I can't think of a more difficult thing to do *properly*
than to end feed a half wave. There are a good may
half-baked feed systems around, but the only reliable and
simple system is a resonant tuning unit at the base of the
vertical with the system worked against a ground system. A
stub would be a good second choice, but a single band affair
and you would be faced with some radiation from the stub.
A much better solution for multiple bands would be a trap
vertical mounted as high as possible worked against a ground
system of drooping radials.
A thick element could have an impedance as low as several
hundred ohms, a very thin half wave element maybe 3000-4000
ohms. The problem is the reactance off frequency, and how to
compensate. It will never be multiband without traps or
complex stub decoupling systems on the element.
Trying to use a decoupling sleeve is almost certainly a
waste of time. The sleeve ID has to be substantially larger
than the internal coax's outside. Folding braid back is a
common method at HF, but the decoupling performance is
horrible. It's almost like no choke at all.
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
Good luck also on building a broadband transformer that
would match 50 to 4000 ohms. That's another almost
impossible task.
If you can put a tuner at the vertical base and work the
vertical against a modest ground, that's the best solution.
An elevated trap vertical worked against multiple drooped
radials is the easiest, most reliable, and best performing
solution if you don't want to use a tuner at the vertical
base and want multiple bands.
73 Tom
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