Hi guys and
Bill,
You didn't say how the transition was made from the tower section to the
whip.
Back in the 30s it was proven that you want a gradual taper from any
broadcast vertical to a whip. Having a sharp transition is not a good
thing to do, you actually have much less current in the whip as a result
of the sharp transition. In other words if your not using a tapered top
section and the connection to the whip is say from a top plate to a 2
inch whip. You probably have a poor radiator on your hands right now.
So you may actual benefit by going to a good top hat.
A good top hat would be 4 AL tubes spoked out from the top plate with a
wire around the ends.
I would have to model the antenna to tell you how long the tubes would
need to be but my feeling is around 10 feet.
So in actuality you may find the top hat vertical will be a better
performing radiator then the whip if the whip is built as I state above.
And you will avoid the coupling and phase issues of having inverted L
wires on your 4 square system. A good top hat design is preferable to
an Inverted L wire for the same reasons of the sharp transition issue
above.
For those who want to dig into the physics, there are numerous articles
in the literature. Probably one of the best is by G.H. Brown," A
critical study of characteristics of broadcast antennas as affected by
antenna current distribution". IRE Proceedings, Jan 1936. But Jasik and
Johnson, as well as any good radio engineering book should also have
reference to this work.
I think Ken K6HPX, who monitors this reflector can state several other
sources of good data.
Jay, WX0B
BILL STRAW wrote:
>
> A quick question:
>
> I have an elevated 80M 4-square made of Rohn 25. There
> is 15' of tower, then an insulator, then 50' of tower,
> then a 20' whip made of aluminum tubing. Every winter
> one or more of the whips break off (even using triple
> wall tubing). This year 3 broke off. I have to do
> something:
>
> 1) Extend each vertical with 1 or 2 more sections of
> tower. This costs $. I have the tower, but the
> Phillstran, and Grips X 4 towers adds up quick.
> 2) Take the whips off and top load each tower with a
> piece of wire strung to the top of another tower.
> Thus I would have 4 inverted-L's, but the top
> horizontal portion would be exactly horizontal.
>
> Would #2 alter the gain or pattern much? I think it
> would decrease the bandwith, but this antenna is used
> mainly on CW.
>
> Thanks for any opinions.
> Bill, WB0O in North Dakota
>
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--
Jay Terleski
Array Solutions - RME Filters
Phased Arrays - Horizontal and Vertical
RF Switches, Antennas & Towers
www.arraysolutions.com
List Sponsor: Are you thinking about installing a tower this summer? Call us
for information on our fabulous Trylon Titan self-supporting towers - up to
96-feet for less than $2000! at 888-833-3104 <A
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