Topband: Update from AG6X shunt feed project

[Carl Braun]

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" <charlie-cunningham at nc.rr.com> 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:topband-bounces at contesting.com] On Behalf Of Carl
> Braun
> Sent: Saturday, February 15, 2014 9:03 PM
> To: topband at 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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