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Topband: Flag Xfmr Measurements End Game

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Subject: Topband: Flag Xfmr Measurements End Game
From: (Tom Rauch)
Date: Mon, 23 Sep 2002 07:44:25 -0400
> I now have one small binocular core transformer, made from two small
> toroids, that gives me a 1.2:1 match at 2 MHz when terminated with a
> 910 ohm resistor.  Then I wound a separate 1:1 transmission-line balun
> on another small core and connected it to the 50-ohm side of the
> transformer.  I still get 1.2:1 through the balun, so I think I'll
> stop there.
> The balun presents about 500 ohms reactance at 2 MHz, so should be
> plenty adequate to decouple the transformer from the feedline.


(You may want to crosspost this the the Flag group.)

We have to be very careful adding reactances in series with the 
common-mode impedance of a system to reduce unwanted current. In many 
cases, we can make the system worse by adding reactance.

The rule-of-thumb that claims using a balun with an impedance several 
times the feedline impedance actually is not a good rule. As a matter 
of fact, it really has almost no relationship to what happens in the 
real world.

It leads to misconceptions, like the common misconception that claims 
moving a balun to the input of a floating tuner will improve balance 
or the other common misconception that several hundred ohms of balun 
reactance is enough for a choke balun.

To improve balance, a choke balun must increase the common-mode 
impedance of the system significantly. If the antenna and feedline 
junction has a capacitive common-mode reactance, like a Flag or 
Pennant presents, and we connect an inductance (choke balun) in 
series with the feedline, it will actually make balance WORSE...not 

That is why almost everyone recommends using an isolation transformer 
with loose coupling, because like-sign series reactances add 
(increase) while opposite series reactances would decrease the 
opposition to common mode currents.

You have to look at each individual system type, and determine what 
type of balun will work. If not, adding a balun can make things 

    73, Tom W8JI 

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