Any idea how much top loading that 5 el 46' boom monster contributes?
At a prior QTH in the 80's I had a 90' 25G toploaded with a 10-15-20M stack
of PV-4 monobanders and about 18' of mast. The 20M boom was 40' and the
tower resonated at 1620KHz if I remember. Sure worked great once I figured
out that 60 radials werent so hot over sand and added a mesh extending 50'
from the base.
The gamma rod was the shield of 3/4" CATV coax about 2' from the tower and
the best tap point was around 60' if I remember.
Carl
KM1H
----- Original Message -----
From: "Carl Braun" <Carl.Braun@lairdtech.com>
To: "'160'" <topband@contesting.com>
Cc: "'Tom W8JI'" <w8ji@w8ji.com>
Sent: Monday, February 03, 2014 11:17 AM
Subject: Topband: Shunt feeding the Skyneedle
Topbanders
If you followed my post on 'In search of resonance' you'll see that I was
struggling with feeding a 160m inverted L up close to my 90' Skyneedle. Tom
W8JI suggested that I cut back the L to 129' but the same phenomenon was
seen...that being the VERTICAL section of the inverted L was being
completely suppressed by the Skyneedle. At 129' long the antenna resonated
nicely on 8.2 MHz or so...indicating 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...
The Skyneedle is 90' tall and has a 13' mast sticking out the top that
mounts a Telrex 20M546 yagi on a 15m boom.
The aluminum gamma arm was attached at the 90' level at 24" away from the
tower and held in place by PVC standoffs. See attached photo if the
reflector lets me post an attachment.
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.
Here are the other resonant points...
15.8 MHz X=0 R=37 with pos and neg reactance on either side of X=0. This
freq showed the sharpest dip of any of the three.
Next was 10.6 MHz X=0 R=23 with pos and neg reactance on either side of X=0.
The last real dip I saw was at 5.3 Mhz with X=0 R=10
No dips below these frequencies but as I stated earlier I tuned the MFJ 259
to 1825 and then played with the variable cap where I saw the big drop in
impedance. (58-60 ohms at X=12 to 22.
Here are my questions for the gurus...
Do I attempt to match the antenna using a gamma match by tapping the
Skyneedle at the 67' level to see how it reacts with regards to R and X?
(Note - I cannot vary my gamma arm height as this is a tubular tower and
there are few places to bolt on the gamma arm...90', 67', 46' and 25' with
the latter being the crows nest platform).
Or
Can I leave the gamma arm at 90' and rely on an Omega match to tune the
antenna?
Carl
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