[Towertalk] Shunt feed tower - Again

[n4kg@juno.com]

Do you have an Antenna Analyzer to measure the feed IMPEDANCE?

My guess is the Resistive component is LESS THAN 50 ohms
which is typical for verticals, especially short loaded verticals.

Use and OMEGA match to step up the low resistance to 50 ohms.

An Omega match has a shunt capacitor from the Gamma arm to
ground, following the series capacitor from the center of the feedline
to the Gamma arm.

Tom  N4KG

On Fri, 13 Dec 2002 12:09:35 -0000 "Bob Henderson" <bob@cytanet.com.cy>
writes:
> Forgive me, I know this subject has cropped up several times here.  I 
> am
> hoping someone among you tower-talkians can give me some guidance on 
> what my
> next move should be.
> 
> I have a crank up tower which is about 65ft fully extended, on top 
> of which
> I have 18ft of HD stub mast.  At 65ft I have dipoles for 160m and 
> 80m, at
> 67ft a F12 C31XR, at 76ft a Cushcraft XM240 and at 85ft a 5 ele 6m 
> yagi.
> 
> I am trying to shunt feed the tower for 160m.  As I want to retain 
> the 80m
> and 160m dipoles at this stage, I have fitted each of them with 
> decoupling
> traps resonant at 1830 kHz.  I have dropped a shunt wire from 65ft 
> at a
> distance of 30 inches from the tower.
> 
> Results so far:
> 
> The lowest SWR I can get at 1830 kHz is 2:1, which I get with 225 pf 
> of
> series capacitance.
> 
> The frequency on which I can get my minimum SWR of 1.3:1 is 2220 kHz 
> with a
> series capacitance of 100 pf.
> 
> Bringing the shunt wire closer to the tower brings down the 
> frequency at
> which I can get minimum SWR, however the minimum SWR increases as I 
> go away
> from 2220 kHz.  I have tried a shunt capacitor and this produces 
> roughly the
> same effect.
> 
> At this point I am unsure whether to reduce shunt height, alter 
> shunt
> spacing or attach some light weight alloy tubing to the top of the 
> stub mast
> to increase overall height.  Perhaps I will need to do all three?
> 
> Every time I need to make an adjustment I have to crank the tower 
> down, luff
> it over and partially dismantle my C31XR and XM240 so I can reach 
> the parts
> I need to adjust.  As you will understand, I'm rather keen to 
> minimise the
> number of times I have to do this.
> 
> Words of wisdom gratefully received.
> 
> 73
> 
> Bob 5B4AGN, P3F
> 
> 
> 
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