Topband: Shunt feeding the Skyneedle

[Carl Braun]

Tom and group

The SWR is 2.4:1 at 1822

I have an old Heathkit tuner that has a pair of air variables that I may temporarily yank to experiment with gamma vs. omega matching.  I've got a wing wang capacitance meter that would tell me the values once I get something to resonate.

The 160pf Johnson variable is 3/4 meshed so I don't see additional C being needed in series.  I think I will have to add a parallel C to get it down to 50 ohms and x=0.  If I can get the tower to 50/x=0 then I'll substitute a vac variable in for the Johnson.  The Johnson SHOULD work as a parallel cap as it's good for 7KV according to the Johnson literature.  See pic.

Also, I've been basing a lot of my values (tap height and gamma spacing) on ON4UNs charts.  But I found some English amateur who did a study on gamma/Omega/Beta matching and found that ON4UNs calculations are up to 2X out of whack.  I also found an old article on shunt feeding towers from Ham Radio magazine that gave tap height, gamma spacing and C curves. His calculations were off quite a bit as compared to ON4UNs.  I attributed this to computer modeling vs. none back in the day and would tend to think the modeling results are more accurate.  See attached article.  I see a good 20-40 degrees difference between the old school article and ON4UNs calculations.

I don't do any modeling though I'd like to try EZNEC or? one of these days to see what my various antennas really look like.

So, i'm assuming you're suggesting that I drop the gamma arm down to the 67' level and see what the impedance looks like?  If so, I'm guessing the series C required to tune would increase in value?

Please advise

Thanks

Carl AG6X


-----Original Message-----
From: Tom W8JI [mailto:w8ji at w8ji.com] 
Sent: Monday, February 03, 2014 8:58 AM
To: Carl Braun; '160'
Subject: Re: Shunt feeding the Skyneedle

the vertical section was still suppressed and the top 29' was resonating. 
With this in mind I decided to run a shunt wire from the top to see where 
the antenna resonates.  Here's what I found..>>>>

I was afraid the tower was messing up the L. This is what happens when they 
are are nearly resonant.

You can't measure tower resonance with a drop wire. The drop wire is a stub 
or shorted transmission line in parallel with the common mode impedance 
presented by the drop wire and tower combination. It is just a mess of stuff 
going on.

 <<<
The MFJ read 380 ohms at 1825 and the X is way off the scale.  I inserted an 
EF Johnson 10-160pf air variable capacitor at the base.in series.and was 
able to tune the antenna to 60 ohms and the X=22.  If I played with the cap 
there was a real sharp drop in reactance showing X=12.  The air variable was 
about ¾ meshed.>>>



Reducing the lenth (tap point height) of the drop wire is a better way to 
get impedance right. Or, better still, use a multiple wire drop to make the 
drop diameter look larger. That will reduce Q, require more C, and should 
reduce impedance.

You are so close at 60 ohms I would not worry. Adjust the cap for lowest 
SWR. What is the SWR??