AIUI, broadcast radio (e.g. FM and AM stations) can spew impressive amounts of power: tens of thousands of watts in some cases. I guess this makes sense, since they are spraying this signal over a very wide area; if you’ve got your car radio 20 miles away from the broadcast antenna, I’d SWAG that maybe the power flux in your area is measured in microwatts per square meter.
But what about radar, which is pointing its beam in a very specific direction to identify the location and position of objects of interest? How much power is being sent out by one of those? Is it very different depending on the purpose? For example:
It depends on the power going into the antenna, the gain of the antenna, and the distance from the antenna.
I don’t know what these values typically are, but here’s how you calculate the power density in front of the antenna:
P[sub]D[/sub] = P[sub]IN[/sub] G / (4 π R[sup]2[/sup])
P[sub]IN[/sub] = power going into the antenna from the RF amplifier (watts)
G = gain of antenna
R = distance in front of the antenna (meters)
P[sub]D[/sub] = power density at R meters in front of the antenna (watts per square meter)
For example, weather radar can be anywhere from a few hundred watts up to tens of thousands of watts. Planes have radar systems that range from a few hundred watts (for small units carried by small general aviation aircraft) up to several thousand watts for a big passenger jet.
I used to work for the plant that made the AWACS radar, but I worked over on the other side on airborne fighter jet radar. I can’t give specifics (security clearance and all that, though you can probably find some rough numbers via google) but I can tell you that the AWACS was made over on the side of the plant that did ground-based radar. AWACS is essentially a huge ground-based radar mounted onto the back of a plane, with its huge antenna and higher transmitting power.
For marine radar, you can get a tiny unit that you can shove on your little personal boat that might be a few tens of watts. A fishing boat will typically carry a bigger and more powerful unit, maybe a few thousand watts, and something like a big military ship could be up in the tens of thousands of watts range.
ATC radar is typically a couple thousand watts or so.
Those huge early-warning radars used by the military are up in the megawatt range.
A man-portable ground radar might have 1W average power. Fighter jet radars often have average power in the kW range whereas warships can be in the MW range. So that’s about 6 orders of magnitude.
A big consideration is the platform you’re using. The bigger the platform, the more power you can call upon and the easier it is to keep it cool. You can also have more antennas in your array which helps to increase power.
Another consideration is the specific use; An F-22 trying to stay stealthy while detecting enemies may use its radar in low probability of intercept (LPI) mode which might involve transmitting at the lowest power that gives you an N% probability of detecting the enemy at N range. The same aircraft trying to lock a missile onto the enemy through jamming, chaff and decoys might use maximum power. http://www.radartutorial.eu/19.kartei/05.perimeter/karte008.en.html : 1W
https://en.wikipedia.org/wiki/AN/APG-77 : 20kW
Note that the 20kW figure is for peak power, not average power but the duty cycle is long so there is less difference between peak power and average power than usual.
Over-the-horizon radar is probably the kind that uses the most power. AFAIK, its range is longer than any other radar. Because of the R[sup]4[/sup] factor pointed out above, the power requirements go very fast with increased range.
I don’t know how realistic it is but in a fiction book I once read an AWACS fended off an attack by a sole fighter plane by narrowing the radar energy from its dome and ‘beaming’ it at the incoming plane.
It didn’t kill the pilot immediately but he certainly felt it enough for it to put him off the attack and he does die a few hours later from massive radiation poisoning.
The author of that book has some fundamental misunderstandings of both radar and radiation.
The “radiation poisoning” type of radiation is ionizing radiation, which is things like X-rays, gamma rays, etc. It’s the really high stuff on the electromagnetic spectrum, so high in frequency that it can strip the electrons off of atoms and create (hence, “ionizing” radiation). Electromagnetic radiation doesn’t become ionizing at all until you get part way through the ultraviolet part of the spectrum. All of the lower stuff, like visible light, microwaves (which are just higher frequency radio waves), and longer wave radio waves, are all non-ionizing.
Radio waves can cook you to death. After all, that’s how a microwave oven works. But there’s no ionizing radiation, so no radiation poisoning.
As far as realism is concerned, it’s a pretty solid fail.
This. If I buy a radar unit and plug it in, I want to know how many volts and amps I have to feed it. My brief mention of distance and power flux in my OP had to do with comprehending why a radio broadcast antenna might use so much power.
That said, I can see that the range at which you want to detect something factors into how powerful a radar unit you want, and that the power requirement will scale roughly with the fourth power of range.
Agreed, and FWIW, the amount of ionizing radiation required to kill someone within a few hours of exposure (as opposed to their dying within two weeks or so) is on the order of 30 grays. (See, https://en.wikipedia.org/wiki/Acute_radiation_syndrome) Which is a gigantic amount of radiation. The kind of dose you’d get from standing next to an unshielded operating reactor. ‘Why is the air glowing blue?’
I do wonder whether an AWACS has enough power though to fry the electronics in a fighter if the AWACS concentrates all of its power along a narrow beam. Isn’t there a similar kill in the literature from, IIRC, an ECM aircraft like an EF-111 doing something similar? I’m aware of the EF-111’s ‘kill’ it induced during Operation Desert Storm where a hapless Iraqi MiG pilot tried to follow the Spark 'Vark through a dive and ended up a CFIT case.
I’m not an expert by any means, but in a brief survey, I’ve not found any candidates for most powerful radar system, with greatest radiated peak power as the criteria, that exceed the SPY-1 on the Aegis. Which is really surprising to me, given a lot of the candidates I was thinking of (Hen House, its successor the Voronezh, SBX, the BMEWS, PAVE PAWS). Maybe I’m interpreting their specifications wrong. The old AN/FPQ-16 PARCS radar that was part of the Safeguard ABM system, did have a maximum radiated peak power of 14.3 MW. (https://mostlymissiledefense.com/2012/04/12/parcs-cavalier-radar-april-12-2012/)
The “Woodpecker” or Duga OTH radar near Chernobyl, (Brain Scorcher for all of you S.T.A.L.K.E.R. fans) allegedly reached over 10 MW radiated power for some of its pulses. Again, not surprising, considering the scale of the thing. https://en.wikipedia.org/wiki/Duga_radar
As already explained, the low-frequency through very-high-frequency radio spectrum and radar (which uses microwave-spectrum RF) are non-ionizing. That does not mean they aren’t potentially harmful.
50 years ago, RF exposure from LF up through VHF was generally considered non-harmful. There were no widely-taught biological exposure limits to high frequency RF fields. Radio broadcast engineers were never taught about this, because that was the current state of knowledge.
Today RF fields are considered potentially harmful at certain field strengths and exposure durations. All broadcast engineers and ham radio operators now learn about this. In most cases it’s not very risky but there are exposure and field strength guidelines.
OTOH, we’ve always known high power microwave RF (as used by most radars) is very dangerous. The first microwave oven was demonstrated in 1947, the 1,600-watt Ratheon Radarange. I’m sure even in 1947 people knew if you jammed the safety interlock and put your hand inside it, that would hurt.
The US WSR-88D weather radars output 750,000 watts. Each installation has a site survey and a hard interlock so it physically can’t point below a certain elevation: https://www.weather.gov/iwx/wsr_88d
Note there is a big difference between radar average power vs peak power. The momentary or peak power of a radar pulse can be very high, but if the pulse width is narrow the average power might be 1/10th or 1/50th of that.
Some space surveillance and missile detection radars are extremely powerful. The US military AN/FPS-85 reportedly emits 32 megawatts peak RF power. However it is elevated upward and is phased array so the beam is electronically steered, not mechanically steered like a rotating dish. The steerable beam is typically rapidly scanning over a wide area: https://en.wikipedia.org/wiki/Eglin_AFB_Site_C-6