Topband: Shunt feeding the Skyneedle

[ZR]

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 at lairdtech.com>
To: "'160'" <topband at contesting.com>
Cc: "'Tom W8JI'" <w8ji at 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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