> If I recall (correctly).<and you should double check> The D40 is
> completely isolated from the mast,
Any coax fed antenna is never isolated from the tower/mast unless the
antenna switch/antenna relay is a double pole design and contained in
an insulated (non-conductive) case. Even then, there is generally
enough capacitive coupling between the tower and shield of the bundled
coax that the "shield leg" of the antenna/driven element will top load
the tower.
73,
... Joe, W4TV
On 12/12/2011 10:29 AM, Mike & Coreen Smith wrote:
> 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
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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