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Re: [TowerTalk] Balun question shield leakage

To: "Tower Talk List" <>,"Jim Lux" <>
Subject: Re: [TowerTalk] Balun question shield leakage
From: "jeremy-ca" <>
Date: Fri, 15 Jun 2007 13:42:30 -0400
List-post: <>
I never stated that heating was ever a part of this thread, perhaps you are 
referring to someone else. However I have come close to burning my hand once 
many years ago while at a friends home tracking down severe RFI/TVI. He had 
made the balun with about 30 FT37 size toroids of unknown mix over small 
Teflon coax. Antenna was a TA-33 and amp a SB220. Replacing with 8 large 
1.25" mix 43 beads over RG213 cleared up the problem entirely. The offending 
balun went into the trash, I never analyzed it.

As far as leakage power I believe that you have confirmed my thoughts. It 
does not take much power, and -35dB down from 1500W can span the globe under 
the proper conditions, to set up a point contact junction on  knife edge 
pieces of contanimated ferrite.


----- Original Message ----- 
From: "Jim Lux" <>
To: "jeremy-ca" <>; "Tower Talk List" 
Sent: Thursday, June 14, 2007 9:09 PM
Subject: Re: [TowerTalk] Balun question shield leakage

> 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. 
>>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 
>>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 asymmetry).
> 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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