Topband: Shunt feeding the Skyneedle

[Carl Braun]

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