On Mon, 14 Feb 2005 05:16:13 GMT, Dan Best
wrote:

A common misconception. If you think about it, you will realize that
the radio energy of flowing off of and being received along the entire
width of the antenna which is much wider than your mast. A healthy
percentage of it is reflected back and to the sides, but most of the
energy just flows right on past the mast. It works the same in reverse
for the reflected energy coming back from a target. Thus, the strength
of the reteurn is reduced, but you are still able to see it. Othereise,
all those boats with mast mounted radars would have a fairly wide cone
to their rear where they pick up nothing.

OK, so it's a function of radome width and radar wavelength then. Is
it fair to say that there is a weaker area of coverage dead ahead and
dead astern, then? The masts must absorb SOME of the signal.

I am thinking that a "watch strategy" for using radar under
unfavourable conditions might be to alter course five degrees or so
every three miles (depending on radar range) or so to confirm the
absence of traffic in such "weak spots", if they exist.

My experience in this is limited (obviously), but many years ago I did
use directional antennas for base-station CB radio of all things and
noticed how signal strengths would fluctuate at various points. I
figure radar is similar.

R.

On Mon, 14 Feb 2005 09:48:35 -0500, rhys wrote:

On Mon, 14 Feb 2005 05:16:13 GMT, Dan Best
wrote:

A common misconception. If you think about it, you will realize that
the radio energy of flowing off of and being received along the entire
width of the antenna which is much wider than your mast. A healthy
percentage of it is reflected back and to the sides, but most of the
energy just flows right on past the mast. It works the same in reverse
for the reflected energy coming back from a target. Thus, the strength
of the reteurn is reduced, but you are still able to see it. Othereise,
all those boats with mast mounted radars would have a fairly wide cone
to their rear where they pick up nothing.

OK, so it's a function of radome width and radar wavelength then. Is
it fair to say that there is a weaker area of coverage dead ahead and
dead astern, then? The masts must absorb SOME of the signal.

I am thinking that a "watch strategy" for using radar under
unfavourable conditions might be to alter course five degrees or so
every three miles (depending on radar range) or so to confirm the
absence of traffic in such "weak spots", if they exist.

My experience in this is limited (obviously), but many years ago I did
use directional antennas for base-station CB radio of all things and
noticed how signal strengths would fluctuate at various points. I
figure radar is similar.

R.

How big is your boat, and in what kinds of waters will your be operating?
My experience with small (39 ft 25,000 lb sailboat) is that your course at
any moment will be off by a lot more than 2 or 3 degrees in any kind of
seaway.

In a bay I might be able to steer a nice straight course. I guess if
"unfavorable conditions" includes calm air and sea with heavy fog, then it
might make sense to snoop around a little with the radar.

But what makes even more sense is to just find out how well the radar can
see straight ahead during good visibility. Then you'll know what you are
dealing with.

Note that I've never used a boat radar, so I can't really comment on the
extent to which it might be blinded by the main mast when mounted on the
mizzen of a ketch.

--Mac

On Wed, 16 Feb 2005 03:54:10 GMT, Mac wrote:

How big is your boat, and in what kinds of waters will your be operating?

34 feet fin keel FG cruiser/racer in Lake Ontario. Boat after that
will likely be steel, 40-45 feet, possibly a ketch, operating
offshore.

My experience with small (39 ft 25,000 lb sailboat) is that your course at
any moment will be off by a lot more than 2 or 3 degrees in any kind of
seaway.

Yes. But I can lash a tiller "tamer" in high summer no wind flat lake
conditions and go several minutes without a tracking error by handheld
GPS...if I don't move around much G. Not an issue as I don't have a
radar, but my scenario posits a foggy, still passagemaking at
dawn/dusk on a calm Great Lake...there's substantial ship traffic from
April to November.

In a bay I might be able to steer a nice straight course. I guess if
"unfavorable conditions" includes calm air and sea with heavy fog, then it
might make sense to snoop around a little with the radar.

That's what I thought...twitch starboard and port 5 to 10 degrees
every 10 minutes or so and check your 6 mile ring or something...see
if it stays empty as you've assumed.

I've since been told that this is unnecessary as the mast even on
something like a Nonsuch treetrunk in front of a radar pole won't
block the signal much.

Thanks,
R.