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@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??
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