Or you can remotely tune the cap .. and bend one rotor blade tip so it
will short out when fully meshed. Used that on an 80m vertical many
years ago.
73
Don
N8DE
Quoting Jim Thomson <jim.thom@telus.net>:
> From: w3oa@roadrunner.com [mailto:w3oa@roadrunner.com]
> Sent: Saturday, November 20, 2010 11:36 AM
> To: towertalk@contesting.com
> Cc: sturpe@gmail.com
> Subject: Tune 160/80 Vertical
>
> I'm helping a friend who has an 80 meter 1/4 wave vertical. At the
> top, he has a 160 meter trap then a wire extending horizontally for
> 160 meters. It is therefore an inverted L on 160.
>
> The SWR on 80 is good and can be adjusted nicely by changing the
> length of the vertical component. 160 meters, however is somewhat
> of a problem.
>
> The minimum SWR he can achieve on 160 is about 1.7:1. Of course the
> object here is to lower the SWR on 160. Here are some readings we
> took with my MFJ -269 antenna analyzer in case this helps anyone.
>
> 1.770, Rs 16, Xs 0, SWR 3:1
> 1.800, Rs 27, Xs 15, SWR 2:1
> 1.825, Rs 45, Xs 26, SWR 1.7:1
> 1.85, Rs 79, Xs 31, SWR 1.9:1
> 1.875, Rs 128, Xs 2, SWR 2.5:1
>
> Can anyone point us in the right direction to find a solution?
>
> Thanks,
>
> Dick, W3OA
>
> ## Install a capacitor in series with the feedpoint of the
> vertical, right at the base. Make the horizontal wire long enough,
> so the resistive component is 50-60 ohms on the freq you are
> interested in. At that
> point you will have some XL. Then tweak the cap so the Xc of the
> cap will cancel out the XL on 160m. Presto, flat swr on 160m.
> On 80m, the cap has to be shorted out. No big deal with a suitable
> spst relay.
> A buddy used a motor driven 2500 pf cap... and a 100' tall
> vertical, with an inverted vee type capacity hat on top. This
> entire mess was suspended on a catenary line, between 2 x trees.
> It's flat swr across the entire 200 khz.
> Note, on some vac caps, they have a feature on em, whereby when the
> cap is fully meshed, and then some, the plates will short out.
> This saves you having to install a relay across the cap, in cases
> where you want it
> shorted out.... as in your dual band set up.
>
> ## a padded air variable and relay will also work, with the
> provisio that the paralleled padder's don't drift about.
>
> Later... Jim VE7RF
>
>
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