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 <firstname.lastname@example.org> 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
> to improve it!! You would pick up more reactance than that by just
> 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
> Charlie, K4OTV
> -----Original Message-----
> From: Topband [mailto:email@example.com] On Behalf Of Larry
> Sent: Wednesday, December 10, 2014 6:14 PM
> To: Charlie Cunningham
> Cc: firstname.lastname@example.org
> 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
> 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
> Larry K4AB
> On 12/10/14, Charlie Cunningham <email@example.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:firstname.lastname@example.org] On Behalf Of
>> Larry K4AB
>> Sent: Wednesday, December 10, 2014 2:41 PM
>> To: email@example.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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