Why not put a small matching network at the base of the parasitic element
with a motor driven capacitor. Then you can tune it above and below
resonance from the shack while watching the signal strength of received
signals. Also a relay to switch it out and you can instantly compare gain or
null.
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Stan Stockton
> Sent: Tuesday, November 24, 2009 12:45 PM
> To: topband@contesting.com; TowerTalk@contesting.com
> Subject: Re: [TowerTalk] 2 Element Parasitic Vertical
>
> Thanks to everyone for the responses to my question. There were posts to
> boith Topband and Towertalk as well as several sent just to me.
>
> The responses ranged from you will probably never make it work or you need
> to drive both elements to it will work according to the computer model.
> Many had suggestions as to how I should proceed in tuning it, cautions
> regarding the ground system, etc. All the responses were thoughtful and
> had information that was useful.
>
> I played with it a little bit last night in the dark. I switched the
> parasitic element in and out while listening to W1AW and it was difficult
> to see whether there was any difference in the forward direction. I had
> no signal source to the Southwest so worked with what I had. I then
> shortened the element by some amount (perhaps five feet or so) and checked
> again. This time I could see a difference and W1AW was best in the Omni
> directional state indicating to me the parasitic element was now a
> director. Then I added back about ten feet or so and W1AW was better with
> the parasitic element switched in. Now I am getting somewhere. When I
> got up this morning I went out and disconnected the shunt feed on the
> tower and used an MFJ analyzer to see what that wire element looked like.
> It looked like it would show least reactance a little below the lowest
> frequency the analyzer would tune - maybe 1750 or so. I took a couple
> feet off and it now shows a small amount of reactance a
> nd about 28 ohms at 1785. Does this give any indication of how adequate
> the ground system may be? I didn't measure anything but think the element
> is probably 2-3 feet longer than it originally was.
>
> The F/B is insignificant and is of no interest to me since I am going to
> listen on Beverages. Maximizing the F/B would only be useful if it would
> correspond to highest forward gain or if it were known to be say 30 Khz
> above the point at which maximum gain would occur so I could trim or add
> accordingly. With the tools I have available I think it would be difficult
> for me to see a small difference in forward gain from one frequency to
> another. It may only vary a small fraction of a dB over 20 khz. The
> computer model would not indicate major changes in what little F/B there
> is over that range either.
>
> I am thinking that if the element shows to be resonant somewhere between
> 1775 and 1790 and I can see a small but noticeable movement up in the S
> meter reading on NE stations when I switch the element in, I have done
> about all I can do unless someone has a practical suggestion as to how I
> can quickly squeeze our that last dB of gain that may be available
> somewhere between 1800 and 1900 and move it to 1830.
>
> Thank you for the suggestions. Since I posted to both reflectors, perhaps
> the best thing would be to send a response just to me in case this is
> boring to some.
>
> 73...Stan, K5GO
>
>
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