900MHz antenna at sea surface
 

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

900MHz antenna at sea surface
 

"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.

900MHz antenna at sea surface
 

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.

900MHz antenna at sea surface
 

"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.

900MHz antenna at sea surface
 

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

900MHz antenna at sea surface
 

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

900MHz antenna at sea surface
 

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...

900MHz antenna at sea surface
 

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.

900MHz antenna at sea surface
 

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 @

900MHz antenna at sea surface
 

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

900MHz antenna at sea surface
 

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.

900MHz antenna at sea surface
 

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.

900MHz antenna at sea surface
 

"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.

900MHz antenna at sea surface
 

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

900MHz antenna at sea surface
 

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

900MHz antenna at sea surface
 

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.

900MHz antenna at sea surface
 

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?

900MHz antenna at sea surface
 

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?

900MHz antenna at sea surface
 

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

900MHz antenna at sea surface
 

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