SHUNT FED VERTICAL
WX0B at aol.com
WX0B at aol.com
Fri Oct 7 10:44:43 EDT 1994
I just ran into an interesting antenna problem, and thought that I'd try out
the expertise that resides on the reflector. I hope this is of interest to
the group in general, please send responces directly to me and I'll summarize
for the reflector. Thereby saving bandwidth.
Problem is a raised radial shunt fed tower on 160m which used to match 1:1 at
1830Khz. Now it matches 3:1 (150 ohms min) after I placed a 15 and 20 meter
4el mono banders above the matching box and below the shunt to the tower. In
other words alot of capacitive loading has been added to the tower.
Statistics. Tower is 160 ft tall and has many 11 beams on it.
So electrically it may exceed 1/2 wavelength at 160 meters.
Shunt feed starts at 25 ft and goes up to the 83 ft point on the tower. The
wire is 12 gauge, and is stood off the tower by about 8 inches.
Using an Autek antenna analyzer I measure at the wire ( without )
the omega match connected to it a Z of 200 ohms and and L of 17.8
I really expected to see about 12-20 ohms here. but its alot higher which
leads me to believe that the tower is around 1/2 wavelenght now electrically
and therefor a very high feed impeadance. I was gg to try an LC network next
to match the higher impeadance to 50 ohms but thought I'd let the reflector
have some fun with it.
....and maybe come up with a better solution. I am also going to beef up the
single wire to be multiple wires to reduce R losses.
Any help out there?
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