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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