Even if the beam is "insulated" from the mast or boom, any device insulating the support members of the antenna should be bypassed at these points:
1) Mast to Boom. 2) boom to longest elements farthest out on Boom. This may not be straightforward with a two element "shorty forty", but definitely connecting the reflector or director on a two element yagi may protect the insulation at the driven element from being arced over or carbon tracked at Remember also that if you have coax to the driven element through a balun of any kind, the half element + coax shield will be connected via the suppression path of the balun, which is NOT designed to stop the series impedance OR QRO voltage found 30 feet from the end of a 160 meter radiator. If anything, at QRO on 160, the suppression path will get heated up, UNLESS there is a more attractive path to the other element. The high beam IS the end of the radiator, whether you want it or not. Do targeted things to let the 160 energy get to the end of the longest elements farthest out on the boom. If not, you may find that it behaves one way with QRP or an analyzer, and quite another with power, all the while polluting your setup with carbon tracks. AND, if you change the stuff on the high half of the tower, it DOES change the 160 feed Z. Worry if it DOESN'T. 73, Guy. On Mon, Dec 12, 2011 at 10:29 AM, Mike & Coreen Smith <[email protected]>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 > > > > ------------------------------------------------------------------------------ > > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 8.5.454 / Virus Database: 271.1.1/4071 - Release Date: 12/09/11 > 20:51:00 > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
