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:topband-boun...@contesting.com] On Behalf Of Carl Braun Sent: Saturday, February 15, 2014 9:03 PM To: topband@contesting.com 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