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Re: [TowerTalk] Balun Recommendation

Subject: Re: [TowerTalk] Balun Recommendation
Date: Thu, 19 Apr 2012 07:42:23 -0400 (EDT)
List-post: <">>
Hi All, this is an observation from an innocent bystander:
With constant power level, if the antenna Z increases, The voltage  
increases as well, right? Conservation of energy then requires the current to  
decrease, right?
The rf voltage on the coax shield where it connects to the  antenna would 
be the voltage referenced to ground at that half of the antenna.  The choke 
current would be that voltage divided by the choke impedance (also  
referenced to ground), Is that right?
So, one might think that a higher Z antenna = higher E = higher choke  
current? If so, power loss is then I^2 X whatever resistive loss is present in  
the choke.
2X Z = 1.414 X voltage = 1.414 X choke current = 2 X loss. if the above is  
Gerald K5GW
In a message dated 4/19/2012 3:08:04 A.M. Central Daylight Time, writes:


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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