Balun at Input or Output of Antenna Tuner?
Dean Straw, N6BV (Senior Assistant Technical Editor, Retired) December 12,
2011
I have been lurking on the Elecraft Reflector monitoring the animated
discussion about where to place a balun -- at the input or the output of a
tuner. I was going to jump into the discussion after I read comments
claiming that a balun at the input of an unbalanced tuner feeding a balanced
transmission line (and balanced antenna) simply "doesn't work." On Dec. 8
Alan Bloom, N1AL, wrote such an elegant analysis that I didn't have to.
If I might paraphrase Alan, the stress across the balun due to unwanted
common-mode current is the same whether the balun is placed at the input or
at the output of an unbalanced antenna tuner network. Other investigators
have come to the same conclusion and have argued that the complexity of
"floating" the antenna tuner components doesn't warrant installing the balun
at the input of the antenna tuner.
As Alan pointed out, when a balun is placed at the input of the tuner, the
differential-mode currents/voltages are well controlled, since the tuner's
input impedance when tuned is 50 ohms. When the balun is placed at the
output of the tuner, the differential-mode currents/voltages are determined
by whatever impedance the load happens to be at the shack-end of the
transmission line feeding the antenna. And that impedance can range very
widely.
For my own curiosity, I wanted to get a handle on the amount of loss
incurred by the differential-mode loss (i.e., the loss "inside the coax")
for a common-mode choke constructed with coaxial cable. (Google Jim Brown,
K9YC's excellent paper "A Ham's Guide to RFI, Ferrites, Baluns, and Audio
Interfacing" Revision 5a, 5 Jun 2010. The section on transmitting baluns
starts on page 25. Photos of typical coaxial CM transmitting chokes appear
on page 29.)
By the way, like Jim Brown, K9YC, I prefer the name "common-mode choke"
rather than the term "balun," whether it is used at the input or at the
output of an unbalanced tuner feeding a balanced transmission line and
balanced antenna. I shall use the abbreviation "CM choke" in the rest of
this paper.
I assumed that the CM choke consisted of three feet of RG-213 wound through
ferrite torroids of the appropriate material and size to achieve K9YC''s
target common-mode impedance of 5000 ohms. At that choke impedance the
effects of unwanted common-mode current would be low and the effect of
differential-mode loss could be explored from a conventional
transmission-line perspective.
A quick review: a CM choke exhibits two main modes: (1) an impedance to
common-mode currents flowing on the outer surface of the coaxial shield and
(2) the effect of differential-mode currents/voltages - that is,
matched-line loss and the additional loss due to SWR of a three-foot length
of RG-213.
My tool of analysis is my own TLW program, but there are other programs out
there that can do the analysis. I assumed that the load presented to the
output terminals of the common-mode choke balun at the output of the tuner
was 3000 + j0 ohms at 28 MHz. This amounts to an SWR of 60:1.
TLW says that the total transmission-line loss in the three feet of RG-213
under these conditions is 0.373 dB. This is comprised of 0.034 dB of
matched-line loss if the RG-213 were matched and an additional loss due to
the 60:1 SWR of 0.339 dB. A total loss 0.373 dB doesn't sound like much, but
when you pump 1500 W into such a tuner this translates into a lot of watts
in a confined space. For 1500 W into the input of the CM choke, 123.5 W must
be dissipated safely in the three feet of RG-213. This is 41.2 W per foot
and at that level the RG-213 would get very warm and could even melt,
especially if the choke were confined in a small box with no circulation of
cooling air.
Times Microwave has a convenient calculator on their web site, where the
power-handling capability of various types of coaxes may be calculated. 100
feet of RG-213 is rated to handle 2.02 kW at 28 MHz with a 1:1 SWR. For a
matched line there would be no "hot spots" along the line due to SWR. The
average power-handling capability of RG-213 is thus 2020 W divided by 100
feet, or 20.2 W per foot.
Interestingly enough, this short length of coax transforms the 3000 + j 0
load to 1.72 - j 47.3 ohms at the output terminals of the tuner, making the
task of the antenna tuner a little more difficult at such a low resistance
level.
If this 3000-ohm load were presented to a tuner with the CM choke at the
input, the loss would be the matched-line loss only of 0.034 dB, which is a
power loss of 11.7 W for 1500 W input to the tuner.
The same kind of TLW analysis can be done for a low-impedance load of 3 + j
0 ohms, for an SWR of 16.67:1. For a CM choke balun at the output of a tuner
this results in an additional loss due to SWR of 0.402 dB and a total loss
of 0.436 dB. This is a power loss of 143.3 W in the three feet of RG-213,
meltdown city again.
While we're at it, let's look at the potential loss due to line
losses at a CM choke balun placed in the wrong place in an antenna system.
Assume the common scenario where a balanced antenna is fed with open-wire
transmission line to a 1:1 common-mode choke balun located at the shack
window. From the balun at the window the ham uses, say, a 20-foot section of
RG-213 to the antenna tuner (which in this case is an unbalanced tuning
network). Assume again that the CM choke balun uses three feet of RG-213
wound on the appropriate ferrite donuts to achieve the target common-mode
impedance of 5000 ohms so that common-mode currents are choked off properly.
The total length of RG-213 is now 23 feet. Again, we'll present the
balun at the windowsill with a load of 3000 ohms. The overall
differential-mode loss in 23 feet of RG-213 is 4.534 dB, nearly 4 dB worse
than connecting the open-wire line directly to a tuner with a CM choke balun
at its output! Ouch, that's a lot of wasted power.
73, Dean, N6BV
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