"jmorash" wrote in message
oups.com...
Steve,

I agree that the sleeve dipole, coax-based J, and whip with radials
should all have the same radiation pattern, but I get the impression
that performance might vary quite a bit (specifically the impedance
matching?).

Nope. I disagree. Commercial sleeve-coaxial dipoles are driven w 50 ohm
coax, I have one - works great to extend the range of my 5-W 2 Meter hand
leld for various temporary Ham communication assisted events. If you are
worried about the so called 50 sv 75 ohm mis match.. Believe me, it is
nothing.
Ditto for the gnd plane 37 ohm mismatch (identical SWR non issue). The
antenna length can even be adjusted to get an improvesd SWR and this may not
be where the antenna is purely resistive. This is way, way down the list
of worries for your situation.

"J" s are matched to 50 ohms. I don't believe the much discussed
feed-line radiation is a significant practical problem.

Something I thought of after clicking last time. Look @ the Arrow antenna
version of the "J". It is also much discussed and argued about, but works.
I haven't studied it enough to have a well reasoned opinion, but highly
suspect it has advantages over the standard "J" that are not only
mechanical. He has no 900 MHz version, but it is a rugged construction idea.
I want to get one and measure the "bad" external feed line currents (yes, I
have Fischer clamp-on RF current probes)
http://www.arrowantennas.com

There's also the "sleeve dipole with the cut shield" referred to previously
on this group which looks easy and interesting. .
http://www.ansoft.com/news/articles/04.05_MWJ.pdf


One more thing I just thought of to worry about. Salt spray:
1- Corrosion of the materials used.
2- Geting into critical locations and causing unwanted conduction. I think
you mentined a radome, which reminds me...
3- A plastic radome (Pipe or whatever) will require you to SHORTEN the
radiating elements a bit or you'll resonate too low in frequency. [[ build,
insert THEN measure]]

73, Steve, K,9.D;C'I


As far as increasing height goes, I agree, the thinner antennas will be
easier to raise than the version with radials.

Thanks all for helping me think this through. Now I need to do some
testing.

OK, impedance mismatches not such a big deal. Got it. I am a newbie at
this, as I said.

Good article on the sleeve dipoles, thanks for that. Hadn't seen it
before.

I think I will just paint the antenna with epoxy or urethane or
something to corrosion-proof it. Think a real thin coat of
waterproofing will affect the resonant lengths?