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 Lusardi" wrote:

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.

Ok Steve, Please tell us all about why this would be important.
Actually, the Vertical Beam-width of a ANY Slot-line Radar Antenna, is
NOT related to the length of the Antenna, at all, but IS related to the
design of the Slot-line shielding, and slot-line position inside the
shielding. If you look at the Furuno Antenna Spec's you will notice
that ALL the Slot-Line Antennas, have a 25° Vertical Beam-width, and
have had since Furuno first Started importing their products to the
USA, back in the KRA-121 days.

--
Bruce in alaska
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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

Bruce,
The 20 degree fan does prevent the helmsman on my boat from being in the
path of heavy radiation and the sidelobe power outside that 20 degree fan
is -28 DB and that is important to me. The specs for Furuno radiator
XN20AF/6.5 Beamwidth (H) 1.23 degrees, Beamwidth (V) 20 degrees. On the
other hand their S band antennas are 25 degrees like the SN30AF. In point of
fact all the Furuno X band radars are 20 degrees, not just the 6 footer,
which is in line with your statement that all Slot-Line anennas have the
same vertical fan, but the number is 20 for X Band not 25, but I do recall
that antennas by other manufacturers are 30 degrees.

By the way, I did listen to your advice last year and scrapped the 6 KW
Decca and acquired an FR2125BB.
Steve

"Bruce in alaska" wrote in message
...
In article ,
"Steve Lusardi" wrote:


Ok Steve, Please tell us all about why this would be important.
Actually, the Vertical Beam-width of a ANY Slot-line Radar Antenna, is
NOT related to the length of the Antenna, at all, but IS related to the
design of the Slot-line shielding, and slot-line position inside the
shielding. If you look at the Furuno Antenna Spec's you will notice
that ALL the Slot-Line Antennas, have a 25° Vertical Beam-width, and
have had since Furuno first Started importing their products to the
USA, back in the KRA-121 days.

--
Bruce in alaska
add path after fast to reply

In article ,
"Steve Lusardi" wrote:

Bruce,
The 20 degree fan does prevent the helmsman on my boat from being in the
path of heavy radiation and the sidelobe power outside that 20 degree fan
is -28 DB and that is important to me. The specs for Furuno radiator
XN20AF/6.5 Beamwidth (H) 1.23 degrees, Beamwidth (V) 20 degrees. On the
other hand their S band antennas are 25 degrees like the SN30AF. In point of
fact all the Furuno X band radars are 20 degrees, not just the 6 footer,
which is in line with your statement that all Slot-Line anennas have the
same vertical fan, but the number is 20 for X Band not 25, but I do recall
that antennas by other manufacturers are 30 degrees.

By the way, I did listen to your advice last year and scrapped the 6 KW
Decca and acquired an FR2125BB.
Steve

"Bruce in alaska" wrote in message
...
In article ,
"Steve Lusardi" wrote:


Ok Steve, Please tell us all about why this would be important.
Actually, the Vertical Beam-width of a ANY Slot-line Radar Antenna, is
NOT related to the length of the Antenna, at all, but IS related to the
design of the Slot-line shielding, and slot-line position inside the
shielding. If you look at the Furuno Antenna Spec's you will notice
that ALL the Slot-Line Antennas, have a 25° Vertical Beam-width, and
have had since Furuno first Started importing their products to the
USA, back in the KRA-121 days.

--
Bruce in alaska
add path after fast to reply

So your inference is that "IF" a RADAR Antennas Main radiation Lobe
intersects with where a human stands, that this is somehow "Bad",
or dangerous? Hmmm, have you ever done the math, to calculate the
Power Density of the RF Energy at the face of the antenna, at a
distance of twice the Slot-Line Antenna Length, or even at 20 Ft
from the Antenna? Then consider that it is Rotating, and the human
is only in the Radiation Field less than 5% of the time. Add to that,
the fact that for non-ionizing Radiation, it is Average Power, and not
Peak Power, that creates harm to biologics. What is the Average RF Power
Output from this Xband Marine Radar? As in all RF Radiation, Marine
Xband Radar Energy follows the Inverse Square Law.

Your concerns, however irrational, are yours, to deal with, but the Math
doesn't compute in your favor, here.

Bruce in alaska

--
Bruce in alaska
add path after fast to reply

I don't know Bruce, but I guess the average power of this is something just
less than 5 KW on long range and the distance between the radiator and
helmsman is around 10 ft and when rotating at 24 rpm the helmsman is in the
zone 14% of the time, so the average power exposure is still 700 watts at
the Slot Line and 300 Watts at 10 ft. Still, that can't be healthy over
time. Perhaps I did not do the math correctly, but that's what I came up
with.
Steve

"Bruce in alaska" wrote in message
...
In article ,
"Steve Lusardi" wrote:

Bruce,
The 20 degree fan does prevent the helmsman on my boat from being in the
path of heavy radiation and the sidelobe power outside that 20 degree fan
is -28 DB and that is important to me. The specs for Furuno radiator
XN20AF/6.5 Beamwidth (H) 1.23 degrees, Beamwidth (V) 20 degrees. On the
other hand their S band antennas are 25 degrees like the SN30AF. In point
of
fact all the Furuno X band radars are 20 degrees, not just the 6 footer,
which is in line with your statement that all Slot-Line anennas have the
same vertical fan, but the number is 20 for X Band not 25, but I do
recall
that antennas by other manufacturers are 30 degrees.

By the way, I did listen to your advice last year and scrapped the 6 KW
Decca and acquired an FR2125BB.
Steve

"Bruce in alaska" wrote in message
...
In article ,
"Steve Lusardi" wrote:


Ok Steve, Please tell us all about why this would be important.
Actually, the Vertical Beam-width of a ANY Slot-line Radar Antenna, is
NOT related to the length of the Antenna, at all, but IS related to the
design of the Slot-line shielding, and slot-line position inside the
shielding. If you look at the Furuno Antenna Spec's you will notice
that ALL the Slot-Line Antennas, have a 25° Vertical Beam-width, and
have had since Furuno first Started importing their products to the
USA, back in the KRA-121 days.

--
Bruce in alaska
add path after fast to reply

So your inference is that "IF" a RADAR Antennas Main radiation Lobe
intersects with where a human stands, that this is somehow "Bad",
or dangerous? Hmmm, have you ever done the math, to calculate the
Power Density of the RF Energy at the face of the antenna, at a
distance of twice the Slot-Line Antenna Length, or even at 20 Ft
from the Antenna? Then consider that it is Rotating, and the human
is only in the Radiation Field less than 5% of the time. Add to that,
the fact that for non-ionizing Radiation, it is Average Power, and not
Peak Power, that creates harm to biologics. What is the Average RF Power
Output from this Xband Marine Radar? As in all RF Radiation, Marine
Xband Radar Energy follows the Inverse Square Law.

Your concerns, however irrational, are yours, to deal with, but the Math
doesn't compute in your favor, here.

Bruce in alaska

--
Bruce in alaska
add path after fast to reply

In article ,
"Steve Lizard" wrote:

I don't know Bruce, but I guess the average power of this is something just
less than 5 KW on long range and the distance between the radiator and
helmsman is around 10 ft and when rotating at 24 rpm the helmsman is in the
zone 14% of the time, so the average power exposure is still 700 watts at
the Slot Line and 300 Watts at 10 ft. Still, that can't be healthy over
time. Perhaps I did not do the math correctly, but that's what I came up
with.
Steve

Yep, you got that right Steve, You don't "Know"...... The Average Power
is not the Peak Pulse Power, but it IS, your Peak Pulse Power. * the
Pulse Length, * the Pulse Repetition Rate. Typically in a Marine Xband
10Kw Radar the Maximum Average Power is about 20 Watts, now if your 4
Foot slot-Line antenna has a Horizontal 3 db Beam-width of say 3°, then
divide that into 360°, and you are being sweeped less than 1% of the
time, so figure, now it is 2 Watts Average Power. Now figure that
Power is at the Magnetron and not the Antenna Face, so you lose, oh
say 1.5 db in the Waveguide, Rotary Joint, etc, so now your down to say
1.5 Watts, spread over the Face Antenna, which is 4Ft X 4" or 192 Sq
Inches, and 1.5 Watts spread over 192 Sq Inches, would be in the
neighborhood of 8 Milliwatts per Square Inch, as the Output Power
Density for the Typical 10Kw Marine Xband Radar, at the Antenna Face.
Now if one figures the Inverse Square Law into Distance from the Antenna
Face, the Power Density is even LESS significant..... Like I said do the
MATH, and it becomes a Non-Issue.... You get more Non-Ionizing radiation
from the Sun, on a Cloudless day, by FAR......

--
Bruce in alaska
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On Sun, 06 Jan 2008 19:28:27 GMT, Bruce in alaska
wrote:

Now if one figures the Inverse Square Law into Distance from the Antenna
Face, the Power Density is even LESS significant.....

To be pedantic, the inverse square law only applies to point sources,
which the antenna is not at some of the distances under discussion.
At a distance of, say, twenty times the source diameter,or more, the
inverse square law is a fair approximation, as the antenna is more
nearly a point source.

Casady

In article ,
(Richard Casady) wrote:

On Sun, 06 Jan 2008 19:28:27 GMT, Bruce in alaska
wrote:

Now if one figures the Inverse Square Law into Distance from the Antenna
Face, the Power Density is even LESS significant.....

To be pedantic, the inverse square law only applies to point sources,
which the antenna is not at some of the distances under discussion.
At a distance of, say, twenty times the source diameter,or more, the
inverse square law is a fair approximation, as the antenna is more
nearly a point source.

Casady

True, it is the Near Field/Far Field reference.... but all the same,
the RF Power Density of a Marine Xband Radar is not a biologic concern.
Your much more likely to be hurt, by the rotating antenna hitting you
in the head, and falling off the ladder than the RF radiating from the
antenna.

Bruce in alaska

--
Bruce in alaska
add path after fast to reply