On May 6, 8:52 am, Sonny Hood wrote:
I have a friend that is putting up a 160 vertical consisting of 100 ft
of Rohn 55G and a 2" length of aluminum pipe to be shunt loaded on 160
meters. He is looking for the K factor for the 18" diameter of the
tower and then the K factor of the 2" needed to resonate the tower as
a quarter wave vertical antenna. So far his calculations have come up
with .86 as for the 100 ft. Section of tower. His estimate for the
2" is approx. 17 ft. He is now looking for the shunt point and method
of connection. He wants to know if there is an internet site with
references to this type of antenna system?


http://search.yahoo.com/search?p=shu...p=mss&ei=UTF-8
http://search.yahoo.com/search;_ylt=...fp-t-501&x=wrt

This is another case of almost being able to do it faster by sense of
smell, vs trying to calculate it all out, and hope it works.
You'll probably still have to tweak it..
I'd probably try the shunt bar at about the 75 ft level for starters,
with about a 2-3 foot separation of the shunt wire from the tower.
Put a variable in series at the feed, and see if it will match. If
not,
just move the wire farther or closer to the tower for best match,
while also checking the variable. If that doesn't work, move the
support up or down a bit. You can just move the lower end of the
wire to see which way is getting better. If moving it closer makes a
better match, then go up and move the upper one too if you want it
parallel. It doesn't have to be though..
I'll irk Art, and quote from a book... The ARRL ant book says
to mount the bar at the top if the tower is 75 ft or less..
Thats why I would probably try it at about the 65-75 foot level
to start with. If it didn't match, I'd probably try going up a bit
first,
vs down.
Once you get it matched, note the spacing of the wire, and
then pull it down tight so it can't move in the wind, etc..
You'll probably need a few standoff insulators.
BTW, that would work even better if he top loaded it with some
wires.. Make it a full quarter wave, or even longer...
Two equal length and spaced wires are enough to minimize
radiation from the wires if a 1/4 wave total.. . IE: T..
An electrical 3/8 wave will raise the max current point
up off the ground a bit, and can reduce losses a bit if the
number of radials/ground quality are fairly low.
A large beam on the tower will also add some top loading.
BTW, he could add a second shunt feed at maybe about 20-25
ft and use 80 too.. Adjusting one is pretty much the same as
adjusting a gamma match on a yagi. Just on a bit bigger scale..
MK