Hi, again, Larry,
Well, first of all the shunt feed, being a shorted transmission line, < 1/4
wavelength in extent, would present an inductive resent at the feed point
that you are correctly cancelling with the 1000 pF series variable
capacitor. You missed ever so slightly and you are showing a series
reactance of j5.6 ohms! Trivially small and negligible!! DON'T bother trying
to improve it!! You would pick up more reactance than that by just changing
frequency a few kilohertz!! It's PERFECT! The 15893 pF is the equivalent
series capacitance that would produce -j5.6 ohms of capacitive reactance -
so the message is that the equivalent series capacitance is NEGLIGIBILY
LARGE!! (LARGE is GOOD!) If you had measured at a frequency where the
reactance cancellatio was exact and perfect, the driving point impedance
would be pure real, with 0 reactance that would correspond to infinite
capacitance or 0 inductance.
The real part of your driving point impedance is 59.5 ohms real, resulting
in a VSWR od 1.23:1!! So near a "perfect match" that you won't improve it
significantly, even if you did an AWFUL LOT of work to make a trivially
small change in the tap point of your wire cage!! The mismatch loss at
1.23:1 VSWR is essentially 0, and the excess loss in a feedline operating
at 1.23:1 VSWR is essentially 0, and the loss in even a very long run of
coax would be essentially equal to the "flat' loss - that is: the loss in a
"flat" line operated at 1:1 VSWR.
So, Larry, you've done a GREAT job on your TX antenna. And do keep in mind
that you have 24 feet of boom up there that's top-loading that 94' of Rohn
25! Just congratulate yourself on a job well done, and ENJOY!! If you want
to worki on topband antennas - work on Beverages or terminated loop receive
antennas to help you HEAR better!
Great job! Have fun and enjoy!
BTW - I AM an RF engineer and I have spent q lot of my career designing and
measuring antennas
73,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Larry
K4AB
Sent: Wednesday, December 10, 2014 6:14 PM
To: Charlie Cunningham
Cc: topband@contesting.com
Subject: Re: Topband: 160 Shunt-Fed Tower Readings
" line
>Why do you feel a need to improve it?
Thanks for the response, Charlie!
I'm concerned about a few things:
1) I'm thinking the antenna is electrically too short. After all, the Yagi
at the top really isn't adding too much top loading. And I'm thinking
adding a couple of top loading wires connected to the tower might be
beneficial.
2) What exactly is the analyzer telling me when it says C:15893pF?
I'm far from an engineer, but something tells me that's way high. Or is it?
73,
Larry K4AB
On 12/10/14, Charlie Cunningham <charlie-cunningham@nc.rr.com> wrote:
> From your description and your measured data, Larry, I don't see how
> you could improve the antenna noticeably! Why do you feel a need to
> improve it?
> I appears to be resonant, well-matched and it appears to have an
> excellent ground image system! I think you might be kidding yourself
> if you change anything! As the old adage says n "If it ain't broke
> -don't fix it"!
>
> 73,
> Charlie, K4OTV
>
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of
> Larry K4AB
> Sent: Wednesday, December 10, 2014 2:41 PM
> To: topband@contesting.com
> Subject: Topband: 160 Shunt-Fed Tower Readings
>
> The antenna is a 94' Rohn 25, with a Force 12 24'boom Delta 6BA on top.
> Note
> that the elements are insulated from the boom. The base of the tower
> is grounded.
>
> 73 radials are attached to the base ranging in length from 130'-250'.
> 15,000 feet of radials, average length of ~200'.
>
> It is fed through a series 1000pF 10KV Jennings vacuum variable.
> The shunt is a 4 wire cage about 3" in diameter and attached at ~45'
> level. It is spaced about 3' from the tower.
>
> Here are the readings I get from a RigExpert AA-30 at the feedpoint:
>
> 1.830 kHz
> Series model: lZI: 59.7 Ohms
> R:59.5 Ohms
> SWR: 1.23
> X:5.6 Ohms
> C: 15893 pF
>
> What are these reading telling me,and how can I improve the antenna?
>
> Thanks, in advance!
>
> 73,
> Larry K4AB
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>
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