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
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|