# How bright are radio and other EM signals?

1. Jan 9, 2010

### CosmicVoyager

How "bright" are radio and other EM signals?

Greetings,

I am wondering how "bright" man made EM signals such as radio are. That is, if they were in the visible range, how would they compare to visible light sources? Would they be barely visible? Would they be daytime sky bright in every direction?

I am also wondering how much energy could be harvested from radio, television, cellular, and WiFi signals.

Here is a new device that charges a battery from WiFi signals:
http://www.ohgizmo.com/2010/01/09/ces2010-rca-airnergy-charger-harvests-electricity-from-wifi/

Thanks

2. Jan 9, 2010

### f95toli

Re: How "bright" are radio and other EM signals?

It really depends on the source. Radio waves can be VERY poweful (standing in front of a radar antenna transmisting at full intensity can potentially be a very bad idea).
That said, the radio waves we have around us all the time are usually quite weak meaning the amount of energy you could extract from them is very small (unless you happen to live right next to a big transmitter, that is)

3. Jan 9, 2010

### Staff: Mentor

Re: How "bright" are radio and other EM signals?

The power density of sunlight, on a surface facing broadside to the sun, is typically about 1500 watts per square meter.

The power density of radio waves on the same surface facing towards a 1000-kW UHF TV transmitter 10 km away (for which a 20-cm circular loop antenna will give you a good picture on your TV) is about 0.0008 watts per square meter.

(1000 kW is the maximum allowable "effective radiated power" (ERP) for a commercial UHF TV station in the USA, by the way. This is for digital TV; analog UHF stations could go up to 5000 kW.)

Last edited: Jan 9, 2010
4. Jan 11, 2010

### cragar

Re: How "bright" are radio and other EM signals?

you could make them any color you wanted depending on how fast you are going .

5. Jan 11, 2010

### Andy Resnick

Re: How "bright" are radio and other EM signals?

One minor detail: the 'brightness' of a source is also a measure of how the emitted power is distributed in angle. That is, a 1 mW laser can be much brighter than a 100W light bulb because the laser light is emitted into a small solid angle while the incandescent bulb emits (essentially) isotropically.

6. Jan 11, 2010

### Staff: Mentor

Re: How "bright" are radio and other EM signals?

Good point. In my TV station example, the actual transmitter power output (TPO) might be something like 35 kW. The transmitter's antenna directs most of this power more or less horizontally so as not to waste energy (and the station's power bill) on broadcasting the Simpsons up into outer space or down into the ground near the transmitter tower.

The effective radiated power (ERP), 1000 kW in my example, is the power that the transmitter would have to produce, if the antenna were completely isotropic, in order to produce the same signal strength at a receiver located some distance away horizontally.

7. Jan 12, 2010

### Doug Huffman

Re: How "bright" are radio and other EM signals?

Mr. Bell, with respect, the Solar Constant is defined outside the atmosphere.

Are we considering Radiant Intensity here, W sr^-1?

8. Jan 13, 2010

### johnhb

Re: How "bright" are radio and other EM signals?

Is there a limit as to how intense electromagnetic radiation can be?

9. Jan 13, 2010

### cragar

Re: How "bright" are radio and other EM signals?

yes .

10. Jan 13, 2010

### Doug Huffman

Re: How "bright" are radio and other EM signals?