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

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

In article .com,
"jmorash" wrote:

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


You might want to look a a 1/2 Wave vertical base loaded antenna
similar to the 1/2 Wave used on sailboat mast tops. Morad makes
one for Vhf. Your real problem will be, when weather causes the
SeaState to be higher than the antenna above the wave troughs.
In this state you range is going to be considerably reduced whenever
the bouy is in a trough. You might just look at designing a
fiberglass antenna mast like 6 Ft long with a counterweight at the
bottom anchorpoint, the electronics package in the middle, and the
antenna at the top, which would give the bouy's antenna 3 Ft elevation
above the SeaState. In a past life, I designed some monitoring bouys
for NOAA, and this is how we solved the antenna problems. Still didn't
solve all the SeaState problems for Heavy Seas, but worked very well
for up to 4 Ft Seas. 900 Mhz isn't exactly the best choice for Maritime
bouys, for just these reasons. You ight also look at the transmission
data protocol, and see if it is compatable with lost packet recovery
techneques, as when the bouy is in moderate to heavy seas, you going
to lose about half the data packets due to SeaState Path losses, which
will seriuously cut into the data thruput your expecting.

Bruce in alaska
--
add a 2 before @

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.