[TowerTalk] Tune 160/80 Vertical

[Jim Thomson]

From: w3oa at roadrunner.com [mailto:w3oa at roadrunner.com] 
Sent: Saturday, November 20, 2010 11:36 AM
To: towertalk at contesting.com
Cc: sturpe at 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