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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
From: Steve Hunt <>
Date: Thu, 19 Apr 2012 09:07:31 +0100
List-post: <">>

Let me try to explain it more simply.

Picture Roy Lewallen's "classic" diagram showing how the 
differential-mode current flowing on the inside of the coax braid splits 
two ways at the feedpoint - some flowing into the dipole leg and some 
flowing the CM path back along the outside of the coax braid. Suppose:

Iant is the current flowing into the dipole leg
Icm is the current flowing along the CM path
Zant is the impedance looking into the dipole leg
Zcm is the impedance looking into the CM path.

We know that the current will split in inverse proportion to the two 
impedances, so:

Icm / Iant = Zant / Zcm
or, rearranging:
Icm = Iant * Zant / Zcm

If Zant changes, thereby changing the SWR, then Icm will also change and 
so will the choke dissipation.

If the SWR is caused by a drop in Zant, the choke dissipation will 
decrease; if it's caused by an increase in Zant, the choke dissipation 
will increase. Either way it's quite wrong to say: "SWR has NOTHING to 
do with dissipation in a common mode choke."

In a practical example like an HF wire dipole, you'll see that the 
increasing SWR as you move away from the resonant frequency results from 
an _increase_ of antenna impedance because the reactance changes at a 
faster rate than the resistance; a choke designer would be wise to allow 
for the extra dissipation that causes.

Finally, if you don't believe my analysis, just try modelling the 
situation in EZNEC. I took an 80m dipole (#14 wire at 35ft) and added a 
4k resistive choke to the CM path at the feedpoint. For 1000W applied 
power these were the choke dissipations at various frequencies:

SWR=1.4:1 at resonance: 2.2W
SWR=2:1 below resonance: 3.3W
SWR=2:1 above resonance: 3.9W
SWR=3:1 below resonance: 5.2W
SWR=3:1 above resonance: 6.4W

As you can see, the SWR directly affects the choke dissipation!

Steve G3TXQ

On 19/04/2012 05:47, Jim Brown wrote:
> On 4/18/2012 12:16 PM, Steve Hunt wrote:
>> Sorry to disagree again, but the CM choke dissipation DOES also depend
>> on the SWR!
> I don't buy your analysis, but I don't have time to pick it apart. :)
> 73, Jim K9YC
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