Hi Bob,
If I recall (correctly).<and you should double check> The D40 is completely
isolated from the mast, so it would be nearly invisible to your 160m
RF...given all the other loading you have going on there.
I don't know a thing about the Optibeam...is it also isolated or grounded?
I am guessing grounded, as you now have a change.
If that IS the case, you'll need more capacitance to counteract the loading
you've added. As to the amount, I could only hazard a guess. If you've
maxxed out your 1000pF air variable, you can add some random length of
RG-214 or other coax in parallel and start hacking off a foot at a time, and
then tweak with the cap as neccessary.
It's cheap and saves climbing ! I forget at the moment how many pF RG-214
or other similar dielectric coax is, but maybe 10-15pF per foot springs to
mind. Try adding 10 or 15' on and see what you can do with your cap.
My brain is still numb from ARRL-10 CW this weekend...I keep hearing "T E S
T, V E 9 A A" in my ears , hi !
DIT dit,
MIke
----- Original Message -----
From: Bob Garrett
To: 'Jim F.' ; 'top Band'
Sent: Sunday, December 11, 2011 1:21 PM
Subject: Topband: Changes To My Shunt Fed Tower
Hello Topbanders,
It is getting cold in the Northeast and as I get older, I dislike the cold
even more.
In April of this year, I replaced my Cush craft D40 40 meter rotable
dipole
with an OptiBeam OB-4030 dipole for 30 and 40 meters right at the top of
the
mast. The overall length of the antenna is about ten feet longer and I am
assuming that it has added some additional capacitance to my 100 foot
shunt
fed tower with a Cush craft X7, A3WS, 5 EL M2 6 meter yagi and the new
OptiBeam dipole.
My match on the tower went from 1.0 x 1. To 1.9 x 1 after I installed the
new antenna. In terms of loss, this isn't a huge change but my amplifier
doesn't like the change. Before I spend a lot of time running back and
forth to the tower making random changes, any thoughts as to what I can do
to improve the match. I'm currently using one series capacitor 1000PF for
the matching network. I have attempted to adjust this capacitor but it
looks like the 1.9 x 1 is the low point. Perhaps some thoughts - increase
or decrease the spacing of the shunt wire? Move the connection up or down
on the tower?
Before I get to crazy, just wanted to look to the base of knowledge on
this
reflector for your input.
73 and tnx in advance.
Bob K3UL
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