RadioBanter - View Single Post

May 13th 04, 07:36 AM

(JGBOYLES) wrote in message ...
Hi,
If you are talking a 1/2 wave vertical better to feed it in the center.
Why ? A 1/2 wave antenna exhibits very high impedance on its end, but
it can be matched to any feeder by a 1/4 wave shorted stub of twinlead
or coax. This stub may be placed horizontally near ground and the coax
one may be even coiled. The antenna would be a sort of shortwave
J-pole. Its big advantage against a 1/4 wave groundplane is the J-pole
has only modest requirements on RF grounding. Lightning and static
protection are still essential, of course, but it is easy - the
shorted end of the matching stub can be grounded directly.

Could a coil be placed in the
center of the endfed wire to shorten it to say 40'
Yes, but you need to remember the radial requirements, and if you start
sticking coils in the antenna, then the matching starts being a problem.
If you cut the coil into halves, you will see a low (about 5 ohms)
balanced impedance there. The better idea seems to be: two identical
whips as long as possible and a matching coil between them. Another
coil (maybe 1 - 2 turns only) wound around the matching coil is
connected to the feeder. Download MIDLOAD.EXE from Reg's superb
collection http://www.btinternet.com/~g4fgq.regp/page3.html#S301" and
play with it a bit. If the total electrical length is 1/2 wave, the
radial requirements should be modest. Remember the bottom end of the
antenna will be near ground - possibly within reach of uninformed
people - and under high RF voltage !

73 Ivan OK1SIP