Date: Thu, 19 Apr 2012 19:12:39 -0700
From: Jim Brown <firstname.lastname@example.org>
Subject: Re: [TowerTalk] Balun Recommendation
On 4/19/2012 9:25 AM, Steve Hunt wrote:
> I was contacted privately on this issue by someone whose engineering
> ability I well respect. He confirmed what I have been saying, but
> suggested that the link between choke dissipation and SWR is "best not
> mentioned" for fear some folk would interpret it to mean that SWR
> *causes* CM current.
I'm very short of time, because I'm entertaining house guests, about to
take them out to dinner, and leaving in the morning for the Visalia
The thing that is WRONG about Steve's statement is that it is not SWR
that changes the voltage on the choke, it is the common mode voltage,
which is the result of imbalance in the system. If there is no
imbalance, there is no common mode voltage, and thus no dissipation, no
matter what the SWR. Consider, for example, a direct short at the
antenna, or an open circuit at the antenna, but no imbalance in the
SYSTEM. No common mode voltage on the choke, no stress.
Steve has cited an example where the 50 ohm coax is terminated by 75
ohms. Yes, there is a mismatch, but the thing that can change the
voltage across the choke is the change in the LOAD impedance, not the
fact that it's a mismatch. And that change doesn't matter if the system
is BALANCED. Changing to 75 ohm coax and a 75 ohm load would preserve
the match, but the higher voltage required for the same power level
would also increase the choke's sensitivity to imbalance.
Bottom line -- dissipation in a common mode choke on a COAX line is
directly, and primarily related to the IMBALANCE in the SYSTEM, the TX
power, and the system impedance. A common mode choke on a parallel wire
line has the additional dissipation due to leakage flux from
differential current. In my experience, the latter effect is small at
(legal) ham power levels.
And, VERY important, the electrical length of the common mode circuit
(that is, Vf approx = 0.99).
73, Jim K9YC
## So why do I measure a temp rise on baluns with my fluke 62 IR gun, when
a high SWR condition exists....vs a lower temp where the swr is dead flat.
I’m talking about yagis and also rotary dipoles.
I’d say they were as about as balanced as you can get, with equal amounts of
## what is causing the temp rise. Mr Brown is implying there should be no
temp rise, even with
a sky high swr.
## as for coax length, try odd multiples of a quarter length, like .25 and
.75 and 1.25 and
1.75 etc. Calculate it for a physical length, not an electrical length.
Then shorten it by 2 percent.
You will end up with a high Z at the ant feed-point. In a lot of cases you can
dispense with the balun. You can make the Z even higher still with the use
a balun at the feed point.
## again, use a clamp on RF ammeter in several places, and measure the shield
current, that’s the
real acid test.
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