Topband: Update from AG6X shunt feed project

Sun Feb 16 11:11:14 EST 2014

FB, Carl!  Well, if you're hearing well on that tall vertical, the noise
environment must not be too bad in your area, It's pretty bad here in
Raleigh, so I had to resort to terminated receiving loops for 160 - and they
help on 80 also.

Yes, I had the same thought about moving that tap point higher as you
improve your radial field. After all, if that 68 ohms drops to 60 ohms
that's still 1.2:1 VSWR, but the BW might be a little better. You might want
to wait until AFTER you've done the radial improvements before adding the
additional gamma wires and selecting the tuning capacitor.

Sounds like your hard work is paying off!  FB on the KH6s! I've worked a
number of them from here in Raleigh, but that's a tougher path than CA-KH6!
Keep at it! Sounds like it's coming together and playing well!  Have a good
day!

73,
Charlie, K4OTV

-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Carl
Braun
Sent: Sunday, February 16, 2014 10:49 AM
To: Charlie Cunningham
Cc: topband at contesting.com
Subject: Re: Topband: Update from AG6X shunt feed project

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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