[Top] [All Lists]

Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
From: Steve Hunt <>
Date: Mon, 23 Apr 2012 23:07:08 +0100
List-post: <">>

That's a gracious comment, and I genuinely appreciate it - thank you! 
But I wouldn't want Dean or anyone else thinking that I was that clever :)

I was never considering differential dissipation in the choke. My point 
all along was that common-mode voltage is determined by 
differential-mode voltage: double the differential mode voltage in a 
system without changing anything else and the common-mode voltage will 
double. The differential-mode voltage in its turn is determined by the 
differential-mode load impedance, as is the SWR on the feedline. There 
are therefore dependencies between the load impedance, the SWR seen on 
the feedline, the common-mode voltage, the common-mode current and the 
choke dissipation, which is why I baulked at your original statement 
that: "SWR has NOTHING to
do with dissipation in a common mode choke". Please note I am _not_ 
saying that standing waves are the _cause_ of common-mode current - I 
made that clear in an early posting!

You and Ian make valid points about the complex inter-relationships 
between the various elements of an antenna system, which is why I 
eventually quoted the results from an EZNEC model rather than using a 
simpler, less complete analysis. If you remember, a relatively small 
shift in frequency on an 80m coax-fed dipole altered the feedpoint 
impedance sufficiently that the observed SWR changed from 1.4:1 to 3:1 
and the choke dissipation increased from 2.2W to 6.4W.

I fully accept that the increased SWR and the increased choke 
dissipation were driven by the increased feedpoint impedance, but then 
to say that SWR has NOTHING to do with the choke dissipation seems to me 
an odd interpretation of the results.

We can debate causality at some length, but folk who simply observe that 
on their real-life antennas an increase in SWR is accompanied by 
increase in choke dissipation will rightly believe there IS a dependency.

In retrospect, perhaps this long discussion resulted from some ambiguity 
in the phrase "has nothing to do with" ?

Steve G3TXQ

On 23/04/2012 21:25, Jim Brown wrote:
> Chatting with Dean, N6BV, at Visalia over the weekend, he observed that
> Steve Hunt had been including differential dissipation in his discussion
> of the effect of VSWR on dissipation in a choke.  My apologies to Steve
> for missing that, and the above discussion is my response to correct for
> that failure.

TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>