[Elecraft] KAT500 update
Wayne Burdick
switchable between balanced and unbalanced.
Recently we decided that a third antenna jack would provide greater
utility overall. (In my case, I'll be keeping a dummy load on the
third jack.) All three jacks will be unbalanced (SO239).
We're hoping to add a high-power, high-performance balun to our
product line sometime next year that will serve the needs of those
using balanced lines.
73,
Wayne
N6KR
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n5...@n5ge.com
Excelent, Wayne and yahoo on the dummy load connection. Thanks.
73,
Tom
Amateur Radio Operator N5GE
ARRL Lifetime Member
QCWA Lifetime Member
"If somebody has a bad heart,
they can plug this jack in at
night as they go to bed and it
will monitor their heart
throughout the night. And the
next morning, when they wake up
dead, there'll be a record."
--Mark S. Fowler, FCC Chairman,
1981 - 1987
Johnny Siu
TNX & 73,
Johnny VR2XMC
Martin Storli - LA8OKA
With some relays it is possible to change the configuration in such a way that say selecting Antenna 1 is SO-239 connector 1, selecting Antenna 2 is SO-239 connector 2, selecting Antenna 3 is SO-239 connector 3, selecting Antenna 4 is SO-239 connector 2 senter conductor and SO-239 connector 3 senter conductor with a balun switched in to the circuit by some relais to make the combined inputs of SO-239 connector 2 senter conductor and SO-239 connector 3 senter conductor a balanced input.
It is of coarse not possible to use SO-239 connector 2 and SO-239 connector 3 as unbalanced (coaxial) antenna inputs at the same time as the antenna 4 configuration is used, so due to this the control firmware should be set up in such a way that when Antenna 4 is NOT enabled, the following selections are available:
Ant 1, Ant 2 and Ant 3,
and when Antenna 4 IS enabled, the following selections are available:
Ant 1 and Ant 4.
I think that something like this would give the users a great flexibility.
Martin Storli
LA8OKA
Oslo, Norway
ARCTICPEAK's Radio pages!
http://www.arcticpeak.com/radio.htm
________________________________
Fra: Wayne Burdick <n6...@elecraft.com>
Til: Elecraft Reflector <elec...@mailman.qth.net>
Kopi: Elecr...@yahoogroups.com
Sendt: Torsdag, 8. desember 2011 2.22
Emne: [Elecraft] KAT500 update
Don Wilhelm
Actually, for my part the 3 SO-239 connectors offer the greatest
flexibility. If a user uses open wire feeders, he likely already has a
balun connected to those feeders. To move that balun inside the tuner
makes no sense to me. In fact, I have an MFJ 962D tuner that I removed
the internal balun just so I could connect a 3rd SO-239 connector. The
baluns live at the center of my antennas or at the end of the
ladder-line (which is not in the shack).
73,
Don W3FPR
On 12/8/2011 8:23 AM, Martin Storli - LA8OKA wrote:
> Have you tought about the possibility to combine two of the SO-239 connectors to form a balanced input.
> With some relays it is possible to change the configuration in such a way that say selecting Antenna 1 is SO-239 connector 1, selecting Antenna 2 is SO-239 connector 2, selecting Antenna 3 is SO-239 connector 3, selecting Antenna 4 is SO-239 connector 2 senter conductor and SO-239 connector 3 senter conductor with a balun switched in to the circuit by some relais to make the combined inputs of SO-239 connector 2 senter conductor and SO-239 connector 3 senter conductor a balanced input.
>
Oliver Johns
When I talked to you briefly at the 2010 Pacificon, you seemed to say that the KAT500 would be a "floating-L" design. That is, it would contain an unbalanced L network, but that network and its circuit board would be floating and not connected to the metal enclosure. You said a balun would be put at the 50 ohm end, feeding a SO239 connector for the coax to the rig. The floating-L could then be connected to balanced lines, or to unbalanced ones by tying one side of it to the case at the antenna end.
Now it seems that you are talking about a totally unbalanced tuner, with no provision for balanced antenna feeders. Do you mean to add an external balun on the *antenna* side of the tuner? Surely not. That seems an inherently bad idea for many balanced lines. The impedance there will be unpredictable, hence the need for the tuner. Not a good place for a balun IMHO. But if you put the balun on the 50 ohm end where it belongs, then the whole enclosure will be tied to one side of the "balanced" line, which will likely unbalance it.
What happened? I had hoped to put the KAT500 in a weatherproof box of my own fabrication and use it between a balanced line to the antenna and a coax feed back to the rig. It could live out near the antenna, where it belongs.
73,
Oliver Johns
W6ODJ
Jim Miller
"balanced" feedline is no more than an appropriately designed common mode
choke.
http://www.audiosystemsgroup.com/RFI-Ham.pdf excellent practical
descriptions, tests of real implementations and cookbook recommendations.
http://fermi.la.asu.edu/w9cf/articles/balun/ excellent mathematical
treatment
http://www.w8ji.com/tuner_baluns.htm bottom of page
73
jim ab3cv
Vic K2VCO
input does not provide better balance than one with the balun on the output. Yes, I know
that there is such a tuner in the ARRL handbook and Alpha is manufacturing one. But the
math doesn't lie. So unless Elecraft wanted to use a balanced network -- which would make
the tuner MUCH more expensive -- I would prefer one without a balun. Then those who do not
use balanced lines would not have to pay for the balun.
On 12/8/2011 8:58 AM, Oliver Johns wrote:
> Wayne,
>
> When I talked to you briefly at the 2010 Pacificon, you seemed to say that the KAT500 would be a "floating-L" design. That is, it would contain an unbalanced L network, but that network and its circuit board would be floating and not connected to the metal enclosure. You said a balun would be put at the 50 ohm end, feeding a SO239 connector for the coax to the rig. The floating-L could then be connected to balanced lines, or to unbalanced ones by tying one side of it to the case at the antenna end.
>
> Now it seems that you are talking about a totally unbalanced tuner, with no provision for balanced antenna feeders. Do you mean to add an external balun on the *antenna* side of the tuner? Surely not. That seems an inherently bad idea for many balanced lines. The impedance there will be unpredictable, hence the need for the tuner. Not a good place for a balun IMHO. But if you put the balun on the 50 ohm end where it belongs, then the whole enclosure will be tied to one side of the "balanced" line, which will likely unbalance it.
>
> What happened? I had hoped to put the KAT500 in a weatherproof box of my own fabrication and use it between a balanced line to the antenna and a coax feed back to the rig. It could live out near the antenna, where it belongs.
>
> 73,
>
> Oliver Johns
> W6ODJ
>
>
> On Dec 7, 2011, at 5:22 PM, Wayne Burdick wrote:
>
>> We originally had two antenna jacks on the KAT500, with one of them
>> switchable between balanced and unbalanced.
>>
>> Recently we decided that a third antenna jack would provide greater
>> utility overall. (In my case, I'll be keeping a dummy load on the
>> third jack.) All three jacks will be unbalanced (SO239).
>>
>> We're hoping to add a high-power, high-performance balun to our
>> product line sometime next year that will serve the needs of those
>> using balanced lines.
>>
>> 73,
>> Wayne
>> N6KR
>>
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
Wayne Burdick
shortly.
Wayne
N6KR
On Dec 8, 2011, at 9:22 AM, Vic K2VCO wrote:
> I t's been shown conclusively that an unbalanced 'floating' tuner
> with a balun on the
> input does not provide better balance than one with the balun on the
> output. Yes, I know
> that there is such a tuner in the ARRL handbook and Alpha is
> manufacturing one. But the
> math doesn't lie. So unless Elecraft wanted to use a balanced
> network -- which would make
> the tuner MUCH more expensive -- I would prefer one without a balun.
> Then those who do not
> use balanced lines would not have to pay for the balun.
______________________________________________________________
Wayne Burdick
Sorry to disappoint you. Here's what happened:
We originally planned a floating L-network with an input balun, and
that's how we wired the prototypes. But recently we studied this
configuration in depth, doing both a theoretical analysis and lab
measurements. We confirmed the findings of other researchers that the
input balun has no advantage over an output balun (more on this
below). Taking the balun out in favor of a third antenna jack will
reduce the cost of the ATU significantly and enhance utility, and
those who need a balanced feed can still use an external balun.
Now for the theory:
When a balun is located at the output, between the tuner and a
balanced line, one side of the balun transformer is connected to
ground at the tuner and the other is connected to one side of the
balanced line. The full RF voltage is then applied to the the balun
transformer. For a given balun design, if that voltage is high enough
then the balun transformer will overheat.
When the balun is located at the input, between the balanced feedline
and an unbalanced network, one side of the balun transformer is still
connected to ground, and the other is still connected to one side of
the balanced feedline. So the voltage applied to the balun is exactly
the same. Therefore there is no advantage to floating the network.
Doing so also greatly complicates the tuner and increases cost.
If anyone is interested in more detail, we could post a drawing, as
well as references.
73,
Wayne
N6KR
Richard Squire - HB9ANM
August, the KAT500 would have a built-in balun on the *input* side. And one
of two outputs switchable between balanced and unbalanced. Now we'll had to
add a balun at the output, which arguably is suboptimal.
On the other hand, I wonder why Elecraft should go into the trouble and
expense of developing a high-power balun. AFIK, there are some good products
already available...
I second Martin's (LA8OKA) suggestion for a relay-switched configuration.
Maybe they'll think it over - or even offer two different versions (just
dreaming...)
73
Richard
wayne burdick wrote
>
> We originally had two antenna jacks on the KAT500, with one of them
> switchable between balanced and unbalanced.
>
> ...All three jacks will be unbalanced (SO239).
>
> We're hoping to add a high-power, high-performance balun...
>
> 73,
> Wayne
> N6KR
>
>
-----
Richard - HB9ANM
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Paul Christensen
Deja vu -- Seems like we had just discussed this on the AMPS list! In this
case, I'm with you on keeping the balun external to the tuner. That gives
the user an option of installing the balun of their choice immediately after
the tuner, or remotely through a short section of high-quality coax to join
the balanced line. It's useful in cases where the shack is adjacent to an
outside wall and the balun can be hung adjacent to a window or other
opening. A symmetrical tuner would still be preferred for balanced lines
where the balun *should* be placed at the tuner input.
It would be an interesting exercise to compare the production cost and
resulting sales price of a switched unbalanced "L" versus a symmetrical
tuner with a balun on the input but with switched, fixed components.
A few design-related problems: (1) ability to switch between balanced and
unbalanced lines with a symmetrical tuner while keeping efficiency high and
cost low; and (2) any commercial version would need to be offered so that it
could accommodate a line input Z range of not only 50-5K ohm but also Z
values less than 50 ohms for folks with short antennas. In the case of a
high-pass design, that requires switched reversal of the shunt C so that it
can be placed on either side of the balanced inductance. And, that's where
it could get messy with cost and component layout to keep efficiency high.
After building a motorized AG6K tuner, I realized that the complex vacuum
relay arrangement could have been omitted. If reasonably "full-size"
antennas are used, it's tough to conceive a condition when using 600-ohm
line where the input Z drops below 50-ohms - even with odd quarter-wave
multiples. After the tuner was built, I worked out several dipole and loop
models using 4Nec2 and TLW software. Change of bands, frequency, and line
length all resulted in reasonable input Z. Another factor when operating
into low Z line inputs -- the lower the line input Z, the greater the
importance of component Q to keep efficiency high, especially on the low
bands. So, now there's a bunch of expensive switching components that will
never be used since I don't operate with short antennas, at least not at
home. Many folks wouldn't have the choice -- and thus the need to offer a
"one size fits all" type of tuner. Probably the best approach is an
unbalanced tuner for coax and "output balun" feeds, and if there's enough
demand, a separate symmetrical tuner just for balanced lines.
It sure is nice being able to toss these ideas around and not worry about
any financial risk, unlike the manufacturers!
Paul, W9AC
Martin Storli - LA8OKA
Hi Don.
I think you misunderstood me, because with my suggestion you will have 3 SO-239 conectors, but you have the choise to use two of them for a ballanced feeder. That should give anybody a good flexibility.
Martin Storli
LA8OKA
Oslo, Norway
ARCTICPEAK's Radio pages!
http://www.arcticpeak.com/radio.htm
________________________________
Fra: Don Wilhelm <w3...@embarqmail.com>
Til: Martin Storli - LA8OKA <arcti...@yahoo.no>
Kopi: Elecraft Reflector <elec...@mailman.qth.net>
Sendt: Torsdag, 8. desember 2011 14.46
Emne: Re: [Elecraft] Vedr: KAT500 update
Arie Kleingeld PA3A
This is exactly what I need! There is only unbalanced coax in my shack.
73
Arie PA3A
Op 8-12-2011 2:22, Wayne Burdick :
> We originally had two antenna jacks on the KAT500, with one of them
> switchable between balanced and unbalanced.
>
> Recently we decided that a third antenna jack would provide greater
> utility overall. (In my case, I'll be keeping a dummy load on the
> third jack.) All three jacks will be unbalanced (SO239).
>
>
hawley, charles j jr
http://www.eznec.com/misc/ibalbrf.txt
Chuck, KE9UW
aka Jack, BMW Motorcycles
________________________________________
From: Elecr...@yahoogroups.com [Elecr...@yahoogroups.com] on behalf of Wayne Burdick [n6...@elecraft.com]
Sent: Thursday, December 08, 2011 11:34 AM
To: Oliver Johns
Cc: Elecraft Reflector; Elecr...@yahoogroups.com
Subject: [Elecraft_K3] Re: [Elecraft] KAT500 update
Hi Oliver,
Now for the theory:
73,
Wayne
N6KR
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Dick Lindzen
is high, and if SWR is high, there are extra losses in the
feedline. The lowest cost low loss feeder is open line, and a
balanced input to the tuner would be useful.
Dick, WO1I
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Alfred P. Sloan Professor of Atmospheric
Sciences
Office: 54-1720, MIT
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1 (617) 253-2432 (voice)
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===============================
Ignacy
The input balun always works at low SWR and at 50 Ohm. It is very easy to
have such balun. A small balun would easily handle a KW without heating.
The output balun can work at high complex impedances. Ferrite baluns are
known to absorb nearly all the power at high impedances (including BL200)
and be very hot even at low power. Iron baluns work well with true balanced
antenna but do not provide enough choking with less balanced antennas.
Even if there is balun on input, there are stray capacitances that can
disturb the balance. Probably bigger for a bigger tuner and also bigger if a
tuner is close to metal boxes.
Those who travel and use portable antennas such as random length wires, they
find ferrite baluns on output nearly always hot on some bands. Now, I use
Z11Pro where both antenna connectors and the control cable are looped
through a toroid many times; antenna wires to the tuner are connected
directly. No heating whatsoever. No hot touch to the radio. But the tuner is
usually on wood 2 ft from the radio.
What can be done with portable tuner with single output cannot be done with
500W tuner in a shack with multiple outputs. So Elecraft's decision to not
use the input balun is a good one. Balun on input would not be very
effective but would increase costs. If an output balun heats up too much or
doe not provide enough choking, change the antenna or the balun. Rules of
QRO are different than rules of QRP or even 100W.
Ignacy, currently VK2/NO9E
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Eric Swartz - WA6HHQ, Elecraft
This is a common misconception. (One which I held until recently. :-) It
turns out there is no advantage to placing the balun at the input of the
L-Network tuner. Since one end of the balun is grounded by the input to
the tuner, it is still presented with the same stresses under high SWR
situations. Baluns at the input and output both drive balanced loads
equally well.
We've now put together a web page describing the impact of placing the
balun at the input or at the output of a L-Network tuner. See:
http://www.elecraft.com/KAT500/input_versus_output_balun.htm
At the bottom of that page are several links providing detailed
technical analysis of these configurations. The first two, by W8JI and
W7EL are very clear discussions of this issue.
73, Eric WA6HHQ
On 12/8/2011 3:17 PM, Ignacy wrote:
> It seems to me that the story is more complex than it sounds.
>
> The input balun always works at low SWR and at 50 Ohm. It is very easy to
> have such balun. A small balun would easily handle a KW without heating.
Jim Brown
> Actually, it seems to me that an antenna tuner is needed because SWR
> is high, and if SWR is high, there are extra losses in the
> feedline.
The mismatch that affects feedline loss is the match between the antenna
and the feedline. Thus, an antenna tuner reduces loss in the feedline
ONLY if it is at the antenna end of the feedline. All the tuner in the
shack does is make the rig happy, so that it can put full power into the
line and not break.
> The lowest cost low loss feeder is open line, and a
> balanced input to the tuner would be useful.
Virtually all the loss in transmission lines at HF (and even VHF) is due
to copper (that is, I squared R). Open wire line (and window line) has
much lower loss than coax because it has much higher impedance, so the
current for the same transmit power is much less than for coax. That is,
you're sending power on the line with more voltage and less current.
It's still possible to have a lot of loss in window line or open wire
line if there is a high SWR. One exception -- window line starts having
significant dielectric loss when it is wet.
73, Jim K9YC
Don Wilhelm
forums (or was it newsgroups at the time), I recall a lot of exchanges
with Tom W8JI on exactly the "balun at the tuner input and isolated
unbalanced tuner". I concur with Tom - it does not work -- both from a
theoretical basis, and also from Tom's measurements.
This was "way back when" - as I recall my situation when all this was
going on, I was running Windows 95 and the year was between 1997 and 1998.
In the timeframe of this discussion, Zack Lau (ARRL engineer) who had
first published the "balun at the tuner input" concept as a QRP tuner,
had retracted that design because it did not maintain balance, but Dean
Straw (ARRL engineer, editor, etc.) published his design of a high power
tuner using the same concepts, and that design can still be seen in the
ARRL publications.
Apologies for the comments into the politics of the ARRL decisions on
what is to be published, but that is both a bit of the history as I know
it as well as my view of the technical side of this issue.
If anyone can tell me how you can run a signal through a balun - and
have equal and opposite currents at its output, and then run it through
an unbalanced network with unequal elements in the two series legs and
still maintain equal and opposite currents and phase, and I will then
concede that an isolated unbalanced tuner with a balun at the input will
work, but until that is presented to me along with detailed engineering
level test data (not just "it works"), I will continue to believe that
using a balun on the input of an isolated unbalanced tuner is a "pipe
dream" that does not mesh with reality.
73,
Don W3FPR
Ron D'Eau Claire
of the line in which it is used?
The justification I had seen for putting the balun at the tuner input where
it will see a load very close to 50 ohms resistive (when the tuner is
adjusted for a "match") while at the output the balun may see anything over
a huge range of impedances.
Ron AC7AC
Ignacy
imagine output resitstance 2500 Ohms and 100W power.
On input, RG174 has a peak voltage of 100V. No problem.
On output, RG174 has a peak voltage of 1000V. It would melt in seconds.
What is wrong here? Assumptions?
Ignacy
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Jim Miller
the conclusion that a common choke on the output of the "unbalanced" tuner
of sufficient impedence would ensure that all of the current on the
interior of the shield of the coax and on the center conductor would be
present on the "balanced" line if the outside of the shield was prevented
from carrying RF.
That equal and opposite current present on the "balanced" line is what is
desired.
No transformer, balun, etc which would be suject to widely varying
impedences from such a line is required.
Just squirt the current in equal and opposite amounts and don't let it leak
anywhere.... <-technical description...
jim ab3cv
Don Wilhelm
Yes, that is valid justification for putting the balun at the tuner
input, BUT it ignores the physics of the tuner itself. If the tuner is
actually a balanced design, I have no problem, but many try to force fit
an unbalanced tuner design after the balun. That latter part is the
part that does not fit. There is no way that an unbalanced network no
matter how well isolated can preserve the balance - just consider the
phase of the signal as it passes through each tuner element if you want
proof.
If you want to use a balun at the tuner input, then please follow it
with a balanced tuner t maintain that balance.
If you want to use the "rule of thumb" that the balun impedance should
be 10 times the load impedance, then refer to K9AY's RFI and balun
information and use those 5 or 6 large cores to provide the proper
choking impedance for up to 500 ohms antenna impedance (5000 ohms
impedance for the balun).
73,
Don W3FPR
Ken
On Dec 8, 2011, at 6:47 PM, Jim Brown wrote:
> Virtually all the loss in transmission lines at HF (and even VHF) is due
> to copper (that is, I squared R). Open wire line (and window line) has
> much lower loss than coax because it has much higher impedance, so the
> current for the same transmit power is much less than for coax.
The impedance of the feedline does not change the impedance of the antenna. A half wave length dipole at the proper height is still a 50 ohm feed regardless of whether it's fed with 50ohm line or 600 ohm line.
Ken WA8JXM
Ignacy
mode. Also that voltage breakage is unimportant. Both are good
approximations when the balun is large and wound by thick wire or with
teflon cable.
Assume we use RG174 for windings. We have SWR 1:1 on input and 25:1 with
2500 Ohm input. Thus 25 lower power ratings on output than on input. Could
be 100W max with balun on on input and 4 W max with balun on output.
One can always substitute a bigger coax (increase the balun size a few
times), or use bifilar windings (and worrying about insulation breakage).
Summarizing, a $2 balun on input would be as effective as a $50 balun on
output, and 10 times lighter. Not important for stationary equipment but
important when portable.
For KAT500 its is better to use a $50 balun as otherwise switching would
cost more.
Perhaps this thread is no longer relevant to KAT500 and needs to be
terminated.
Ignacy
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Don Wilhelm
Actually, the feedpoint impedance for a half wave center fed dipole is
70 ohms if it is "in the clear". The proximity to earth and other
physical objects will lower that impedance. For antennas typically used
at HF, yes, the feedpoint impedance will be closer to 50 ohms than the
ideal of 70 ohms.
73,
Don W3FPR
Ken
Yes, I know, that's why I said "at the proper height".
73, Ken WA8JXM
Don Wilhelm
I read the W8JI comments presented in that link on the Elecraft website.
I was familiar with Tom's work and comments from emailing with him on
this particular subject many years ago.
The one thing that struck me in W8JI's paper is at the end and embodied
in Tom's comment that --
"The irony is, moving the balun to the input mostly works only when the
balun is not needed!"
For those who want a really high efficiency balanced tuner, go to ARRL
publications back several years to find a link coupled tuner. With
switching between series and parallel tuning, it can handle impedances
from very low to very high. The input is DC isolated from the output,
and the input can be either unbalanced or balanced. The output can be
either balanced or unbalanced depending on where you connect the output
tuned circuit. The venerable Johnson Matchbox is an example of a link
coupled tuner. The taps onto the output tank circuit were created by
the differential capacitor in the output tank circuit - that allowed it
to be boxed up into a bandswitchable box without need for connecting the
antenna feeders to taps on the tuner coil. That works quite well , but
it restricts the range of matching impedances to significantly less than
would be obtained by the basic tuner with taps on the inductor. Yes, I
do use these tuners, and they are configured for single band use at the
antenna - 3 coax feeds for HF and 3 for VHF/UHF give me access to all my
antennas The nearest antenna is 200 feet of coax away from the
hamshack, and that is why I use tower mounted preamps for VHF/UHF.
I will never give up my Matchbox until it is pried from my cold dead
hands - I use mine mostly as a test instrument - as bandpass filter as
well as an impedance transformation device. After experimentation asnd
development, the permanent tuners are designed and installed in the
antenna field - I have tuned coax fed antennas for each band from 160
meters through the 432 MHz bands. I use the Matchbox tuner only for
experimental antennas until I can develop a permanent and dedicated
tuner for any one antenna.
OK, those are the advantages - the drawbacks are that that the link
coupled tuner arrangement does not lend itself well to bandswitching,
but i would suggest it be the tuner of choice for situations where the
antenna is used for only a single band - put the tuner on the antenna
feedline and tune it to resonance (and minimum SWR) once and be done
with the settings.
73,
Don W3FPR
On 12/8/2011 6:28 PM, Eric Swartz - WA6HHQ, Elecraft wrote:
> Hi Ignacy,
>
> This is a common misconception. (One which I held until recently. :-) It
> turns out there is no advantage to placing the balun at the input of the
> L-Network tuner. Since one end of the balun is grounded by the input to
> the tuner, it is still presented with the same stresses under high SWR
> situations. Baluns at the input and output both drive balanced loads
> equally well.
>
> We've now put together a web page describing the impact of placing the
> balun at the input or at the output of a L-Network tuner. See:
>
> http://www.elecraft.com/KAT500/input_versus_output_balun.htm
>
>
Ron D'Eau Claire
of thumb is to have choking impedance 10X the line impedance. That's easy if
you have a 50 Ohm line, such as the input to the tuner. It's quite a
different story if the line impedance is 1000 or 2000 ohms, such as can
happen on the output of the tuner feeding an "all band doublet" or similar
antenna with open wire line.
Ron AC7AC
Jim Miller
it up. Google "k9yc", "w9cf"
Alan Bloom
complicated analysis.
_________ _________
I1 --> | | | | I3 -->
--------| |--| |-------
Transmitter | BALUN | | TUNER | Antenna
--------| |--| |-------
I2 --> |_________| |_________| I4 -->
The purpose of the balun is to eliminate the common-mode current. The
CM current is the NET current, that is, I1 + I2 at the input and I3 + I4
at the output. The balun does not store electrons, so it must be that:
Input common-mode current = output common-mode current.
That is true for both the balun and the tuner. No matter whether the
balun is on the transmitter or antenna side of the tuner it does an
equally good (or bad) job of choking the common-mode currents.
The advantage of putting the balun at the input is that the
differential-mode voltages and currents (the ones you want) are
well-controlled because of the constant 50-ohm impedance. The DM
current or voltage at the output can be much higher, depending on the
load impedance. Of course, the common-mode current and voltage are the
same at the input and output, but even so the worst-case stress on the
balun should be less when it is at the input.
The disadvantage of putting the balun at the input is that none of the
tuner circuitry can be grounded. For example, the control shafts of the
variable capacitors have RF voltage on them, so the knobs must be
isolated. If it is an automatic antenna tuner, that is less of a
problem.
Bottom line? The ARRL is not necessarily "wrong" to put the balun at
the tuner input. It's just a matter of whether you think the lower
worst-case current/voltage on the balun is worth the hassle of floating
the tuner ground.
Alan N1AL
dave
Don't forget that MFJ now makes a balanced tuner. Three versions -
974B, 974HB, 976. They do have what is needed - a balun at the input
followed by a fully balanced tuner. Or at least they appear to be
fully balanced.
73 de dave
ab9ca/4
Bob Cunnings
with choke balun at input. I've been using one for years here. Build
quality is excellent and it has served me well matching doublets fed
by 450 ohm window line running into the shack. Right now it's being
used with the KPA500.
Bob NW8L
>Don't forget that MFJ now makes a balanced tuner. Three versions -
>974B, 974HB, 976. They do have what is needed - a balun at the input
>followed by a fully balanced tuner. Or at least they appear to be
>fully balanced.
>
>
>73 de dave
>ab9ca/4
Jim Brown
of a half dozen different devices/circuit elements that are commonly
called baluns, and most are very different from each other. I abhor the
word "balun," and try to avoid using it. I have done considerable
research, and published what I've learned, on the subject of COMMON MODE
CHOKES wound on ferrite cores, especially LOSSY ferrite cores. The
efficiency of such a common mode choke is essentially independent of the
SWR, except to the extent of the increased loss in the short length of
transmission line wound through the core to form the choke.
Since I have no idea what sort of "balun" Elecraft plans for their
tuner, I have no comments on it. I have, however, measured some common
mode chokes bifilar wound using #14 and #12 THHN to form a parallel wire
transmission line that have excellent choking performance for the HF
bands. The data are included in the latest version of my RFI tutorial
(since the summer of 2010).
The concept of a totally isolated tuner with common mode chokes at input
and output is so non-nonsensical to me that I've never contemplated it.
It is also nonsense to consider parallel wire line as a balanced system
unless everything connected to it is also balanced -- that is, tuner,
line, and antenna. Since most ham antennas are at least somewhat
unbalanced by their surroundings, even when fed by these parallel wire
lines, there will be SOME imbalance in the line, and thus some common
mode current. That common mode current causes trouble in at least three
ways -- it puts RF in the shack to excite Pin One Problems (very common
in ham gear); the common mode current will radiate, potentially causing
RFI to consumer gear in your living room (and your neighbor's); and by
reciprocity, the transmission line will act as a receiving antenna,
coupling noise from your neighbor's computers and battery chargers to
your receiver. THAT"S why we need common mode chokes at the feedpoint
of EVERY antenna, whether fed with coax or parallel wire line.
Loss in common mode chokes is addressed in the sections of my RFI
tutorial where transmitting chokes are discussed.
http://audiosystemsgroup.com/RFI-Ham.pdf
73, Jim Brown K9YC
On 12/8/2011 4:36 PM, Ron D'Eau Claire wrote:
> Is it true that the efficiency of a balun is not affected by the impedance
> of the line in which it is used?
>
> The justification I had seen for putting the balun at the tuner input where
> it will see a load very close to 50 ohms resistive (when the tuner is
> adjusted for a "match") while at the output the balun may see anything over
> a huge range of impedances.
______________________________________________________________
Jim Brown
> "sufficient impedance". A normal rule
> of thumb is to have choking impedance 10X the line impedance.
In my tutorial, I've shown that for several important reasons, 5,000
ohms is a far better design goal, that 10K may not be enough for certain
extreme conditions running maximum legal power, and that 500 ohms is
quite inadequate.
73, Jim K9YC
Ken
On Dec 8, 2011, at 10:51 PM, Alan Bloom wrote:
> The disadvantage of putting the balun at the input is that none of the
> tuner circuitry can be grounded. For example, the control shafts of the
> variable capacitors have RF voltage on them, so the knobs must be
> isolated.
If you have the balun on the input to the tuner, can you have ANY SO-239 outputs and still be balanced? Or it it limited to balanced line output ONLY (e.g. open wire or ladder line), no coax outputs?
Ken
Geoffrey Mackenzie-Kennedy
the use of a properly designed link coupled tuner (we used to call them
Antenna Matching Units or AMUs) are:
1) This type of tuner being a bandpass circuit helps to reduce the level of
unwanted signals reaching the receiver, e.g. Medium and Shortwave broadcast
signals, which if not reduced in level could result in receiver
generated intermodulation products being heard. This type of tuner also
reduces the level of the transmitter's harmonics reaching the antenna.
2) That if a ferrite or iron cored balun (sorry Jim) is used with this type
of tuner, it would be placed in the link circuit where the impedance is 50
+j0 ohms - or close to this value, and it would not be exposed to
large values of reactance.
3) Placing a balun at the end of a feeder exposes the balun to all of the
signals arriving down the feeder, which in some situations could be strong
e.g. Medium Wave and / or Shortwave BC signals. This could result in
numerous intermodulation products being generated by the balun's core, some
of which might end up in our bands. One has to careful
as well that a ferrite/ iron cored balun connected to the feeder or
antenna's feedpoint does not increase the level of radiated transmitter
harmonics.
73,
Geoff
LX2AO
On Dec.09, 2011 at 3:53 AM, Don Wilhelm wrote:
> For those who want a really high efficiency balanced tuner, go to ARRL
> publications back several years to find a link coupled tuner.
______________________________________________________________
Jim Brown
Change the word "balun" to common mode choke, and I'm with you up to here.
> If it is an automatic antenna tuner, that is less of a problem.
Except that the automatic circuitry, and it's power supply must also be
isolated from ground. That part of the design problem seems non-trivial.
> Bottom line? The ARRL is not necessarily "wrong" to put the balun at
> the tuner input. It's just a matter of whether you think the lower
> worst-case current/voltage on the balun is worth the hassle of floating
> the tuner ground.
I agree with your analysis, Alan, with the exception noted.
73, Jim K9YC
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2012.0.1873 / Virus Database: 2102/4668 - Release Date: 12/08/11
Alan Bloom
connected at the output. I guess that's another reason to put the balun
at the output.
Alan N1AL
Ross Primrose N4RP
>
>> If it is an automatic antenna tuner, that is less of a problem.
> Except that the automatic circuitry, and it's power supply must also be
> isolated from ground. That part of the design problem seems non-trivial.
>
Seems fairly simple to me, use a transformer in the PS, and float the
secondary side...
73, Ross N4RP
--
FCC Section 97.313(a) “At all times, an amateur station must use the minimum transmitter power necessary to carry out the desired communications.”
Ian White GM3SEK
>On 12/9/2011 6:09 PM, Jim Brown wrote:
>>
>>> If it is an automatic antenna tuner, that is less of a problem.
>> Except that the automatic circuitry, and it's power supply must also be
>> isolated from ground. That part of the design problem seems non-trivial.
>>
>
>Seems fairly simple to me, use a transformer in the PS, and float the
>secondary side...
>
Nope - there will be a large capacitance between the transformer
windings. In parallel with this will be the capacitance between the
entire RF sub-chassis and the outer metal case. If the choke balun is
connected on the input side of the ATU, these quite large parallel
capacitances will partially unbalance the output.
Worse yet (as Alan points out), if you select a coaxial output with the
SO-239 socket grounded to the case, the balun becomes short-circuited
from end to end. Alpha are working on a very similar auto-ATU but their
proposed schematic shows exactly how 'broken' this concept is:
<http://www.rfconcepts.com/PRODUCTS/New-Products/Alpha4040>
Elecraft have done well to avoid this trap, and to decide that the
KAT500 will be all-coaxial - because there simply isn't a "one way fits
all" solution for a balanced ATU. Different user installations will
require different solutions on different bands, so the optimum solution
will always involve some customization, swapping or switching between
1:1 balun, 4:1 balun and 'no balun'.
Please don't be confused by some ATU manufacturers' efforts to sell us a
"one way fits all" balanced ATU. They're only doing that because a fully
versatile engineered solution would be too complex and expensive. So
Engineering steps back and Marketing takes over instead - bending *your*
perceptions to fit what they have to sell.
Again, Elecraft have done well to avoid this.
Some time ago it was suggested that an all-coaxial KAT500 should also
include programmable band-switched relay control outputs (rather like
DIGOUT0 and DIGOUT1 on the K3) so that users have the option to
configure their own custom relay switching if they need it. Did anything
happen to that suggestion?
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
Al Lorona
1. has to be plugged in to AC power to work
2. has cooling fans
3. runs the Linux operating system
I am just old enough to find that somewhere between humourous and preposterous.
Al W6LX
________________________________
> Alpha are working on a very similar auto-ATU but their
> proposed schematic shows exactly how 'broken' this concept is:
> http://www.rfconcepts.com/PRODUCTS/New-Products/Alpha4040
--
73 from Ian GM3SEK
Matthew Pitts
Matthew Pitts
N8OHU
Sent from my Wireless Device
Mike
73, Mike NF4L
On 12/10/2011 11:46 PM, Al Lorona wrote:
> I followed the link, Ian. I think it's amazing that an *antenna tuner*:
>
> 1. has to be plugged in to AC power to work
> 2. has cooling fans
> 3. runs the Linux operating system
>
> I am just old enough to find that somewhere between humourous and preposterous.
>
> Al W6LX
>
>
>
>
> ________________________________
>> Alpha are working on a very similar auto-ATU but their
>> proposed schematic shows exactly how 'broken' this concept is:
>> http://www.rfconcepts.com/PRODUCTS/New-Products/Alpha4040
ai6ii
am starting my pre-purchase Jonesing for the missing piece in my K-line
shack.
..mike AI6II
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