loop antenna question(s)
Lyle Koehler
>
> Hi,
>
> I've been doing some reading about loop antennas before I go ahead
> and build one for my shortwave listening... 6MHz to 21MHz....
>
> I've seen many that aim at the AM/MW band consisting of several
> wraps of wire and tuned with a 365pF cap. No problem. However, I've seen
> one that is aimed for the SW band made of a 'wide' single loop, also
> tuned with a cap... So, if a single wide loop 'works so well' for SW,
> why is that not done for the AM band? Is it just the lower amount of
> inductance needed to get the lower resonance frequency? any comments
> about either antenna and whether a loop antenna is good for my
> application?
First of all, you have answered one of your questions -- the reason for
more turns on the AM band is simply to get more inductance so that the
loop can be tuned with a reasonable sized variable capacitor. It's hard
to say for sure whether a loop will help you much in the 6 to 21 MHz
range. On the AM band, the directional characteristics of a loop are
useful for nulling out strong local stations and man-made noise sources.
Above a few MHz, you may find that the loop doesn't seem to show much
directivity, and a short (20 feet or more) piece of wire might receive
just as well.
>
> Also, I've seen large area, single wire loops (on the order of a
> wavelength) and then the much smaller multi-turn loops.. and comments
> that indicate the length of wire in the small loop is related to the
> wavelength...is this true? Isn't the loop just an inductance, regardless
> of the amount of wire used to make it? What I'd like to do is try to
> make a smaller loop with more turns to keep the inductance the
> same....am I just shooting myself in the foot by reducing the area?
>
Full-wave loops are great for transmitting, but are overkill for
receiving. Reception on frequencies below 21 MHz is usually limited by
external noise rather than receiver noise, unless the receiving antenna
is really inefficient. I've had very good reception with a two-turn,
1-foot diameter loop on the 7 MHz amateur band, but it incorporated a
preamplifier to make up for the low efficiency of the small loop. It
isn't necessary to use a wavelength of wire in a receiving loop.
However, efficiency is roughly proportional to the cube of the diameter,
so you can't make the loop *too* small. Reg Edward's RJELOOP3.EXE
program is very useful for "what-if" loop designs. It's free and can be
downloaded at http://www.btinternet.com/~g4fgq.regp/ For those of us on
this side of the pond, it can be a bit of a nuisance to think of wire
sizes in millimeters. Just make all of your loops out of #18 wire, which
is almost exactly 1 mm in diameter, and you won't have any trouble using
Reg's software :-)
--
Lyle, K0LR
AJ
I've been doing some reading about loop antennas before I go ahead
and build one for my shortwave listening... 6MHz to 21MHz....
I've seen many that aim at the AM/MW band consisting of several
wraps of wire and tuned with a 365pF cap. No problem. However, I've seen
one that is aimed for the SW band made of a 'wide' single loop, also
tuned with a cap... So, if a single wide loop 'works so well' for SW,
why is that not done for the AM band? Is it just the lower amount of
inductance needed to get the lower resonance frequency? any comments
about either antenna and whether a loop antenna is good for my
application?
Also, I've seen large area, single wire loops (on the order of a
wavelength) and then the much smaller multi-turn loops.. and comments
that indicate the length of wire in the small loop is related to the
wavelength...is this true? Isn't the loop just an inductance, regardless
of the amount of wire used to make it? What I'd like to do is try to
make a smaller loop with more turns to keep the inductance the
same....am I just shooting myself in the foot by reducing the area?
Also, I've read that the large loops respond to the electric field
while the smaller loops respond to the magnetic field...true? and if so,
what is the transition region? eg.. how would I know which field I'm
responding to for a given design?
Any references or comments would be appreciated.
Thanks :)
aj
AJ
... hmmm I saw one site that had what looked like just a bulky multi turn loop
... you were supposed to just put your AM/MW radio beside it and all your
troubles would go away... I was thinking a similar thing would happen for my
bands as I don't have an antenna connection for my radio...just a whip...
I've tried the wire already too.. I ended up with some more stations for my
SWL, but ended up with a lot of garbage too.. so I was hoping the loop would
provide some increased selectivity...I thought of a tuner, but my load and
source Z's would both be unknown.. seems pretty tricky to go over all the
bands that way...what about using a feedback loop to increase the Q of the
loop instead of a preamp?
thanks for the size/efficiency rule of thumb.. exactly what I was looking
for...
aj
> First of all, you have answered one of your questions -- the reason for
> more turns on the AM band is simply to get more inductance so that the
> loop can be tuned with a reasonable sized variable capacitor. It's hard
> to say for sure whether a loop will help you much in the 6 to 21 MHz
> range. On the AM band, the directional characteristics of a loop are
> useful for nulling out strong local stations and man-made noise sources.
> Above a few MHz, you may find that the loop doesn't seem to show much
> directivity, and a short (20 feet or more) piece of wire might receive
> just as well.
Lyle Koehler
>
> ... hmmm I saw one site that had what looked like just a bulky multi turn loop
> ... you were supposed to just put your AM/MW radio beside it and all your
> troubles would go away... I was thinking a similar thing would happen for my
> bands as I don't have an antenna connection for my radio...just a whip...
The passive loop trick can work very well with portable AM/MW radios
because they use a small internal ferrite-core loop as an antenna. When
a larger, more efficient loop is placed near the radio and tuned to the
proper frequency, it re-radiates energy that is coupled into the radio's
internal loop. But I don't know of any radios that use an internal loop
antenna on the shortwave bands. I was gonna say that coupling from an
external loop to a whip wouldn't help at all, but decided to try a quick
test before making the statement. My HF receiving loop is simply two
turns of #14 insulated building wire, 1 foot diameter and self supported
-- in other words, a double loop of stiff wire. With a 500 pF variable
capacitor, the loop will easily tune from below the 40m band to above 10
MHz. Actually, there is a significant improvement in sensitivity on my
old Sony ICF-2001 when the whip is fully retracted and the tuned loop is
held tangent to the whip, both on the 40 meter ham band and on WWV at 10
MHz. However, the loop provides very little sensitivity improvement when
the whip is fully extended. Looks like my small loop is less efficient
than the 4-foot whip on the Sony -- no big surprise. A larger loop might
be worth a try, however.
> I've tried the wire already too.. I ended up with some more stations for my
> SWL, but ended up with a lot of garbage too.. so I was hoping the loop would
> provide some increased selectivity...I thought of a tuner, but my load and
> source Z's would both be unknown.. seems pretty tricky to go over all the
> bands that way...what about using a feedback loop to increase the Q of the
> loop instead of a preamp?
>
It would take a very high Q antenna to improve the selectivity of even a
fairly broad shortwave receiver. At 10 MHz, you need a Q of 2000 to
provide a 5 kHz bandwidth. You might achieve that kind of Q with a loop
made of large copper tubing and with careful construction techniques.
However, tuning will be very touchy, and the "skirt selectivity" will
still be poor because the response of a single-tuned circuit falls off
rather slowly outside the passband. On the plus side, a moderate-sized
loop with that kind of Q should be more efficient than the radio's
built-in whip. Yes, regenerative (positive) feedback can also be used to
improve the Q, but amplification is required in order to achieve
regeneration, so some kind of preamp is required if you want to try that
approach.
AJ
that I'd go through the whole process and be very disappointed with the whole
excercise... thanks for the comments on your set up too.. it'll give me something to
compare to...
The comment of the Q enhancement is well taken... I may try it when I do my loop...I
just have to avoid oscillations I guess :)
It seems that I would be better off trying a 'broomstick' antenna for indoor SWL
then eh? If I compare size, capabilities, tunability etc... it seems to be steering
me away from the loop...comments?
aj
> external loop to a whip wouldn't help at all, but decided to try a quick
> test before making the statement. My HF receiving loop is simply two
> turns of #14 insulated building wire, 1 foot diameter and self supported
> -- in other words, a double loop of stiff wire. With a 500 pF variable
> capacitor, the loop will easily tune from below the 40m band to above 10
> MHz. Actually, there is a significant improvement in sensitivity on my
> old Sony ICF-2001 when the whip is fully retracted and the tuned loop is
> held tangent to the whip, both on the 40 meter ham band and on WWV at 10
> MHz. However, the loop provides very little sensitivity improvement when
> the whip is fully extended. Looks like my small loop is less efficient
> than the 4-foot whip on the Sony -- no big surprise. A larger loop might
> be worth a try, however.
>
>
Gary Coffman
>Yes, regenerative (positive) feedback can also be used to
>improve the Q, but amplification is required in order to achieve
>regeneration, so some kind of preamp is required if you want to try that
>approach.
And be wary that a regenerative preamp doesn't deal well with a large
dynamic range. That means you'll have troubles when strong signals
are intermixed with weaker ones, as is the norm on the SW broadcast
bands (or today's ham bands for that matter).
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
Guillermo C.W.
AJ wrote:
..
>
> I've seen many that aim at the AM/MW band consisting of several
> wraps of wire and tuned with a 365pF cap. No problem. However, I've seen
> one that is aimed for the SW band made of a 'wide' single loop, also
> tuned with a cap... So, if a single wide loop 'works so well' for SW,
> why is that not done for the AM band? Is it just the lower amount of
> inductance needed to get the lower resonance frequency? any comments
> about either antenna and whether a loop antenna is good for my
> application?
Loops, both large and small, are some of the quietest, easiest to live
with antennas yet designed. Small loops, of either the 1/2-wavelength
variety, or the miniature "magnetic" type, have the advantage of having
nulls that are steerable, so you can stamp out interference and noise
coming from directions *other* than the direction of the desired signal.
The disadvantage of small loops is narrow bandwidth and difficulty in
tuning. meaning that every time you change band or move a substantial
distance within a band, you have to readjust the tuning. This can get
tedious after while.
>
> Also, I've seen large area, single wire loops (on the order of a
> wavelength) and then the much smaller multi-turn loops.. and comments
> that indicate the length of wire in the small loop is related to the
> wavelength...is this true? Isn't the loop just an inductance, regardless
> of the amount of wire used to make it? What I'd like to do is try to
> make a smaller loop with more turns to keep the inductance the
> same....am I just shooting myself in the foot by reducing the area?
While you can look at a small loop (single or multi turn) as an
inductance alone for purposes of calculating size et al, it is more
useful to consider the fraction of a wavelength that the total conductor
represents at the frequency you are interested in. Also, the "amount" of
wire one uses determines the amount of capacitance required to resonate
it, and especially on the lower bands (<10 MHz), the length of conductor
hugely influences the loop's value as an antenna at all. While you can
resonate a 1-meter diameter (about 3.1 M "length") given enough
capacitance on 800 KHz, such an antenna will not hear much, and will be
a real bear to tune. So, if you make a 4-turn 1-M loop, it will have
greater inductance, and resonate at a lower frequency, given the same
amount of capacitance, as the 1-turn loop will. It will also yield much
higher signal strengths at a given frequency.
>
> Also, I've read that the large loops respond to the electric field
> while the smaller loops respond to the magnetic field...true? and if so,
> what is the transition region? eg.. how would I know which field I'm
> responding to for a given design?
Since radio waves are a composite electro-magnetic field, the question
is really moot. Whether small loops are truly "magnetic" (and I doubt
that) that makes no difference to you as you use the antenna. Small loop
makers and dealers have often claimed in their advertising that their
antennas "respond" only to the magnetic component of a signal, and thus
they are more immune to electrical noise that is generated near the
antenna site. I dont think this has been demonstrated at all, and then
you still have to deal with electrical noise originating more than a few
loop diameters from the antenna. No effect is even claimed (thank god!)
for this. The nulls of small loops do offer some noise cancellation, as
I noted above.
Given the purpose you have in mind (i.e. SWL listening, no
transmitting), I think you will obtain better results with a closed
full-wave loop cut for 6 MHz (approx. 156 feet in circumference, though
dimensions are *much* less critical in fullwave loops than in dipoles,
verticals, inverted-Ls, Zepps, Windoms, and other end or center fed
"open" conductor elements). For receiving purposes, you don't even need
a tuner, though that would help. A large loop would be much easier (and
cheaper!) to build, install, and operate than a small one would be,
since fullwave loops don't require the high-voltage wide-range variable
capacitor and remote control. Bear in mind also that since your priciple
interest is in receiving AM SWL signals, with a typical bandwidth of
about 6 KHz, a small loop may not even pass a signal that wide on the
low bands. Typical bandwidth on my 1.25 meter (in diameter) loop at
7.150 MHz is only about 3.3 KHz, enough for SSB but not for AM.
>
> Any references or comments would be appreciated.
There are any number of designs and analyses of small transmitting
loops, as well as large loops (horizontal or vertical; circular, square,
or delta; single or multiple elements [i.e. quads]) in QST, CQ, and 73
magazines, and in recent editions of the ARRL antenna handbook and
various other antenna books (CQ magazine publishes a few titles). You
might also examine Reg Edwards DOS program for small loop modelling,
"RJELOOP1.EXE," available from his web site.
Best of luck in choosing and building your loop!
Will
KD7BFX
AJ
were particularly helpful and some of the comments were great food for
thought. The practical answers on building are really helpful.
I have some further questions as well if we're all still interested :)
In follow up to the loops , I have been thinking about the way the EM
couples to the loop... and, looking at other designs, specifically inductor
loaded dipoles, does anyone have any comments on an inductively loaded loop? I
haven't seen any designs that do this, (maybe for good reason :)...
Comments?
Also, several people have hinted that the small loop isn't very 'efficient'
e.g., the signal that you get is pretty small and that it is low bandwidth
('high' Q ).. any comments on using the loop antenna for a table top crystal
set as opposed to the standard coil and cap attached to the " 100 feet of
antenna " ? Would enough signal be captured with a loop to allow similar
operation ? or, a loop antenna nearby a standard crystal set without the " 100
feet of wire" ? would enough signal get coupled into the coil? (the idea here
then would be to have a coupling loop form the loop antenna onto the crystal
set coil...)
That being said, doesn't coupling these two loops sort of equal an
inductively loaded loop antenna? sort of a double tuned transformer I would
think, no?
aj
Guillermo C.W.
Let's keep our loops straight: large loops (nominally full-wavelength)
can and are coil-loaded. You need to dig into a good book like the ARRL
Antenna Book for the real dope, but I do know that all four sides of
such a loaded-loop must be equal and each have an identical coil.
I am unsure what you mean by a "standard cap and coil," and crystal
below. Do you mean an analog VFO tuned receiver? Or do you mean a "L" or
"T" section antenna tuning unit? A Xtal receiver? Please define which
circuit each is in and for which purpose.
For all but LW (below 500 KHz) reception, the 100 feet of wire will
almost certainly yield better signal strength, and yes, higher
Signal/Noise ratio (the real figure of merit).
Considering the tiny signal voltages in crystal radio sets, I think the
(small) loop would be too lossy. Although, come to think of it, xtal
rigs are so un-selective that the high-Q/narrow-bandedness of the loop
might help you there, especially if a signal you are interested in is
getting squashed by another closer, stronger signal.
Will
KD7BFX