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.
Carl
KM1H
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "jeremy-ca" <km1h@jeremy.mv.com>; "Tower Talk List"
<towertalk@contesting.com>
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.
>>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:
> http://lists.contesting.com/pipermail/towertalk/1997-August/008139.html
>
>
>
>> >
>> > 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 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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