Thanks Charlie I haven't transmitted on the antenna yet but I did hear KH6XX, KH6LC and WL7E this morning along with a bunch of stateside guys participating in the contest.
N7GP and K7FA in AZ were +20 this morning and N7XM was loud too. I'm looking forward to seeing improvements with additional radials but I do anticipate the R dropping too. If things "improve" too much I may be moving the gamma arm up to the 90' level yet. More later. Thanks again Carl AG6X Sent from my iPhone > On Feb 16, 2014, at 7:37 AM, "Charlie Cunningham" > <[email protected]> wrote: > > Wow!, that's perfect, Carl!! That should result in better than 1.2:1 load > VSWR!! No wonder you see a "flat" line at the transmitter end! Looks like > either 90' or 67' would be good tap points, but 67' is better! Can't do > much better than 42 ohms feed-point resistance! > > If you're seeing a flat line from 1800-1850 that's pretty good bandwidth!! > > I'm not sure how much you might gain by adding additional gamma wires, The > loss in 67' of 14 ga wire can't be that great! The impedance of your gamma > may decrease with the additional wires, requiring a larger capacitor to tune > it! There's a lot to be said for "If it ain't broke -don't fix lt"! > > Regarding the gamma capacitor - if I did my quick "back of the envelope" > 'rithmatic right, the 140 pf of capacitance should have about 36.2 ohms of > capacitive reactance at 1.8 MHz. Now, if you are delivering 1500 watts of RF > into a 42 ohm load, that's a little less than 6 amps RMS of RF current. > That would result in an RMS voltage of about 220 volts across the 140 pF, or > about 330 volts peak. So your 4500 volt Cardwell should have no problems > dealing with it. No need to use your vacuum variable! > > If you add additional gamma wires, you may need something a bit larger than > your 160 pF capacitor, but the voltage requirements wouldn't increase > significantly, if at all. Of course whatever capacitor you use, you, of > course, need to protect it from moisture, insects etc. > > Sounds like you have it working well, and you can concentrate on workin' on > your radial field. Of course, as you improve the radials, you may see that > 42 ohm resistance drop a little and your BW decrease as you reduce the > ground losses and the Q of you antenna system increases! I've had that > experience in the past. > > Anyway, Carl, sounds like you have it playing pretty well! Have fun! > > 73, > Charlie, K4OTV > > > > > -----Original Message----- > From: Topband [mailto:[email protected]] On Behalf Of Carl > Braun > Sent: Saturday, February 15, 2014 9:03 PM > To: [email protected] > Subject: Topband: Update from AG6X shunt feed project > > > All > > I decided to take a chance at tapping the tower at 67' and apply some series > capacitance to see how the structure would work there before tapping it at > the 90' level. Here is what I found: > > The gamma arm spacing is at 33" and with 140pf in series I see 42+j0 ohms at > the feedpoint. Inside the shack at the end of the LMR 400 I see basically > FLAT SWR from 1800 to 1850 and 1.5:1 at 1865...with the cap fixed at 140pf. > > All of that with my skimpy (single 14AWG) gamma wire. > > Tomorrow I plan on dropping the tower again to add the additional 2 or 3 > wires to create the gamma wire cage. My current PVC standoffs have been > modified to accept three gamma wires spaced approx. 10" apart (though I'm > only using one now as I said before). I'm assuming this MAY provide me with > a couple more ohms getting me closer to the magical 50 but bandwidth is what > I'm truly after. If I still need a few more ohms I may extend the gamma and > standoff arms out another 6" or so...which would be the MAX reach without > installing new arm and standoffs. > > So...with these low capacitance requirements (140pf now and possibly less > with the multiple gamma wires) will I still need to scrounge my vacuum > variable out of storage or will my 4500V Cardwell cap get the job done at > 1500W? > > Thanks to all who offered their advice and look for an update from me after > the gamma cage is assembled and additional radials are installed > > Carl AG6X > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband > _________________ Topband Reflector Archives - http://www.contesting.com/_topband
