ON4UNs graph in his book states that my 27m tower with 13' mast and 5 ele 20m
monobander (@ 28m high) is good for about 115 degrees. Other old school
references place it at 140 degrees.
Per my past posts I have a gamma arm at 90' and 25-28 inches from the tower. I
have 380 to 400 ohms at the bottom of the gamma wire to gnd. If I insert my EFJ
160pf air variable I can get the antenna to tune to 60 ohms and X=20 or so.
This morning I was copying Asian stations on 160 and the tuned into the BC
band. Using my 40m vertical array as a reference I switched back and forth
between my shunt fed tower and the array. At 600AM the signal strength on the
40m antennas were stronger. At 1200AM the array and the shunted tower were
equal at 1700AM ESPN radio was a good 30 to 40db stronger on the shunted tower.
Then the sensitivity decreased as I approached 1800 but the tower was still
20db stronger than the 40m antenna when listening to the FT5 pileup.
More experimentation with gamma arm placement today
Carl AG6X
Sent from my iPhone
> On Feb 4, 2014, at 8:13 AM, "ZR" <zr@jeremy.mv.com> wrote:
>
> 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
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2014.0.4259 / Virus Database: 3684/7058 - Release Date: 02/03/14
>
>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|