[Antennas] Common Mode Choke

[Dan Richardson]

At 05:25 PM 3/9/2003 -0800, you wrote:
>"The only instance I know of that uses an isolated
>(floating) tuner is when
>the choke/balun is placed on the input side of an
>unbalanced tuner that in
>turn is connected to balance line (ladder line). This
>is under the mistaken
>belief that this reduces the stress on the balun which
>is does not."
>
>This subject keeps coming up and everyone keeps
>telling me that putting the 1:1 choke balun on the 50
>Ohm (transmitter) side doesn't do *anything*.

No, it does the same thing.


>I've build three QRO tuners with 1:1 current baluns on
>the transmitter side, wound them for a 50Ohm Zo and
>they handle 1500W continuous duty for 30 minutes. No
>matter how high or low the load Z gets, the balun
>doesn't see it -- at these power levels, a 1:1 on the
>output side is prone to arcing over when the Z gets
>high.
>
>For that reason alone, I chose this tuner
>configuration and it works very well for me.

I have built and use one also and it work well.


>This is available commercially, Palstar makes a true
>QRO tuner in this configuration - they use tall
>standoff insulators to isolate the components from the
>chassis. I made mine from 0.236" thick Lexan.
>
>For what it's worth.
>
>73, Steve N4SL Machias, WA CN88xa
>
>__________________________________________________

Steve,

While that approach does address the deferential mode impedance you can not 
be assured that common mode impedances are improved and, under some 
circumstances they can be made worse. It took me a little time to 
understand this. But if you consider carefully the common mode current path 
to ground I think you'll see what I am talking about.

Consider your 500Z choke impedance trying to block a very high (2,000 to 
3000Z) common model impedance.

On the plus side the tuner is an L tuner and thusly has a little better 
efficiency than a T type tuner. If you wish to have the best unbalanced to 
balanced tuner then consider a link type tuner. They don't make them any 
better than those for unbalanced to balanced matching.

And finally, remember I use one, however, I have increased the choke's 
impedance to over 3000-Ohms and try to keep the antenna system 
configuration as balanced as I can.


73
Danny