[TowerTalk] OWAs or Fans?

[Jim Thomson]

Message: 4
Date: Wed, 15 Jun 2016 20:32:22 -0400
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk at tm.net>
To: towertalk at 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