Finally someone mentinoed the curvature of the sea...

Answers:

The 1/4 wave ground plane with a flat plate or 3-5 radials [drooping or
not]; the coaxial "sleeve" dipole and the "J" are all pretty much the same
thing / pattern-wise. The "J" and sleeve/coaxial dipole may be easier to
get up higher, however.

The sleeve dipole is a 1/2 wave TOTAL, just like a regular dipole and coax
feed is standard. The "j" is nothing more than an end fed 1/2 wave (where
the 1/2 wave dipole is a center fed half-wave).
These are all fairly simple to make and the "J" shouldn't be a big problem,
even with it's much debated so-called feed-line radiation 'problem'. They
work just fine.
I think the main issues a
1- As Richard correctly points out, sea / earth curvature means there is a
minimum altitude required for a given receiver distance. With this case of
the low "bouy", it will be the larger of determining factors and the other
end will require greater altitude to make up for small changes at the
"bouy".

2- Sea action means an omni gain antenna is contraindicated (not a good
idea) because there will be larger variations in signal strength as it bobs
around. This is because this "gain" is obtained by compressing the
radiation pattern more toward the horizontal and as the "bouy" rocks, you
more quickly get to the points where the signal drops off - above and below
the horizontal where the signal peaks. Trying to get a "stable" platform
with non active means (perhaps a weighted pendulub under the antenna on a
gimble) won't work well due to the accelelrations involved - it may rock
more than the "bouy". I would first try a stabilizing "keel" if this is an
option, to provide a little stability to the "bouy". Depending on its
construction, it may rock moerthan the surface, but be stabilized by being
"anchored" to the water below (which win't be moving as much as teh "bouy"
can.

Just like in real estate, the three most inportant things in antennas are
altitude, altitude altitude...

Hope this helps & good luck
73, Steve, K,9.D;C'I









"jmorash" wrote in message
ups.com...
Richard (& Bob),

I didn't think of simply calculating the distance to the horizon...
oops.

The problem is that this is a mobile device that will spend most of its
time underwater. It will not be particularly stable at the free
surface, hence it will be tough to support a tall antenna mast without
it waving around wildly. The taller it is, the more it's going to move
around.

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?).

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.

"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.

jmorash wrote:

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?).

Impedance matching isn't much of a problem with any of those antennas.
The main issue is feedline decoupling. Current will end up on the
outside of the feedline unless, of course, your antenna is mounted
directly on a metal structure -- in which case the current will end up
on the outside of the metal structure. This current radiates just like
the current in the antenna, and this added radiation can cause a number
of problems. One is that it can modify the pattern and reduce the
radiation toward the horizon. Another is that the current can get back
into the transmitter and other circuitry where it doesn't belong.
Finally, it can effect a change in feedpoint impedance, since the coax
is part of the antenna you probably didn't account for.

The current can originate by two mechanisms, conducted and coupled.
http://eznec.com/Amateur/Articles/Baluns.pdf explains the conducted
mechanism. Current can be coupled from the antenna to the feedline even
if you're using a solid ground plane of moderate diameter, and all the
popular implementations have coupled current to some extent. If you
search for a while, you'll find people both raving about and raving at
J-Poles. I suspect this is at least partially due to the amount and
phase of coupled current they ended up with due to their particular
installation. The amount of coupled current depends on the length and
path of the feedline, as well as the path to ground or some large body.
If I were designing the antenna you describe, I'd use a "current balun"
(common mode choke -- see the balun article) at the feedpoint and about
a quarter wavelength down the line. At that frequency, a good size
ferrite core or two of the right type might provide adequate impedance.

Half wave antennas have relatively little conducted current because the
feedpoint impedance is so high. But feedline current can still exist due
to coupling.

Roy Lewallen, W7EL

In article .com,
"jmorash" wrote:

....

thanks for any suggestions
--Jim Morash


Jim, I've read through the responses you have had and allow me to inject
some real world additions. You see I work for the Navy and we have built
things a lot like what you are talking about and getting the antenna
some height above the water is a requirement. It's actually rather easy
to do. A simple fiberglass mast will do it, counter balancing the added
weight is easy, since there needs to be enough mass below the surface to
balance some sea state conditions


Here is a link to the Newport tracking system
http://www.npt.nuwc.navy.mil/autec/barts01.htm This isn't the one we
did, but if shows you some ideas

The system people I work with developed, use a different buoy system
but works almost the same.

I did some experiments with antennas (900MHz) just as sea level (OK 3
feet above) and could still get a solid connection at about 1 mile, when
I raised them to 6 ft I got a solid connection at 5 miles.

Other considerations.. the antenna mast whips a lot and makes recovery
(in rough water) a little interesting. There is going to be a switch
from the simple ground-plane antenna to a disc-cone due to the danger
the ground radials pose.

We also found that putting a LED based flasher on the bouy a real
good idea (to see it in low light conditions)

--
--------------------------------------------------------
Personal e-mail is the n7bsn but at amsat.org
This posting address is a spam-trap and seldom read
RV and Camping FAQ can be found at
http://www.ralphandellen.us/rv

Thanks Ralph, good to get some actual numbers on use of 900MHz at sea!
I'm not sure if we can quite manage a 6' mast (this is not a buoy, but
a mobile vehicle) but clearly more height is better. We already use a
strobe to see the vehicle at night, it does help a lot.

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?

Thanks Roy! I think I understand baluns quite a bit better now. Seems
like, indeed, a couple ferrite beads could make quite a difference to
my radiation pattern.

This question is for Roy or anyone else who might know the answer: what
tool(s) do I want to try and gauge how good of an antenna I've built?
An SWR meter? Do they make those for 900 MHz? What else?

jmorash wrote:
Thanks Roy! I think I understand baluns quite a bit better now. Seems
like, indeed, a couple ferrite beads could make quite a difference to
my radiation pattern.

This question is for Roy or anyone else who might know the answer: what
tool(s) do I want to try and gauge how good of an antenna I've built?
An SWR meter? Do they make those for 900 MHz? What else?

An SWR meter doesn't measure antenna goodness. The only thing it tells
you is how close the antenna's impedance is to 50 ohms, which has
nothing to do with the important measures of its performance such as
gain and pattern. The best test instrument is a low power transmitter
and field strength meter, one at each end of a simulated communication
link. For the other end of the link, it would be best to use a setup
typical of what you'll actually be using in the field. You'll be able to
get some idea of the antenna's effectiveness with over-ground tests, but
the ultimate test will be the strength of the signal received over water
with the antenna mounted as it will be for the real application. Precise
quantitative measurement isn't trivial at all, but qualitative relative
measurements are fairly easily made.

Roy Lewallen, W7EL

On 2005-11-03, Steve Nosko wrote:

2- Sea action means an omni gain antenna is contraindicated (not a good
idea) because there will be larger variations in signal strength as it bobs
around. This is because this "gain" is obtained by compressing the
radiation pattern more toward the horizontal and as the "bouy" rocks, you


True --- but I would go with the omni directional anyway, for the same
reasons that you mentioned.

Sea action will move the antenna about in the xy plane as well as up and
down. There is more than simple up and down bobbing going on here.
There would be no guarantee that the forward lobe of a directional
antenna would be pointing towards the receiving antenna.

--
Alex/AB2RC