You can temporarily use an inductor in series with the cap to extend the
range. It will not be a good idea for transmitting, but OK for tuning.
----- Original Message -----
From: "Carl Braun" <Carl.Braun@lairdtech.com>
To: "Tom W8JI" <w8ji@w8ji.com>
Cc: "160" <topband@contesting.com>
Sent: Tuesday, February 04, 2014 10:27 PM
Subject: Re: Topband: Shunt feeding the Skyneedle
Thanks for the input Tom
The only variable cap I have is the EF Johnson which is 60-160pf. I have
some ham radio stuff at my parents house not the least is a Jennings
1000pf vac variable rated at 5KV or 7.5kv. I was hoping to use that with a
12v motor for QSYing up the band for contesting. I'll have to ask mom to
send it to CA in a pkg with some cookies.
When the gamma arm was at 90' I was able to add 160pf to get a resonance
point around 1825 but the resistance was still high at 58-60 and X was
20++. Maybe the big vacuum cap would bring that R and X down to where it
needs to be.
ON4UNs figure 9-85 on page 9-71 of his third edition shows that a tower
that is electrical 110 to 130 degrees should have a tap height around 20
meters and a matching cap of 400pf. That being said it may be a good idea
to get the vac variable into service. I would assume I would want to raise
the gamma arm back up to 90' as it resonated closer to 1825 than the
latest iteration which shows a Fr near
1.977
Sent from my iPhone
On Feb 4, 2014, at 6:40 PM, "Tom W8JI" <w8ji@w8ji.com> wrote:
Here's what changed though...when I had the gamma arm at 90' with the 14
gauge gamma wire 24" away from the tower I was able to insert my Johnson
60-160pf variable cap in series with the gamma wire to get approx 58-60
ohms at X=20. The cap was 2/3 meshed at this point. >>>
That's the right way. You have to cancel the reatcance of the drop arm to
get a good reading. Maybe you need a larger capacitor to hit the bottom
of the band? Resistance normally goes up in a case like yours as
frequency is drecreased.
<<<NOW that I've lowered the gamma arm to the 67' level...I insert my
variable cap and the antenna resonates at 1.970 MHz with R=36 ohms and
X=0. For some odd reason the MFJ SWR reading shows 1.0:1 with this 36 ohm
reading and, inside the shack, the Ft1000D shows 1.0:1 swr from 1.988 to
1.950 and a 1.5:1 range of 2.007 to 1.930>>>
What does more capacitance do?
<<<<It now appears that the antenna is a bit short but why am I seeing
these crazy high resistance readings with no variable cap in line?>>>
You should see them. The MFJ detector is a 50 ohm bridge. It will
overflow and give all kinds of goofy readings when impedance is far away
from 50 ohms.
<<How can I lower the resonant freq without moving the gamma arm up?
Increase the spacing of the gamma wire from the tower? Add more
radials?>>>
I would have left it at the top and shorted the wire to the tower at
different places until I found the sweet spot. But you have to dip the
reactance out to really know what you have.
<<<
I was going to build a three conductor wire cage with the wires spaced
10" apart or so once I had an idea where the antenna resonates. Would a
fatter gamma trio drop the resonant freq or just change the capacitance
value of the antenna?>>>
A fatter shunt wire will lower reactance and resistance. You will need
more C, and the tuned resistance will be a bit lower.
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