Radars transmitted power

[Lars_L]

I remember an old statement that you want there to be at least 2000 W transmitted power in the radar for it should have a chance to penetrate through the fog. Broadband radar transmit with a power of 0.165 W, how can you get any echo at all from such signal? If the sky is clear (even if it’s dark), yes, but if there is any fog?

[wellin]

.165 watts? Transmit power for radar? Maybe 1.65Kw is more like it.
What radar has this spec.? Must be wrong.

Andy

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[colemj]

FMCW (broadband) radar uses different frequencies that are not absorbed/deflected by water or water vapor to the extent that pulsed radar frequencies are.

While I don't know the exact transmit power output of FMCW units, it is definitely not in the kW range. These units use a low-power continuous output, whose frequency is modulated - rather than a high kW power millisecond pulse.

Mark

[transmitterdan]

The so-called "broadband" radars are chirp radars. It's not worth going into the math but the radar is always on unlike the more traditional pulse radar. If the pulse radars were rated at their average power as the chirp radars are there would be a lot less difference in the powers.

Take a high end Furuno radar unit.

https://www.furunousa.com/ProductDoc...%201-19-06.pdf

It has a PRF of 500-600Hz in the ranges > 3nM. The pulse length is 800nS. So there are 500-600 pulses of 800nS each every second. That is a total on time of 400-480 microseconds out of every second. So a 6kW peak power radar is actually only transmitting 6000*480e-6=2.88 watts average EIRP. Because the antenna is very narrow beam (<2 degrees) the gain is quite high so the actual power out of the magnetron is much less than 3 watts average.

The chirp guys can't get away with using peak power ratings because they transmit 100% of the time. Even so, the chirp radars don't go as far as the big pulse unit.

[Lars_L]

I have a JRC 1500. It’s quite comparable with the broadband radar. Both are low-end radars and boat has a horizontal beam width of 5.2° and they sends on almost exact same frequency 9445 MHz (as all small radars). But JRC is a pulse radar and have for example a:

Pulse length 0.3 μs
Pulse repeat frequency (PRF) 1200 Hz
Mean transmission time 0.3 μs * 1200 Hz = 0.036%
Peak power 2000 W
Mean power 2000 W * 0.036% = 0.72 W

Even if the pulse radar only transmits during a short time, isn’t it the peek power that gives the signal it’s power to penetrate through the fog? At the same time it does sounds logical that it should be the energy in the signal that’s matters, and then it should be the mean power you compare. Independently ofhow you count, 2 kW was the recommended minimum power, and the broadband radar (which sends with 0.165 W) is significantly below that.

[snotter]

Also, by passing the chirped returns through a Fourier transform, the returns will stack up in the time domain, causing the returns to be much larger in amplitude (assuming I am remembering correctly from my old radar class in grad school in the early 90's). Thus, the low-powered wide-band radars can be much more effective than the non-chirped, higher-powered radars.

[Lars_L]

Originally Posted by snotter

Also, by passing the chirped returns through a Fourier transform, the returns will stack up in the time domain, causing the returns to be much larger in amplitude (assuming I am remembering correctly from my old radar class in grad school in the early 90's). Thus, the low-powered wide-band radars can be much more effective than the non-chirped, higher-powered radars.

If you sample the signal for a short time and with that get a stronger signal sounds technical possible. With that you get advantage of the continues signal. But that advantage has you already taken in the calculation since you count with the mean power.

A stronger echo means a signal with higher energy and the Fourier transformation can’t increase the energy. It’s my only be a help to process the signal.

[transmitterdan]

The main claim to fame for chirp radars in the recreational marine market is close in resolution. They can resolve more detail closer to the boat. So for navigating close quarters in fog or at night they can be very useful. But generally they don't have the range of pulse radars. So for ocean sailing where you want maximum advance notice of approaching weather or ships the pulse radars still seem the better choice. Over time the chirp radar designs will achieve longer range.

[Dockhead]

I don't know the physics, but I have the Navico "4G" [sic] FMCW radar, and it penetrates fog and rain with no problem. The range is similar to my previous 4kW Pathfinder pulse radar -- very reliable and with a lot of resolution out to 12 miles or so; easy targets like the sides of ships out to 20 -- 25 miles.

For some reason, it does not show weather as distinctly as the pulse radar, but I can see squalls.

It has really good close-in resolution -- I can recognize the mast and forestay of my neighbor's boat less than 10 meters away. Is this practically useful? Maybe not all that much. But in general, the radar is entirely satisfactory.

[colemj]

Originally Posted by Dockhead

I can recognize the mast and forestay of my neighbor's boat less than 10 meters away. Is this practically useful?

I should think you would hope not…

Mark

[Dockhead]

Originally Posted by colemj

I should think you would hope not…

Mark

LOL.

[Neptune's Gear]

I'm with Dockhead. Mines a 3G, but range has never been a problem. How far does a relatively slow Yacht need to see on Radar?? The low power consumption and instant on feature of the Broadband radars is what sold it to me. I'm happy :-)

[onefastdaddy]

Low emissions is a big plus for me. With kids aboard I would like something low power and low emissions. If it's even roughly the same as 4kw hd radar why not is my thought. I am still shopping myself.



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[Lars_L]

If you have hade a spotlight with a lamp of 1/6 W, how much do you see on at target that is 10 km away? If you put a lamp of 2000 W in the spotlight, how big is the chance that you see the target that is 10 km away?

You should bee aware that high-emitted power can bee of danger. That’s one of the reasons that you should mount the radar a bit up in the mast. But you should also bee aware of that a low emission gives a weaker echo.

Of the more nuanced statements, I have understood that a pulse radar with 45 cm antenna (2 kW) and a broadband radar provides equally clear images at 3 M. On shorter distances the broadband radar gives a sharper image, an on longer the pulse rader gives the sharper. Now do the boarband radar only existis with 45 cm antenna, but if you take a pulse radar with a 60 cm antenna (4 kW), then will they provides equally clear images at 1.5 M, on longer distances the pulse rader will provide the sharper image.

[transmitterdan]

Lars,

That's not exactly how it works with radar. The new chirp radars are on all the time whereas the traditional ones pulse high power for extremely short intervals. The average power is not thousands of times different as in the spotlight example.

Onefast,

Regardless of choice of radar you should install where people are not in the direct path of the antenna. The radiation should not be a concern for any correctly installed recreational radar.