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

"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.

Hi Jim

Along with comments from Dave I thought it might be worth mentioning a
few more points.

Think of 900MHz as line of sight and any obstruction is an issue to plan
for. It is worth keeping the antenna gain low because of wave slap
action tilting the structure and radiation pattern. Also consider
mounting it as high as you can off the water surface, say a few metres.
Hopefully this will allow for the device rolling into a wave trough
whilst still giving you reasonable range. The best analogy here is to
replace the antenna with a lamp and se how far you can see it from.

If you end up needing high antenna gain consider gimballing the antenna
to keep it oriented properly. I'd stick to providing the gain at the
ship/shore end though.

If multipath reflections and cancellations are a problem (and honestly I
dont think they are in a water/wave environment - more an issue between
trees and buildings) then consider a horizontal antenna like a halo,
bent dipole or "3 leaf clover". The plus with a horizontal is that the
antenna aperture is wide rather than high so phase cancellations are
much less of a problem.

You should also consider that what ever data you are sending might be
lost periodically so whatever it is make sure there is a forward error
correction or retry mechanism where needed.

Cheers Bob VK2YQA

jmorash wrote:


thanks for any suggestions
--Jim Morash

Jim Morash wrote:
"It will be tough to get the antenna more than a foot (maybe two) out of
the water."

It may be worth the effort for the sake of your hardware.

Distance to the horizon in miles is the square root of teice the antenna
elevation in feet.
Two feet elevation puts the horizon at two miles. 20 feet slevation on
the other end of your path extends it by 6 miles. Low obstructions may
block a very low path at times as 900 MHz requires a line-of-sight path.
You might anchor a buoy to hold your antenna at a convenient height.

A 1/2-wave coaxial dipole works about as well as any VHF or UHF antenna
on a boat or float. Commercial versions use 1/4-wave of rigid tubing as
a skirt over the coax feedline. This tubing is the lower half of the
antenna. The upper half is a 1/4-wave whip mounted on the lower half.
There are no radials to get in the way or poke out any eyes.

Best regards, Richard Harrison, KB5WZI

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...

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.

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 @

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