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[TowerTalk] 160m shunt feed followup

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Subject: [TowerTalk] 160m shunt feed followup
From: (Tom Rauch)
Date: Mon, 1 Oct 2001 10:30:00 -0400
First on suggestions of high-pass filters...or the even worse the 
useless suggestion of a small variable capacitor.

Also, BC interference normally gets strong at night because then 
you have the combined signals of many BC stations propagated in. 
The only possible advantage is you might have a strong local 
daytime-only station that pulls the plug at dark. 

Virtually all filters will shift the resonant frequency and readings, 
unless you use a filter that does not act like a transmission line 
and is tunable. The MFJ filter is virtually transparent, because you 
set the tuning so the reactance change is zero. It does not look 
like a long transmission line electrically, and will solve all but 
severe cases of problems.

> Just for fun, I decided to substitute a small roll of RG-8X as the
> series capacitor (440 pf measured) and see what SWR my VFD measures at
> the shack end of 200 feet of 75-ohm CATV hardline.  To my pleased
> surprise, it indicates an SWR minimum of ~2.1, at the same frequency
> where the Autek indicated a Z minimum, and the radio's internal tuner
> found it very easy to match.

It is very odd (improbable) for RF to cause an error in SWR and not 
in Z reading. The reason is a diode that mainly is used in Z is right 
across the feed terminals, and the SWR stuff is further in.  

Z is by far more sensitive than SWR to external voltages, because 
it involves squares in the calculations.

Z is notoriously unreliable for finding resonance, especially if Q is 
high or transmission lines involved (and ALL antennas act like they 
contain transmission lines). 

Since Z is scalar unless you can sort out the real and imaginary 
parts, you have no idea what you are measuring.

> Can I declare victory and start laying down radials, or do I need to
> drag a radio out to the base of the tower to verify what I think I'm
> seeing in the shack?

It might be useful to get a better idea what the impedance is, but it 
sounds like you are close.
73, Tom W8JI 

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