Topband: High Z vs Low Z shunt feed tower

Ford Peterson
Mon, 6 Nov 2000 13:17:24 -0600



 "That is what I have.  One 15kV 200pf door knob cap in series with the
> ----------
> Plus one other component -- the 9:1 un-un.

I measured the transformer with the autek.  It seems to be very low
insertion loss and doesn't get more than just warm after many minutes of 800

> "It seems to me that as long as I can get the tower to take power
> (resonate it) at the frequency of interest (1832), messing with the
> shunt, etc., is just alot of work.  Rauch seems to feel that I'm
> splitting hairs."
> ----------
> The tower does not have to be resonant to take all of the power you give
> it.  When you adjust the shunt feed, you are not resonating the tower,
> you are matching it to your feedline.

Good point.

> If you move the tap point down from the 450-ohm point to the 50-ohm
> point, you'll be able to get rid of the un-un and the required gamma
> capacity will be about 600 pF, three times what you're using now.  Tom
> basically said to use the fewest number of components to reduce losses
> and that the greatest amount of capacity is the most efficient.

But, will it provide a better pattern or ERP?  That's the issue.  I am
certain I could figure out 50 different ways to get a match.  Does it
improve my SNR?  I figure I could easily spend a weekend up and down the
tower and drop a few bucks in components.  I am investigating whether
anybody would "hear" any difference.  I have a system that seems to work.  I
could easily modify things and get them to the point where I can't go back
to as good as what I've got.  I am nervous about messing with it.  I will if
it makes the difference between a QSO or not.

At this point I am leaning towards Rauch's response--Splitting hairs...

Thanks for the input.


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