[Top] [All Lists]

Re: Topband: Shunt feeding the Skyneedle

To: Tom W8JI <>, '160' <>
Subject: Re: Topband: Shunt feeding the Skyneedle
From: Carl Braun <>
Date: Mon, 3 Feb 2014 17:26:08 +0000
List-post: <">>
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 

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


Carl AG6X

-----Original Message-----
From: Tom W8JI [] 
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 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??

Topband Reflector Archives -

<Prev in Thread] Current Thread [Next in Thread>