Topband: 160 Shunt-Fed Tower Readings
charlie-cunningham at nc.rr.com
Wed Dec 10 21:29:32 EST 2014
You're quite welcome, Larry! Hope it helped! You're certainly on the right track and doing some good work!
FB on the Beverages!! Wish I had real estate for beverages, but I live on a city lot, so I had to resort to terminated receiving loops that, surprisingly, worked quite well and enabled me to hear LOTS that I had been missing on 160. Generally, with my inverted-L and 500-600 watts, if I could hear 'em, I could work 'em! My problem before putting up the KAZ loops was hearing! The terminated loops were great receive antennas on 80, 40 and 30m also.
The Beverages should help you a LOT! Have fun and have a Merry Christmas!
From: Larry K4AB [mailto:larry.k4ab at gmail.com]
Sent: Wednesday, December 10, 2014 8:23 PM
To: Charlie Cunningham
Cc: topband at contesting.com
Subject: Re: Topband: 160 Shunt-Fed Tower Readings
TU, Charlie on a great explaination for us non-engineers!
The topband reflector is a great resource for guys like me trying to understand this band, even after decades in the hobby.
And yes, I'm working on the Beverages, as we speak!
On 12/10/14, Charlie Cunningham <charlie-cunningham at nc.rr.com> wrote:
> 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
> 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
> Charlie, K4OTV
> -----Original Message-----
> From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of
> Larry K4AB
> Sent: Wednesday, December 10, 2014 6:14 PM
> To: Charlie Cunningham
> Cc: topband at 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?
> Larry K4AB
> On 12/10/14, Charlie Cunningham <charlie-cunningham at 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"!
>> Charlie, K4OTV
>> -----Original Message-----
>> From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of
>> Larry K4AB
>> Sent: Wednesday, December 10, 2014 2:41 PM
>> To: topband at 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.
>> 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!
>> Larry K4AB
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