"jmorash" wrote in message
oups.com...
Hi folks,

I've got some background in EE, but know very little about antennas
(though I have a copy of the ARRL Antenna Book I'm reading now), so
please bear with me if these are newbie questions.

I need to communicate with a device floating on the surface of the
ocean, at ranges of several miles or more, using a 900MHz radio link.
Vertically polarized seems to be the way to go, to get true
omnidirectional reception. My shore- or ship-side antenna can easily be
placed 20' or more above the surface of the water, and I can use an
off-the-shelf, moderate gain (5-6 dB) product with a nice fiberglass
radome, etc.

It's the remote side that's the problem. I need to fabricate my own
antenna, rather than buying one, for packaging and waterproofing
reasons. I have a 50 ohm coax transmission line coming out of the
electronics housing; right now I'm just modifying the end of that cable
into a "coaxial dipole" (design I found on the internet). This is a 1/2
wave section of core, with the corresponding 1/2 wave section of shield
folded back down over the feedline, to form a simple dipole.

The coaxial dipole works OK at short ranges (up to a mile or so), but
there must be a better way to do this. In fact, based on the stuff I'm
reading, it seems that a dipole is a "balanced" antenna, but a coax
feed is "unbalanced" ... would I be better off with a whip and small
metal ground plane? How would I match this type of antenna to 50 ohms?
What sort of instrument would I need in order to check the impedance?
Essentially I'm looking for something easy to build without much (if
any) tuning required, doesn't need to be high gain.

It will be tough to get the antenna more than a foot (maybe two) out of
the water, and the platform will be rolling and bobbing around a lot,
so I'm reluctant to use the seawater as a ground plane. Figure the
tuning would change too much. I also expect that in general, a low-gain
antenna on the remote side will be better - generous vertical
beamwidth.

thanks for any suggestions
--Jim Morash


first the dipole as you describe it is probably not what you think it is.
if you fold back 1/2 wave worth of shield you end up with an antenna that is
1 wavelength end to end... this should result in a very high feedpoint
impedance and a poor match to your transmitter. the normal way to do this
is to fold back 1/4 wave worth of shield thus giving you a 1/2 wave end to
end dipole which should have a reasonably low impedance, though not always a
great match to 50 ohms it should work better.

for other options... a simple 1/4 wave ground plane is probably the easiest.
a 1/4 wave vertical wire with 3 or 4 1/4 wave radials is a reasonable
antenna. there are other variations that would be good on 900mhz, one takes
the vertical wire and winds it into a small coil then has another 1/4 wave
above it so you end up with 2 phased 1/4 wave sections giving a bit more
gain at lower angles that should help you. if you can get the ground plane
on a 1 to 2' mast out of the water that should help the range quite a bit,
especially with waves and bobbing around. another option might be a J-Pole,
look that up in the books, it has some small advantages over a 1/4 wave
groundplane i think. you can also do a phased coaxial vertical where you
have multiple 1/4 wave sections with coax phasing for more gain, but also
more complexity to tune.

Dave, you're absolutely right, the existing antenna is 1/2 wave
end-to-end, not 1/2 wave per section.

As for your other suggestions - based on other reading, it seems like
J-Poles are finicky to attach to coax. And I've looked at
phased/stacked dipoles, looks promising but I worry about the narrow
vertical beampattern.

For an elevated ground plane scenario - what's better? Radials, trimmed
to resonant length, bent down 45 degrees or so (I've read since that
this is required for 50 ohm impedance) -- or a simple ground plane, 90
degrees to the axis of the 1/4 wave whip, min. 1/2wave diameter? 900MHz
is a pretty small wavelength, I'd be fine with using a solid 6.5"
ground plane if it would get me better performance than radials.

"jmorash" wrote in message
oups.com...
Dave, you're absolutely right, the existing antenna is 1/2 wave
end-to-end, not 1/2 wave per section.

As for your other suggestions - based on other reading, it seems like
J-Poles are finicky to attach to coax. And I've looked at
phased/stacked dipoles, looks promising but I worry about the narrow
vertical beampattern.

For an elevated ground plane scenario - what's better? Radials, trimmed
to resonant length, bent down 45 degrees or so (I've read since that
this is required for 50 ohm impedance) -- or a simple ground plane, 90
degrees to the axis of the 1/4 wave whip, min. 1/2wave diameter? 900MHz
is a pretty small wavelength, I'd be fine with using a solid 6.5"
ground plane if it would get me better performance than radials.

a solid ground plane would be more efficient, but is of course bigger,
heavier, and more wind load if you are worried about it blowing around. the
3-4 radials bent down gives an easy way to fine tune the impedance... of
course if they aren't stiff enough they can get bent if slapped around in
waves.

That makes sense. Thanks for the tips.

On page 16-23 of the ARRL Antenna Book, 19th ed, Figure 35B shows a
very simple vertical J design. It's open-stub, direct fed from (in
fact, pretty much made out of) 50ohm coax, with a 3/4 wave length of
core conductor and a parallel 1/4 wave wire coming off the shield.
Looks almost too easy... any thoughts on how this would compare to the
1/4 wave whip with ground radials? It's got a larger aperture, but I
don't understand how the grounding is meant to work...

Dave wrote:

a solid ground plane would be more efficient, but is of course bigger,
heavier, and more wind load if you are worried about it blowing around. the
3-4 radials bent down gives an easy way to fine tune the impedance... of
course if they aren't stiff enough they can get bent if slapped around in
waves.

Why would a solid ground plane be more efficient? What's the loss
mechanism in radials?

Roy Lewallen, W7EL