Question for electrical guru's regarding RADAR power output

AK Angler

OK. I've never had the inclination to do much research on RADAR systems until now (the Cape Cruiser I recently purchased came with a Raymarine electronics package that includes RADAR). So, suffice it to say, I don't really know anything about RADAR, but since I now own such a system, I figured it was time to learn. So, I start down that path and, blammo! I run into information that I just can't wrap my head around.

How, exactly, do radomes have such high output power ratings, yet such low input power requirements?

For instance, the published specs for the Garmin GMR-18 radome indicates a transmit power of 4000 watts, using only 33.5 watts of input power. What the...? How can this be? It seems to defy the laws of physics. (i.e., power out can't exceed power in... can it?)

I'm sure it's something obvious, but what am I missing here? Where do they get all that 'extra' power?

(BTW, I would love to find such a device. I would immediately hook it to the electrical meter on my house while I decide what else to use it for!!!)

potter water · Posted: Wed Dec 04, 2013 7:45 pm Post subject:

They are advertising "effective radiated power," meaning the antenna design creates a beam that is effectively 4000 watts. When I was in the television station business, our transmitter was a 50 thousand watt thing, but effective radiated power was almost 300 thousand watts because of the antenna design. Antenna designs can up the "gain" of radiated power, even with your marine radio, you will see antennas that have 3db of gain or even more. So, the best way to get your arms around radar is to just read the manual and enjoy. You don't need the EE degree with radar specialty to use radar effectively.

Sometimes it is best to just accept that most technical things we use, including radar operate by magic. It is less stressful.

potter water · Posted: Wed Dec 04, 2013 7:49 pm Post subject:

I guess I could add that radar is a pulsed system. So you could think of that 33 watts charging up a circuit over a short period of time then, firing off a big old bunch of saved up watts at one time. Each of the pulses that come off the radar can thereby be more powerful as a periodic pulse than if they were just running a continuous transmitter. THAT and antenna design gets you effective radiated power.

thataway · Posted: Wed Dec 04, 2013 8:04 pm Post subject:

Correct--the energy is sent in one very small beam--for the 18 it is about 5 degrees, for the 24 about 3.6 degrees, and the big 6' 12,000 watt, it is 1 degree. The second is that the pulse is in milli seconds. So focused beams in a fraction of second at a very high frequency-and that takes a small amount of power.

AK Angler · Posted: Wed Dec 04, 2013 8:43 pm Post subject:

Ahhhh.... magic. I knew it had to be something simple!

But seriously... Using an 'effective' power rating makes sense. Kind of like how music would effectively sound louder when standing directly in front of a speaker versus when standing off to the side. Or how a focused flashlight would better illuminate a small area some distance away than a bare unfocused light bulb of the same wattage would at the same distance. I can wrap my head around that. Thanks for the explanation guys.