Sorry Marc, you are not correct, power is not reduced at lower range
selections. The transmitter tube is a magnetron, which effectively is a
resonant cavity within a permanent magnet. It is excited by driving the
cathode severely negative with a high voltage pulse. There are no other
controls. The cavity will resonate only as long as the cathode sees the high
energy. Perhaps you are referring to average power as opposed to peak power.
In which case you would be partially correct, but this is a function of
pulse length and pulse recurrent frequency (range selection). Normally, as
the pulse length decreases, the PRF increases proportionally, so even the
average power doesn't change too much.
Steve

"Marc Heusser" d wrote in
message ...
In article ,
"Steve Lusardi" wrote:

Marc,
Excellent advice, but I would also like to add that power is your freind
and
it is not just for distance, as the radar mount will always be low in
height, but it is for small partially submerged objects that are close
that
when struck could ruin your day. (like containers)

Power is automatically reduced in all radars for closer ranges. And with
the typical 50 microsecond pulse at short range a range of 15 metres
around your antenna is blind anyway.

The key issue is learning to adjust and read the radar, again and again,
under good conditions blocking the radar operator's sight.

There is an excellent simulator to train by the way from
http://www.lightmaster.co.uk/Radar/L..._Simulator_Mk2
I found it when training for the Long Range Certificate, they have a
simulator too for GMDSS DSC VHF, very useful to train distress calls.

If had to start again, I'd first get the simulator, then go through a
course, and only then select and buy the radar. I had the luck to get
good advice from a trainer.

From own experience in training for the radar certificate (Rhine at
Basel, 200 meters wide, commercial ships up to 135 m long crossing at 30
m lateral distance, bridges with support towers, narrow ports etc...),
objects vary wildly in reflection, eg you may easily mistake a goose on
the water or a duck flying by for a small rigid bottom inflatable power
boat WITH a radar reflector (- emergency stop of a 30 metre fire boat
when cruising under radar only - real time simulation of dense fog :-)
or the next navigation mark with radar reflector at times. On the lake
of Zurich we have ferries for cars - they have two parallel faces in
their superstructure - this makes for a great multiple echo, even of
your own boat ... the same can happen with oil storage tanks, regularly
planted trees, a motorway bridge ... All this on a river quietly
streaming. We were trained for failures too, be it rudder broken, radar
failing or motor. It does give you confidence when you know what to do.

I learnt too that you must not have a radar without VHF to call
approaching vessels - on a river to call around bends, or negotiate
crossing port to port or starboard to starboard crossing. AIS goes one
step further as it broadcasts your position, heading, speed, size of the
vessel, destination continously and includes the MMSI, so you can
selectively call approaching vessels. Simrad has a unit for recreational
boats.

A narrow river is much more demanding than open waters in this respect.

Knowing the waters (or at least having a good chart, preferrably
overlaid) is key. A flat coast will be much closer as it seems because
the reflections do not come from the waterline but from the next tree,
house, boulders, ...

Another important fact
about 6' radiators is that the vertical fan is close to 20 degrees, as
opposed to a 4' radiator's 30 degree fan. If, as in my case, mounting the
antenna on an aft bridge, it means that a 6' radiator will not radiate
the
center cockpit and a 4' will. Marc has advised anyone to witness the
radar
you intend to buy before you buy it is imperitive. I helped install two
Kelvin Hughes commercial radars on a super yacht last summer. Both of
these
drove 17" marine LCD screens at a very high resolution. They had 4'
radiators. The ability of these radars to discriminate targets at close
range was appalling.

You can get that on a small boat if you have the same 4' antenna.
(They might have some digital filters that help some more, but you get
most of it.)
Actually on a super yacht I'd get a larger open array (IF I had the
money for the yacht...) - at least 6', preferrably more.

I am not a radar expert, but I do know that everybody
that is anybody buys Furuno for a reason. It is not unusual to see 20,000
hours of transmit time without failure. That says a lot.
Steve

There are other choices, Furuno is not the only one. Swissradar has some
pretty nifty digital filters for example.
(Check eg
http://www.apple.com/downloads/dashb...wissradar.html if
you have a Mac - 3 weather radars map the current rainfall for the whole
of Switzerland, 100 by 200 sm)
Koden (Sitex) might actually try hard and be successful at it.

On a small boat the combinations become important, as you might not have
the space for a separate echo, chart plotter, radar and AIS display.
A good balance of technical specs with space requirements and user
interface is what you are typically after.

My Simrad has a rather good auto setting on tuning/gain/etc, which for
me is a big advantage as I expect other people to operate it with even
less experience than myself. Along with the good chart overlay and a
very flat unit to mount this tipped the scale. It does not have a MARPA
on the other hand, but its trails work well as a substitute. The user
interface is clear but not as nice as Mac OS X or my TomTom GPS for the
street. (BTW their manual at
http://www.simrad-yachting.com/Produ...Fishfinding/CX
44-NavStation/Downloads/
has a good explanation of radar operation and what sorts of displays can
be handy - off centre display is one of them, what is behind you is less
interesting than what's ahead).
And I knew Koden as an electronic engineer, so I did not have any doubts
there. Simrad's key expertise is actually echo sounders but their radars
have a good reputation for a reason too. In yachting they are well known
for their autopilots.

On my small boat I could install a 4' open array antenna, which still is
useful. I would not want anything less. Bear in mind also that larger
antennas often have a smaller vertical beam, which is NOT what you want
on a small boat, because it is rolling and you still want to see.

It might be worthwile to check some of the articles referenced in
http://marinedirectory.ybw.com/reprints/results1.jsp
a search for "radar chart" from 2005-2007 in Category Electronics turns
up this eg
Buying a radar plotter Practical Boat Owner Jul 2006 p76-79 (4.00
pages) etc
before testing and buying equipment.
The reprints are not free but most likely worth the money.

HTH

Marc

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In article ,
"Steve Lusardi" wrote:

Sorry Marc, you are not correct, power is not reduced at lower range
selections. The transmitter tube is a magnetron, which effectively is a
resonant cavity within a permanent magnet. It is excited by driving the
cathode severely negative with a high voltage pulse. There are no other
controls. The cavity will resonate only as long as the cathode sees the high
energy.

Yes, I know that as an MSEE and anyone can verify it watching their
microwave oven that operates on the same principle. I did not want to
make the discussion more technical than necessary. I was referring to
average power to be precise.

Perhaps you are referring to average power as opposed to peak power.
In which case you would be partially correct, but this is a function of
pulse length and pulse recurrent frequency (range selection). Normally, as
the pulse length decreases, the PRF increases proportionally, so even the
average power doesn't change too much.

Actually average power is reduced usually because the pulse length
decreases more than the pulse repetition frequency:

Data from Swissradar JFS364C that I currently have at hand:

Range 2 km 8 km 64 km
Pulse repetition frequency 3000 Hz 2000 Hz 1000 Hz
Pulse length 50ns 150 ns 600 ns
Average emitted power 0.6 W 1.2 W 2.4 W

So for a close range the average power is reduced to one quarter of the
power at long range.

Anyway most people would not care about power, and they do not have to
because the available units take care of that design decision.

Marc

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Agreed Marc. My point that I was trying to make was that not only does the
6' antenna with its narrow beam help disciminate targets, so does higher
transmit power, even at short range, because of the increased echo strength
increasing the signal to noise ratio of the returning echo. This is very
important, especially in rough water, where the receiver needs all the help
it can get to discriminate a small, perhaps radar absorbing target from the
white caps. It is my belief that these two factors are the most important
points to be aware of when in the market for a new unit. To make my point
even stronger, a 25KW FR2125 Furuno will pick up a paper dixie cup at a 100
meters and an FR2115 at 12KW will not. Both radars are identical in every
respect except the transmitter. You may not attach much importance to this
ability, but the fellow I purchased my masthead rig from decided he would
change from a masthead rig to a fractional on his 70' aluminum sloop the
Dance II out of Southhampton in the UK. Right after the conversion, he set
out to Gibralter for some chartering. At 0200 off the coast of Portugal
making 7 knts he hit a partialy sunken shipping container that he never saw
on his 4KW radar with a 4ft radiator. The Dance II was lost. According to
Lloyds, this is a regular occurance and in a single calendar year hundreds
of containers are lost overboard. Maybe this ability is not important on a
lake, river or canal, but in the open ocean it certainly is.
Steve

"Marc Heusser" d wrote in
message ...
In article ,
"Steve Lusardi" wrote:

Sorry Marc, you are not correct, power is not reduced at lower range
selections. The transmitter tube is a magnetron, which effectively is a
resonant cavity within a permanent magnet. It is excited by driving the
cathode severely negative with a high voltage pulse. There are no other
controls. The cavity will resonate only as long as the cathode sees the
high
energy.

Yes, I know that as an MSEE and anyone can verify it watching their
microwave oven that operates on the same principle. I did not want to
make the discussion more technical than necessary. I was referring to
average power to be precise.

Perhaps you are referring to average power as opposed to peak power.
In which case you would be partially correct, but this is a function of
pulse length and pulse recurrent frequency (range selection). Normally,
as
the pulse length decreases, the PRF increases proportionally, so even the
average power doesn't change too much.

Actually average power is reduced usually because the pulse length
decreases more than the pulse repetition frequency:

Data from Swissradar JFS364C that I currently have at hand:

Range 2 km 8 km 64 km
Pulse repetition frequency 3000 Hz 2000 Hz 1000 Hz
Pulse length 50ns 150 ns 600 ns
Average emitted power 0.6 W 1.2 W 2.4 W

So for a close range the average power is reduced to one quarter of the
power at long range.

Anyway most people would not care about power, and they do not have to
because the available units take care of that design decision.

Marc

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In article ,
"Steve Lusardi" wrote:

Agreed Marc. My point that I was trying to make was that not only does the
6' antenna with its narrow beam help disciminate targets, so does higher
transmit power, even at short range, because of the increased echo strength
increasing the signal to noise ratio of the returning echo.

It may - if the noise is coming from an external source. If the noise is
the water reflecting the transmitter's pulse, it probably does not help,
or does it?

This is very
important, especially in rough water, where the receiver needs all the help
it can get to discriminate a small, perhaps radar absorbing target from the
white caps. It is my belief that these two factors are the most important
points to be aware of when in the market for a new unit. To make my point
even stronger, a 25KW FR2125 Furuno will pick up a paper dixie cup at a 100
meters and an FR2115 at 12KW will not. Both radars are identical in every
respect except the transmitter. You may not attach much importance to this
ability, but the fellow I purchased my masthead rig from decided he would
change from a masthead rig to a fractional on his 70' aluminum sloop the
Dance II out of Southhampton in the UK. Right after the conversion, he set
out to Gibralter for some chartering. At 0200 off the coast of Portugal
making 7 knts he hit a partialy sunken shipping container that he never saw
on his 4KW radar with a 4ft radiator. The Dance II was lost. According to
Lloyds, this is a regular occurance and in a single calendar year hundreds
of containers are lost overboard. Maybe this ability is not important on a
lake, river or canal, but in the open ocean it certainly is.

I wonder if he would have caught it only because of a more powerful
radar. Anyway, containers are a major danger, and I would not want to
hit one.
I guess digital signal processing can make more of a difference, but
this is unfortunately only available at a price.

Marc

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In article ,
"Steve Lizard" wrote:

Perhaps you are referring to average power as opposed to peak power.
In which case you would be partially correct, but this is a function of
pulse length and pulse recurrent frequency (range selection). Normally, as
the pulse length decreases, the PRF increases proportionally, so even the
average power doesn't change too much.
Steve

Well, Not Exactly Steve....as the Pulse Length decreases the Peak Power
in each Pulse decreases, and the Effective Radiated Power for each
Pulse, corresponds directly to the Peak Power of the Pulse, as does
distance that pulse can travel, and be detected after reflecting back
from a target. A Marine Radar has much better Target Resolution at
Shorter Ranges, due to higher Pulse Repetition Rates, but if you do the
Math, you will see, even the Average Power is somewhat Higher for the
Longer, but less Frequent, Pulse Lengths. Because Radars use RF, they
are slaves to the Inverse Square Law, and at Short Ranges, it isn't
the Peak Pulse Power, that is the Limiting Factor, for either Resolution,
or Minimum Effective Target Acquisition.

--
Bruce (semiretired powderman & exFCC Field Inspector for Southeastern Alaska)

Bruce Gordon * Debora Gordon R.N. Bruce's Trading Post
P.O. Box EXI Excursion Inlet South
Juneau, Alaska 99850 Excursion Inlet, Alaska 99850
AL7AQ * KL7WJ www.btpost.net www.99850.net