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Re: [TowerTalk] Balun question shield leakage

To: "jeremy-ca" <>,"Tower Talk List" <>
Subject: Re: [TowerTalk] Balun question shield leakage
From: Jim Lux <>
Date: Thu, 14 Jun 2007 18:09:41 -0700
List-post: <>
At 05:32 PM 6/14/2007, jeremy-ca wrote:

> >>With
> >>a reactive load as would exist with many commercial and home brew antennas
> >>leakage is not only possible but very likely.
> >
> > I do not think that's true.  Can you give an example and the mechanism by
> > which anything connected at the end of the coax would change the
> > propagation "through" the shield?
>Ill answer that with another question. Assuming high VSWR conditions as
>would be found on a typical 80M dipole being used over the whole band. What
>would be happening to the currents on the inside of the braid and also on
>the outside?

The fact that it's mismatched isn't going to put any current on the 
outside (unless it's also unbalanced), but looking at the current on 
the inside of the braid..

OK.. a dipole in free space cut for about 3.75 MHz (VSWR 1.23:1), 
VSWR at 3.5MHz is 5:1, about 4:1 at 4.0 MHz
50.05-j85.1 at 3.5
61.72+j0.034 at 3.75
76.12+j85.45 at 4.0

So.. assuming you have some sort of matching network at the 
transmitter end, you have about 1.5x reactive power circulating as 
you do active power  (since the X is about 1.5 times the R)

Running the calculations for LMR400 (chosen because it's low loss): 
as one might imagine, the current varies 4:1 from about 8.5A to 2A.

Now, comparing that to the matched case (at 3.75 MHz)... The current 
is around 4.5Arms

So, if the leakage were due to the current flow, it would only be 
twice as much (4 times the power) at the current peaks, and 1/2 as 
much (1/4 the power) at the current minima.  Depending on the length 
of the line, etc., it will be pretty much a wash (the maxima will 
balance the minima).

I've seen coax isolation numbers >60 dB at 100 MHz (I'm not sure what 
the measurement condition is).. So, even if the leakage were 
quadrupled from a mismatch, it would still be 54 dB down.

Times Microwave's catalog shows 55dB isolation for one foot of single 
braid copper coax at frequencies below 100 MHz.  100 ft would be like 
coupling out -35dB.. less than a watt if you run 1kW.  I suspect that 
you'll have other sources that induce more stray fields (like the 
near field of the antenna?)

In the balun context, you're looking at the leakage through a few 
feet of cable.. tens of milliwatts?

See, for some more information:

> >
> > I can believe currents induced on the outside very readily.
>VSWR's up to 5:1 would be more realistic especially with the above mentioned
>80M dipole and many trap dipoles and verticals.

And that would be 25 times the radiated power, if you happened to 
have the choke at just the wrong place.. but even then, you're still 
in the <1 Watt area. And though that power leaks out, nothing says it 
gets all would be absorbed in the ferrite cores.   I don't think 
leakage is a big contributor to heating of the cores. (compared to, 
for instance, coupling to the outside from the antenna itself due to 

Sure, if the choke has a too low impedance, significant current could 
flow, and heating could occur (which was Jim's point)


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