A computational problem here... 3/4 wave at 28 MHz will be about 24
feet, definitely too tall to run down a road under power lines. A deadly
hazard. But there's no need for that bottom quarter wave of a J, its
simply a quarter wave line transformer to match the coax to the high
impedance of the end of the half wave radiator that would be only 16
feet for 10 meters. Power lines over roads have to be 21 feet up but
telephone an cable aren't that tall. Its not good to run an antenna over
13 or 14 feet tall.
You can match that 16 feet of 10 m vertical with a tuner or a coil. that
coil needs to be about a quarter wave of wire. That 16 feet could be
shortened by wadding up a couple feet at its middle. Or better wadding
up wire at the top and bottom of the radiator because its the maximum
current region at the middle that does the greatest part of the
radiation.
The key to working any antenna without a decent ground plane (whether 15
miles of wire dangling from an airplane or 14" of wire at 440 MHz) is to
match it as an end fed half wave which means high impedance.
My big objection to a J (and that's what it was called in the 30s when
it was first used as a straightened end fed zep) is that there's no
radials to decouple the feedline from the antenna so in home situations
where the J is elevated a ways, the antenna acts to end feed the outside
of the coax and so end up with a very high radiation angle and poor
performance compared to a properly isolated antenna and feed line.
73, Jerry, K0CQ
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