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-bounces@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
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