Sounds like this could be acting like a 160m L with a good
radial field should act. If you REALLY want to lower the
SWR on 160m, one approach would be to place an appropriate
L network at the base which matches on 160m and is
switched out on 80m. Several antenna handbook tools can
be used to get an idea of the network since you have some
impedance data for the antenna, but a little cut and tweak
of the components will most likely be needed. If you
don't worry about the insignificant loss, some THN wire on
PVC and some fairly low value military micas or doorknobs
can be used for the network. Or, if you are well heeled a
nice adjustable vacuum variable would be good. One of the
limitations of the old micas is that many of them give up
the ghost when you ram more than half an amp or so through
them, so if you are going to run legal limit a parallel
combination would be better because of the current
sharing.
Bob W2WG
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Thomson
Sent: Saturday, November 20, 2010 9:51 PM
To: towertalk@contesting.com
Subject: [TowerTalk] Tune 160/80 Vertical
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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