Message: 4
Date: Wed, 15 Jun 2016 20:32:22 -0400
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] OWAs or Fans?
75/80 meter slopers are very different than those on 40 and have a much
greater imbalance.
The reason why I'm considering so many cores is experience with a half
wave, center fed, 80 meter fan which though it's a center fed dipole,
it's highly unbalanced.
http://www.rogerhalstead.com/ham_files/AntennaFeed2.htm When I count
passes through the core it's actually 7 rather than 6 turns.
With that set up, it worked well at 200 W, but as I increased the power,
around 1 KW (maybe a little less) every LED in the system lit up.
Instead of hanging more weight at the feed point, I added another choke
with 5 cores and 6 turns ( wound much tighter) where the feedline
reaches the tower. That completely cured the problem up to full power out.
Because of experience and Jim's tutorial, I'm a bit skeptical of the
really high common mode resistance claims. 5,000-7,000, I can believe.
The photo shows two chokes, with different number of turns to get the
frequency range which is understandable. I only need one octave so one
stack, with 6 or 7 turns looks good. I could wind a single stack tight
wound on 3 cores using RG400 to try on the fan that was a problem, or
"bundle" the LMR400 winding that's there. "Bundling" is easy (no, not
that kind of Bundling) and as that choke is already there, it only
requires lowering the antenna, which is easy with the rope and pulley
arrangement.
With a substantial imbalance on 75, core heating would likely be a
problem with only two, or three cores, so my concern is with the heating
more than the common mode Z although common mode has proven to be a problem.
As I have the one antenna that is easily accessible and an AIM for
measuring all parameters to a finer accuracy than needed, I plan on
using this antenna as a test bed. I definitely will try 3 and 5 cores
with 1/8th in spacing (0.125") for cooling and tightly wound using RG-400.
I use a milling machine to cut slots for the cores in quarter inch thick
Lexan with 1/8th inch spacing between slots. The slots need to be 0.003
to 0.005" over the half inch of the core thickness so they can be
epoxied in place.
My weatherproofing gets complicated as I do not want the chokes enclosed
because of cooling concerns.
## How much stuff are you trying to hang off of just one tower ? Are you
planning on a 80/75/40
sloper plus 4 x more 1/2 wave 40m slopers + a 160m 1/4 wave sloper ?
That’s a lot of stuff
to hang off one tower. The 1/4 wave 160m sloper would possibly screw up the
80m sloper.
## The oem ARRL 5 x sloper array didnt use any baluns at all. Dunno, but I
would consider a 1/2 wave
sloper an unbalanced ant. Dispense with the balun at the feedpoint, and just
feed the unbalanced
slopers with unbalanced.....coax. Then maybe a single CM at the input of
the switching box.
## Both danny and also Bill hyder N3RR are obtaining sky high Z with their
latest CM designs, both using
accurate test gear. N3RRs latest creation is > 20K, and covers 160-80-40m.
RG-303 is also teflon coax,
and last time I looked, was aprx $1.00 per foot.. that was a few years ago,
but even then it was a lot cheaper vs
RG-400. Loads of goods deals on RG-400 on ebay too. Your AIM has too much
stray C to obtain legit Z and RS
readings.
## Another option, simpler, is to just use a rotary dipole on 40m. You wont
have any FB, but you will get 14+ db of FS.
It will also be dead flat across the band. With your soon to be 2 x towers,
why dont you consider a 40m yagi + perhaps a
80m rotary dipole ?? Even the 80m rotary dipole would be dead flat across the
entire band. You would have a lot of fun with
that.
Jim VE7RF
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