[TowerTalk] Roller inductor sources

[Michael Tope]

----- Original Message ----- 
From: <cgerarden at atomix.com>
To: <towertalk at contesting.com>
Sent: Wednesday, October 19, 2005 8:16 AM
Subject: Re: [TowerTalk] Roller inductor sources


>I must respectfully disagree with Tom Rauch about the concept of a balun on
> the output side of a tuner vs the input side. I currently have 4 tuners 
> that
> I have basically fried due to legal limit power and mismatches on 450 ohm
> line. These 4 tuners all had output baluns that saturated and got hot, but
> this is not always the failure point. Mostly HV failures at switches or
> caps.
>
> A critical part of 450 ohm line matching is to vary the feedline too. If 
> you
> really want to match 450 ohm line use a balanced tuner like the Palstar 
> 1500
> BAL. the T and L networks are fine for coax line, but the secret is to
> provide a low SWR into the balun output befor you connect it to the 
> antenna.
> this is all based on real world testing. I have a stack of MFJ, Dentron, 
> and
> 1 Nye-Viking that I smoked. If you are only runing 100 watts, then almost
> anything will work, but with higher losses.
>
> Chuck
> W0DLE

What you say, Chuck, is true for so-called "voltage baluns". I have fried
at least one myself (Ten-Tec 4229 tuner). If, however, you use a current
balun, then it shouldn't matter much if you place it at the input or the 
output
of the tuner since there is little or no magnetic flux in the balun of a 
current
that is due to differential mode excitation (all the flux will be due to 
common-
mode). Almost all of the older tuners I've seen (Dentron, Ten-Tec, etc) use
4:1 voltage baluns. The only one I have seen that uses a 1:1 current balun 
is
the Ameritron ATR-30. There maybe others, but this is the only one I have
seen. I suppose there maybe some benefit to the balanced tuner if the
common-mode voltage gets really high, but then again, the balanced network
is normally set to transform the value of the differential-mode impedance
down to 50 ohms, so I don't know if you can necessarily make any claims
about what it will do to the common-mode impedance. For all I know it
could transform it (the common-mode impedance) to a higher value on the
input-side. W9CF wrote a paper on this where he goes through the math.
Perhaps the answer to that question is in Doug's paper.

Anyway, sounds like the Palstar is doing a good job for you, but it would
be interesting to test it against the ATR-30 to see if there are any 
situations
where the current balun at the output of the ATR-30 gets stressed when
the balun at the input of the Palstar does not.

73 de Mike, W4EF...........................................