 60 kHz.

True, I looked at it more earlier this evening, at 3V supply you'd be lucky
to get much more than a gain of 40 I think, so some specific and discrete
transistor fix might be best.

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2rd7l$p0t$1...@dont-email.me:

> For an opamp to work as an opamp it needs to have
> significant gain over the BW in use.

Ok, how about just enough gsain to get a buffered output of some oomph to
survive integration to slow clean pulses? That might not take so much to do,
and if it works, it really takes the strain off the real gain stage
which follows it because that will be operating pretty much at DC capability.
:)

[rickman]

This is new to me. I guess I have been mistakenly using the E field
formula. The field strength at optimum times is estimated at 100 uV/m
at my location which is at the weak end of the CONUS map. I will plug
the numbers into your H field version of the equation.

> The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q
> is the difficult part. You can't just use resistive loss (even when
> corrected for skin effect). As you have a multi-turn loop there is an
> eddy current loss due to proximity of the turns (the so-called proximity
> loss). At these frequencies loss due to radiation is negligible, unless
> you make very large coils.

I have not seen the proximity effect taken into account in any
calculations for similar antenna, so I assumed it was also not
appreciable at this frequency. I'm not at all sure about the radiation
resistance. I will be plugging the numbers into the equation I have. I
assume this resistance would be in parallel with the inductor so a high
value is better. Or would it appear in series with the inductor and a
low value is better?

> Practically spoken you can't model the proximity loss in spice. In my
> opinion you should measure the Q of your loop, or do some search on
> Q-factor of VLF/MF coils for your coil geometry. That result you can put
> into spice together with the induced EMF.

I'm surprised you feel the Q can't be calculated. When originally
digging into this I found that the calculation of inductance is an
amazingly complex thing. There are lots of equations out there each of
which simplifies some aspect of the phenomenon and have different
applications. I would not expect the proximity effect to be any more
complex.

> At these frequencies, external (induced) noise is the dominant factor,
> think of man made noise. Only the resistive loss part of the capacitor
> generates thermal noise. Using a coaxial cable as tuning capacitance
> will not give the highest Q as you have a long/thin conductor. A
> parallel plate capacitor has less resistive loss.

Q is important, but not the only factor. The coax was chosen to be
inexpensive and easy to work with. RG-6 with an 18 ga solid center
conductor is just slightly bigger than the skin effect and so is about
as usefully large a conductor without it being hollow. So I'm not sure
what might be better. I suppose Litz wire could improve the Q, but I'm
already looking at a Q of ball park 100 or more. Once you get a very
high Q it become hard to use the device without ruining the Q.

> Are you able to use good quality RG58? As far as I know RG6 for consumer
> CATV has low copper content and may have a CCS center conductor.

I picked an RG-6 with a solid center conductor. The specified
resistance is 6.5 mohm per foot. Funny, I'm sure most RG-6 is used for
cable TV where the center conductor is steel for strength with copper
plating for conductivity at high frequencies. One vendor argued with me
that solid copper cores were not available in RG-6. lol

BTW, I measured the resistance of my 50 foot of cable and it is in the
right ball park for 6.5 mohm/foot. The shield measured in the same
range as well. I thought the shield might have had a lower resistance
because it would amount to a larger cross section, but I guess not. I
don't think the shield resistance factors into the Q, but I'm not
certain of that.

--

Rick

[rickman]

What off the shelf answer? I have not seen any all digital receivers
for any frequency. I think it may only be practical for this case and
I"m not sure of that. lol

This signal is very unique in that it has a very low data rate. This
allows integration in the digital domain over a large number of samples.
Theoretically the signal would be detectable with a negative SNR.
There are actually a number of issues I need to solve to get a prototype
working. The big one is being able to get a large enough signal that
even statistically it is noticeable at the receiver input.

--

Rick

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2rhm3$bo2$1...@dont-email.me:

> What off the shelf answer?

I just meant in terms of interfacing. :) Never mind, one of my other replies
might be far more useful. While you can integrate digitally, why do so? It
seems to me (if I haven't missed something I shouldn't) that you might get
away with much less gain before analog integration, then you can boost the
resulting slow signals with much less struggle with gand bandwidth products
and slew rates for low power and such. If you can do it this way, the
resulting slow pulses can be boosted with CMOS which at those speeds will be
pretty much nanopower.

[Wimpie]

El 29-10-14 21:03, rickman escribió:

Based on your 100 uV/m, H = 0.27 uA/m Using a coil with 2 ft
diameter, this would result in EMF = 35 nV for a single turn.

>
>
>> The EMF is boosted with the Q-factor of your tuned loop. Guessing the Q
>> is the difficult part. You can't just use resistive loss (even when
>> corrected for skin effect). As you have a multi-turn loop there is an
>> eddy current loss due to proximity of the turns (the so-called
>> proximity
>> loss). At these frequencies loss due to radiation is negligible, unless
>> you make very large coils.
>
> I have not seen the proximity effect taken into account in any
> calculations for similar antenna, so I assumed it was also not
> appreciable at this frequency. I'm not at all sure about the radiation
> resistance. I will be plugging the numbers into the equation I have. I
> assume this resistance would be in parallel with the inductor so a
> high value is better. Or would it appear in series with the inductor
> and a low value is better?

What are you going to make (a link to a drawing may be helpful)?
What equations do you have for the Q factor for your geometry?

>
>
>> Practically spoken you can't model the proximity loss in spice. In my
>> opinion you should measure the Q of your loop, or do some search on
>> Q-factor of VLF/MF coils for your coil geometry. That result you can
>> put
>> into spice together with the induced EMF.
>
> I'm surprised you feel the Q can't be calculated. When originally
> digging into this I found that the calculation of inductance is an
> amazingly complex thing. There are lots of equations out there each of
> which simplifies some aspect of the phenomenon and have different
> applications. I would not expect the proximity effect to be any more
> complex.

If calculation of L is very difficult, Q will be also, as they are
related. Many formulas for Q factor for certain geometry are (partly)
empirical. Formulas for Q for real coils take proximity into account.

You may know that Q-factor heavily depends on frequency.

If you use the cable dielectric as part of the tuning, it is good that
you have cable with solid copper instead of CCS, otherwise lots of the
current would be into steel instead of copper. Your DC resistance
value is correct for copper (assuming about 1 mm diameter).

Your probably found that turns should not touch (increases proximity
loss and loss due to the jacket) to get highest Q factor. A high Q
factor helps you rejecting out of band signals. What values of
inductance do you expect?

In parallel equivalent circuit, the loss resistance (Rp) equals:
Rp = XL*Q = w*L*Q.
When the output goes directly to the input circuitry, Zin >> Rp to
avoid reduction of Q.

[rickman]

Before integration comes demodulation. How would you demodulate and
integrate in the analog domain on a 100 uW power budget? The signal is
PSK. But that is not the real reason. My goal is to show it is
possible to do this entirely in the digital domain.

The devices I have available are not 100% optimized for low power at low
clock rates, but they are pretty good. If I can find devices that have
lower quiescent current the digital design has potential of being lower
power than the analog approach.

--

Rick

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2tq72$5ul$1...@dont-email.me:

> Before integration comes demodulation. How would you demodulate and
> integrate in the analog domain on a 100 uW power budget? The signal is
> PSK. But that is not the real reason. My goal is to show it is
> possible to do this entirely in the digital domain.
>

Low Vf diode in feedback loop of op-amp? I'm curious though, it's an
interesting thought, doing it all in digital equipment, but why? The main
drive behind me 'off-shelf' remark is that I suspect the best answer already
exists in many forms. I'm curious about what makes a need to keep searching.
:) I'm not denying it, far from it, there's usually more than one good way to
do something, I'm just not sure what the differentiating factor is in this
case.

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2tq72$5ul$1...@dont-email.me:

> The signal is PSK.

I missed that bit. :) I thought it would be simple AM.. If the integrated
signal (after feedback diode demod) differ enough in amplitude (or AC
content) with frequency, threshold detection might be enough. I'm just
pondering it though, I have no idea if it can be done with less power than
you can give it.

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2tq72$5ul$1...@dont-email.me:

> The signal is PSK.

My sight isn't very good. That's Psk, not Fsk... Phase? What did I miss. :)
I've been hung up on the notion that this is an MSF time signal thing, and I
just looked at the spec for the UK one which is a simple switch on/off of a
carrier, so easy to detect efficiently. Yours is something else entirely, but
what? You may need to lay a lot more cards down before you find an answer you
can use, unless you hunt in the dark. (No reason not to, I usually do, on
most things I do, as the net usually makes some light at greatest need).

[rickman]

I don't know about "best" but you can buy a time code receiver chip that
spits out a demodulated signal to be decoded by an MCU. At that point
the data rate is pretty low so an MCU can run at very low power levels,
likely dominated by the quiescent current.

When you suggest an op amp, we already covered that ground and they
aren't low power enough. I'm curious how they amplify the signal in the
receiver chip with the whole circuit drawing a very low power level.

--

Rick

[rickman]

The signal is also AM, but the PSK is supposed to be detectable at lower
signal levels.

--

Rick

[rickman]

I have not studied the international time signals extensively, but I
believe they all use AM. The US located beacon added PSK a few years
back to make the signal easier to receive. The US is large enough that
reception is poor in some of the east coast areas. I am east coast and
would like to see just how much I can do to optimize the antenna to make
this work well.

--

Rick

[Lostgallifreyan]

rickman <gnu...@gmail.com> wrote in news:m2u07f$u30$1...@dont-email.me:

> The US located beacon added PSK a few years
> back to make the signal easier to receive.

I went Googlong after I wrote that last one. I'd have thought PSK would be
harder to detect than AM. So much for what I know. :) I read that
wristwatches can detect the PSK signal too, so low power must have been done,
with a small antenna too, but I don't knpow what they did so I'll leave it
there.

About op-amps, I just found it hard to let go of a favourite idea. :) Too
many transistors though...

Just one thought left that might be a kernel of a new idea: if you have a
tiny resonant circuit at 60KHz, with high Q, then the change of phase ought
to make some kind of detectable upset, a spike maybe, whose polarity you can
use to determine mark or space in the signal. The combination of resonance
and short spike duration might give you a usable combination of low power and
detectable threshold.

I'll stop there because I don't think I have anything you can use.. I'm
interested in what you come up with though, especially if it avoids a large
antenna.

[rickman]

Define large... lol. I'm already looking at a 2 foot diameter which is
just a wee bit too large for my wrist watch.. lol. Actually I really
don't know how practical this is. The digital approach will depend
greatly on pulling a very weak signal out of the noise. Not that the
noise is in the signal on the antenna, but noise in terms of poor
detection of such a weak signal.

I will happily report back if/when I get any sort of results.

--

Rick

[George Cornelius]

Motorola's app notes on the old 4000 series CMOS included
various analog circuits, including use of a CMOS inverter
as an amplifier. I'm enough of a packrat that I keep those
things.

4000 series may not be useful in your case, but the circuits
or variants of them may apply in newer CMOS implementations.

'Course calling it all digital may be just a game if your
input stage is a digital circuit biased to operate in an
analog mode.

George

[Rob]

George Cornelius <gcorn...@charter.NOWHERE.net> wrote:
> Motorola's app notes on the old 4000 series CMOS included
> various analog circuits, including use of a CMOS inverter
> as an amplifier. I'm enough of a packrat that I keep those
> things.

I'm sure this guy (who is coming back on this subject regularly) is
not going to consider that low-power. The inverter was driven into
the area between switching to '1' and to '0' by using a feedback
resistor, and so both output fets are conducting and drawing current
from Vcc to Gnd.

[rickman]

If you I am "the guy", whether or not this is low power enough depends
on the power. My understanding is that when operated in the linear mode
significant current can flow in a CMOS device. So likely this isn't low
enough power, no.

I'm very curious about how they do it in the commercial chips. I have
seen block diagrams and they show an amplifier as the first part of the
chip. Maybe the design really isn't all that low power. Rather than
running at low power all the time, they just limit the duty cycle of the
receiver. "Atomic" clocks don't need to monitor the signal except for a
few minutes each day.

--

Rick

[Rob]

I have several battery-powered "atomic clocks" and all of them enable
the receiver only for a few minutes, either every hour or twice a day
depending on the particular design. The receiver I have connected to
my computer is of course enabled all the time.

Many years ago I worked on a "shop-shelf tag" system that used a low
frequency receiver in a single-chip design, and it also had a power
saving mechanism. The tags (powered by single lithium cell like those
used as a BIOS backup battery) were usually in a sleep mode only driving
the LCD, and once every so many seconds they briefly enabled the receiver.
To run an update, the controller sent a wakeup signal that lasted long
enough to get the attention of all tags, then it sent the updates
addressed to each tag, and finally an end-of-transmission signal that
put everything back into sleep mode. The lithium cell lasted several
years, I think.

[rickman]

On 11/5/2014 8:29 PM, Jeff Liebermann wrote:>
> Incidentally, I don't believe using a high impedance loop and amp are
> good ideas. While there are benefits, my experiences from the marine
> radio biz convinced me that high voltage is an invitation to problems
> from condensation, salt fog, and PCB leakage. In other words, it
> works on the bench, but craps out in the field. I'll probably end up
> with a large high Q loop, and a separate low-Z coupling loop (i.e. a
> step down xformer).

Not sure why you can't discuss this in the right thread of this group.
I've posted my reply to your post in the loop antenna thread.

First, I'm not sure what you are talking about connecting high impedance
antennas to condensation and salt fog. If you are transmitting, then
maybe you could get such high voltages as to attract microscopic
objects, but this is a receiver design.

Also, the antenna is not high impedance, just the input to the receiver.
The transformer I am looking at is a high turns ratio current sensor.
It spans the right frequency range and is a nice compact package easy
to mount on a PCB.

My main concern is lowering the Q because of the loading from the
receiver input, especially with the change in impedance as reflected
through the transformer. I think when I simulated it, I found the max
signal strength came with a 25 or 33:1 turns ratio because with higher
turns ratios the Q was spoiled enough to bring the voltage down at the
receiver input.

This simulation didn't include the effect of the radiation resistance,
so I will need to add that in. I expect this will lower the Q as a
starting point which means the affect from the receiver input loading
will not be as significant, possibly making a higher turns ratio in the
transformer more useful.

--

Rick

[Jeff Liebermann]

On Wed, 05 Nov 2014 20:50:31 -0500, rickman <gnu...@gmail.com> wrote:

>Not sure why you can't discuss this in the right thread of this group.
>I've posted my reply to your post in the loop antenna thread.

Because I prefaced my comments by mentioning that a 60 KHz loop is on
my "agenda". I guess that's a bit vague. What I meant to say was
that I'm not very well read on the technology involved, a total clutz
with LTspice, and I haven't built another loop so I can measure how it
acts. In other words, I'm not ready to discuss it (unless you can
tolerate my guesswork).

>First, I'm not sure what you are talking about connecting high impedance
>antennas to condensation and salt fog. If you are transmitting, then
>maybe you could get such high voltages as to attract microscopic
>objects, but this is a receiver design.

Well, a 33:1 turns ratio is a 1000:1 impedance ratio. Using 75 ohms
as the coax cable and the characteristic impedance, that's 75K ohms.
In general, board leakage and conduction problems start around 100K
(depending on trace spacing etc), so I suspect you can make it work,
at least on the bench. However, in the typical marine atmosphere,
with ionic crud in the water, there will be leakage issues. I don't
recall the typical sheet resistivity for a standing salt water puddle
on a PCB, but I suspect it will be a problem. Of course, you can
conformal coat the board, hermetically seal the package, wax dip it,
or pot the antenna amplifier in epoxy to avoid the problem. However,
the favored method is to design with low impedances and not create new
problems with conformal coatings and sealed boxes.

There are also some PCB layout tricks that will help. For example,
here's part of a book on PCB design issues:
<http://www.analog.com/library/analogdialogue/archives/43-09/edch%2012%20pc%20issues.pdf>
See Pg 12-15 to 12-19 on "Static PCB Effects" with examples of PCB
guard patterns.

Incidentally, my unofficial test for decent design was to immerse the
radio in a bucket of genuine San Francisco Bay salt water. If the
board continued to operate normally, it passes. If not, I get to
spend the evening with the bucket and a megohmmeter looking for the
culprit.

If you're building this loop as an academic exercise, you can probably
ignore all the aforementioned comments on PCB leakage. However, if
you're going to sell it, think carefully about such environment
problems.

>Also, the antenna is not high impedance, just the input to the receiver.
> The transformer I am looking at is a high turns ratio current sensor.
> It spans the right frequency range and is a nice compact package easy
>to mount on a PCB.

Why not just make it a 40 KHz tuned xformer? You get the same
impedance transformation with the added bonus of additional bandwidth
reduction (increased Q) to eliminate as much atmospheric and man made
noise as possible. It's also much less lossy than a broadband
xformer.

>My main concern is lowering the Q because of the loading from the
>receiver input, especially with the change in impedance as reflected
>through the transformer.

Well, you're stuck with matching the loop to the receiver input
anyway, so there's no way around that with passive components. You
can insert an emitter follower to do the impedance transformation.

Incidentally, the typical loaded Q for such loops seems to be around
100. Some claim 200 or more, but for small loops, 100 seems to be the
target. At 40 KHz, that's a -3dB bandwidth of 200 Hz, which is rather
wide for a 1Hz wide WWVB signal. You could probably increase the Q
somewhat, mostly be reducing the resistive losses, but that might
create drift and tuning accuracy problems. Higher Q is possible, but
I suspect will require a much more rigid and beefy design.

>I think when I simulated it, I found the max
>signal strength came with a 25 or 33:1 turns ratio because with higher
>turns ratios the Q was spoiled enough to bring the voltage down at the
>receiver input.
>
>This simulation didn't include the effect of the radiation resistance,
>so I will need to add that in. I expect this will lower the Q as a
>starting point which means the affect from the receiver input loading
>will not be as significant, possibly making a higher turns ratio in the
>transformer more useful.

I can't comment on that without seeing the design. Actually, I'm not
sure seeing the design will help as I need to do some more reading
before I can understand exactly how it works.

11:30PM. Time for dinner.

--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Jeff Liebermann]

On Wed, 05 Nov 2014 23:23:32 -0800, Jeff Liebermann <je...@cruzio.com>
wrote:

>Why not just make it a 40 KHz tuned xformer?

Somehow, my brain converted the 60 KHz WWVB frequency, to the 40 KHz
frequency of the vehicle detection system I'm working on. The
frequencies mentioned should be 60 KHz and as usual, my arithmetic
sucks.

>target. At 40 KHz, that's a -3dB bandwidth of 200 Hz, which is rather
>wide for a 1Hz wide WWVB signal.

At 60 KHz, that's a -3dB bandwidth of 600 Hz, which is rather

wide for a 1Hz wide WWVB signal.

[rickman]

On 11/6/2014 2:23 AM, Jeff Liebermann wrote:
> On Wed, 05 Nov 2014 20:50:31 -0500, rickman <gnu...@gmail.com> wrote:
>
>> Not sure why you can't discuss this in the right thread of this group.
>> I've posted my reply to your post in the loop antenna thread.
>
> Because I prefaced my comments by mentioning that a 60 KHz loop is on
> my "agenda". I guess that's a bit vague. What I meant to say was
> that I'm not very well read on the technology involved, a total clutz
> with LTspice, and I haven't built another loop so I can measure how it
> acts. In other words, I'm not ready to discuss it (unless you can
> tolerate my guesswork).

I'm happy bouncing things off you. I did some reading on this back a
year or so ago and feel like I got a lot, but not enough to really
optimize it for my application. One thing I was missing was an
understanding of the radiation resistance which I now have a formula for
and can include in my LTspice simulation when I get to it.

I don't quite have a feel for radiation resistance in terms of its
effect on the receive antenna, but I'm sure that will come once I look
at the equations. I expect it will be small, hopefully small compared
to the wire resistance.

One thing that gave me fits early on is the calculation of the loop
inductance. Seems there are a lot of equations out there and most of
the sources don't talk about where they got them or what they assume. I
finally got one from Lundin that seems pretty good and covers the widest
range of coils I might be using.

>> First, I'm not sure what you are talking about connecting high impedance
>> antennas to condensation and salt fog. If you are transmitting, then
>> maybe you could get such high voltages as to attract microscopic
>> objects, but this is a receiver design.
>
> Well, a 33:1 turns ratio is a 1000:1 impedance ratio. Using 75 ohms
> as the coax cable and the characteristic impedance, that's 75K ohms.

Forget 75 ohms. There is no cable. The antenna connects directly to
the receiver circuit through the transformer. The characteristics of
the antenna are defined by the inductance of the loop and the resonance
with the tuning capacitor and the Q.

> In general, board leakage and conduction problems start around 100K
> (depending on trace spacing etc), so I suspect you can make it work,
> at least on the bench.

100k? I will be using up to 10 Megohm parts but even that is not very
sensitive to board leakage unless you leave a lot of rosin on the board
and it collects dust for a few years.

> However, in the typical marine atmosphere,
> with ionic crud in the water, there will be leakage issues. I don't
> recall the typical sheet resistivity for a standing salt water puddle
> on a PCB, but I suspect it will be a problem.

I won't be in salt spray, it will be in my living room. Still, any
aquatic electronics would be in a sealed enclosure.

> Of course, you can
> conformal coat the board, hermetically seal the package, wax dip it,
> or pot the antenna amplifier in epoxy to avoid the problem. However,
> the favored method is to design with low impedances and not create new
> problems with conformal coatings and sealed boxes.
>
> There are also some PCB layout tricks that will help. For example,
> here's part of a book on PCB design issues:
> <http://www.analog.com/library/analogdialogue/archives/43-09/edch%2012%20pc%20issues.pdf>
> See Pg 12-15 to 12-19 on "Static PCB Effects" with examples of PCB
> guard patterns.

I am familiar with guarding, but that is not going to be needed with an
antenna. The voltage will be very low level even when the Q is
optimized, so no appreciable leakage currents.

> Incidentally, my unofficial test for decent design was to immerse the
> radio in a bucket of genuine San Francisco Bay salt water. If the
> board continued to operate normally, it passes. If not, I get to
> spend the evening with the bucket and a megohmmeter looking for the
> culprit.
>
> If you're building this loop as an academic exercise, you can probably
> ignore all the aforementioned comments on PCB leakage. However, if
> you're going to sell it, think carefully about such environment
> problems.

I don't think it will ever see duty on a sea vessel.

>> Also, the antenna is not high impedance, just the input to the receiver.
>> The transformer I am looking at is a high turns ratio current sensor.
>> It spans the right frequency range and is a nice compact package easy
>> to mount on a PCB.
>
> Why not just make it a 40 KHz tuned xformer? You get the same
> impedance transformation with the added bonus of additional bandwidth
> reduction (increased Q) to eliminate as much atmospheric and man made
> noise as possible. It's also much less lossy than a broadband
> xformer.

What would that entail?

>> My main concern is lowering the Q because of the loading from the
>> receiver input, especially with the change in impedance as reflected
>> through the transformer.
>
> Well, you're stuck with matching the loop to the receiver input
> anyway, so there's no way around that with passive components. You
> can insert an emitter follower to do the impedance transformation.

You aren't in tune with this design. The goal is to minimize power.
There won't be a preamp of any kind unless absolutely required.

> Incidentally, the typical loaded Q for such loops seems to be around
> 100. Some claim 200 or more, but for small loops, 100 seems to be the
> target. At 40 KHz, that's a -3dB bandwidth of 200 Hz, which is rather
> wide for a 1Hz wide WWVB signal. You could probably increase the Q
> somewhat, mostly be reducing the resistive losses, but that might
> create drift and tuning accuracy problems. Higher Q is possible, but
> I suspect will require a much more rigid and beefy design.

There is only so much that can be done to increase Q. The wire I am
using in the antenna is already pushing the skin effect at 1 mm
diameter. If I am reading the equations correctly increasing the number
of turns on the loop does increase Q. I am currently looking at 8 turns
(50 feet of RG-6) and may increase it to 100 feet (16 turns). But I've
already built a support and 16 turns will be hard to add without a
redesign.

>> I think when I simulated it, I found the max
>> signal strength came with a 25 or 33:1 turns ratio because with higher
>> turns ratios the Q was spoiled enough to bring the voltage down at the
>> receiver input.
>>
>> This simulation didn't include the effect of the radiation resistance,
>> so I will need to add that in. I expect this will lower the Q as a
>> starting point which means the affect from the receiver input loading
>> will not be as significant, possibly making a higher turns ratio in the
>> transformer more useful.
>
> I can't comment on that without seeing the design. Actually, I'm not
> sure seeing the design will help as I need to do some more reading
> before I can understand exactly how it works.

The equations are pretty simply once I found them (and could trust I had
the right ones).

Lundin's formula for inductance of a solenoid
L = N^2 * a * Correction Factor * μ0

N is the number of turns
a is the loop radius in meters
the correction factor based on the coil shape is a bit complex but comes
to 3.3 ballpark with the loop shape used.
μ0 is the permeability of free space

He is the effective height of the antenna, an expression of the
effectiveness of the antenna in converting the field into a voltage.

He = 2pi * N * A / λ, ignoring the orientation factor cos θ.

N is the number of turns
A is the loop area in meters^2
λ is the wavelength of the 60 kHz signal

Inductance and frequency get the reactance which when compared to the
total loss resistance yields the Q.

Multiply the effective height by the field strength (on the east coast
it's ~100 uV from WWVB) to get the antenna voltage. Someone was trying
to get me to use an equation based on the magnetic field but I believe
once you combine the equations you get the same calculation.

Multiply by Q and the transformer ratio and you have the voltage at the
receiver input.

Wire resistance goes up with the product of N and a, or in other words
the length of the cable. The loop inductance goes up with N^2 and a.
Effective height goes up with N and a squared (area). So a bigger loop
will get a larger signal but the same Q. Adding turns will get a larger
signal *and* a higher Q. Obviously the size of the loop has an upper
limit based on practicality, but more turns gets improved performance
with less impact on the size.

--

Rick

[Wimpie]

El 06-11-14 10:37, rickman escribió:

Depending on what is in your "Correction Factor", I would expect a^2
isntead of "a" (coil radius). I also expected to see the length of the
coil in the formula.

>
> He is the effective height of the antenna, an expression of the
> effectiveness of the antenna in converting the field into a voltage.
>
> He = 2pi * N * A / λ, ignoring the orientation factor cos θ.
>
> N is the number of turns
> A is the loop area in meters^2
> λ is the wavelength of the 60 kHz signal
>
> Inductance and frequency get the reactance which when compared to the
> total loss resistance yields the Q.
>
> Multiply the effective height by the field strength (on the east coast
> it's ~100 uV from WWVB) to get the antenna voltage. Someone was trying
> to get me to use an equation based on the magnetic field but I believe
> once you combine the equations you get the same calculation.

That someone was me, and you are right, results should be the same.

>
> Multiply by Q and the transformer ratio and you have the voltage at
> the receiver input.
>
> Wire resistance goes up with the product of N and a, or in other words
> the length of the cable. The loop inductance goes up with N^2 and a.
> Effective height goes up with N and a squared (area). So a bigger loop
> will get a larger signal but the same Q. Adding turns will get a
> larger signal *and* a higher Q. Obviously the size of the loop has an
> upper limit based on practicality, but more turns gets improved
> performance with less impact on the size.
>

More turns (that is more copper/copper area) give higher Q, but there
are other effects that will cause deviation from this reasoning. I
still can't draw on a piece of paper what you have in mind, however
you may search for coil/inductor design and Q factor together with
names of researchers/experimenters (Medhurst, Nagaoka, Wheeler, Corum,
etc).

If you are able to make an LC circuit with Q say over 10.000 (10k),
radiation resistance will have some influence. When discussing Q<1000,
size around 2 feet, forget radiation resistance, resistive loss
dominates.

[rickman]

It is not "my" correction factor, it is Lundin's. It is based on the
ratio of loop diameter to coil length and the formula I used applies for
diameters larger than the coil length. Here is the note in my spread
sheet...
Lundin's Formula for 2a>b, Proc IEEE, Vol 73, No. 9, Sept 1985
If you google it I'm sure you can dig up all sorts of references.

Of all the many inductance formulas I found none used the area rather
than coil radius (not squared). Here is one for a single loop from
http://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/current_lectures/L12_Loop.pdf

The inductance of a single circular loop of radius a made of wire of
radius b is
L = μ a (ln(8a/b)-2)

Notice the 'a' factor (loop radius) is not squared.

When I did my research, Lundin's formula appeared to be the one that
gave the best results over the largest range of coil diameter to length.
It was also fairly simple to program in a spreadsheet. There is even
one web page I found that discusses some of the attempts to do better
which actually failed for various reasons. I found this very interesting.
http://www.g3ynh.info/zdocs/magnetics/part_2.html

I have found most of those although more when looking for inductance
formula rather than Q formula.

> If you are able to make an LC circuit with Q say over 10.000 (10k),
> radiation resistance will have some influence. When discussing Q<1000,
> size around 2 feet, forget radiation resistance, resistive loss dominates.

Ok, that is what I expected. Still, I want to add radiation resistance
to my simulation just for completeness. It shouldn't be hard. It is
just a bit more math to type in.

If I get a Q of 10,000 (10k) I don't think the design would be usable.
A degree or two of temperature drift and it would be out of tune. I
would like to see a Q of over 100 though.

--

Rick