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I know that [the commercial broadcast] radio spectrum is divided into several chunks and each chunk is assigned to be used by different radio stations. Suppose that radio station S1 is using 93MHz and radio station S2 is using 94MHz. My question is, which radio station's signal strength is higher? As far I know:

  • signal strength is high if wavelength is high
  • wavelength is low if frequency is high
  • So signal strength is low if frequency is high

Is my understanding correct? If so, why would some radio stations pick a higher frequency? Is there a competition among companies to acquire lower frequencies? I am a computer science student and only have a bit of knowledge in signals.

JYelton
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No, your understanding is not correct.

First, just "signal strength" by itself is meaningless. Signal strength where? If you mean at some distant receiver, then yes, frequency is one factor in how strongly a station is received at the same distance and transmitter power. However, there are many such factors and the relationship with frequency is not monotonic. The difference between 93 MHz and 94 MHz will be irrelevant in a practical sense. Sometimes, for FM radios receivers ( when difference between waves frequencies is "low") , there can be what is called a "capture effect" . It is caused by the way how radio receivers synchronize their local oscillator via AFC)

Long wavelengths, like are used by commerical AM (around 1 MHz) are long enough that they refract around the earth to some extent. This doesn't really happen at commercial FM frequencies (around 100 MHz). Different wavelengths also get absorbed, passed, or bounce off of layers in the atmosphere. There is much more to this than lower frequencies magically have more "signal strength", whatever that actually means.

Antonio51
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Olin Lathrop
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Quantized energy of a photon can be described by E=hv, where h is Planck's constant, and v is the frequency of the wave. I believe this is what you are thinking of: If a wave has a higher frequency, it also has higher energy per photon (as The Photon clarified), but not the way you were thinking.

When it comes to radio, which is an electromagnetic (EM) wave, the energy of the carrier wave, such as radio, is a function of how much energy is put into it. The average radio station pumps quite a bit of power into their radio signal, anywhere from a few kW to more than 100kW. Thus, the different frequencies all have whatever power they want, even though in theory the higher frequencies carry more energy.

This idea of "signal strength" depends on a whole suite of factors, and you really need more info to determine what exactly your signal strength is defined as. How far away from the transmitter? What sort of geographical location? Is strength defined as raw power returned, or is the integrity of the data being transmitted also being taken into account?

Jay Greco
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  • Good point. I'll clean it up and clarify, but I think this is the root of the OP's initial understanding/the basis of the question. – Jay Greco Jun 26 '13 at 17:05
  • Removed my -1, but I still think there's room for improvement. For example, "basic physics of waves" to me means classical electromagnetism, which says nothing about the quantization of energy. – The Photon Jun 26 '13 at 17:18
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As far I know:

signal strength is high if wavelength is high

Nope, unrelated.

wavelength is low if frequency is high

Nope, they are inversely proportional. Wavelength is short if frequency is high. (EDIT, if by low you mean 10 cm is lower than 20 cm, then you're correct. Saying it's a low wave length is confusing because you don't usually use low or high for distance but short and long. Low and high is usually reserved for describing time or height not length.)

So signal strength is low if frequency is high

Nope, see first nope.

Signal strength and frequency (or wavelength--they are the same thing really) have nothing to do with each other. Signal strength is due to the transmission effective radiated power, the pass loss to the receiver, then the receiver's antenna+system gain. Factoring into this is the bandwidth used, the modulation type and the channels noise floor (the commercial FM broadcast band has a higher noise floor than just the thermal noise floor due to the extreme power used in that band). However for the geeks out there: the receiver signal strength is usually a measure of the IF amplifier’s amount of saturation through multi-stage amplifiers and is envelop based. This is actually a relative amount and is meaningless by itself until it is calibrated against a known input power level, then the amount of saturation can be compared to a calibrated input power level and thus a "signal strength" in dB can be estimated.

Anyway, among other things path loss can vary over frequency, but the FM broadcast band does not experience path loss differences from the low end to the high end because the band is too small to experience propagation differences.

The FCC tends to try to separate high power signals (commercial) from low power public broadcast signals (College radio, etc.) by putting them away from each other in frequency. That is why you often see the college/public stations in the 89-91 Mhz range and the big nationally syndicated stations at higher frequencies.

user6972
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    Actually, wavelength is low (short) when frequency is high. This is because, as you point out, they are inversely proportional to each other. – Olin Lathrop May 31 '16 at 13:41
  • @OlinLathrop perhaps you're correct. The semantics of low and high for distance is usually used to describe time. This post is 3 years old...:/ – user6972 May 31 '16 at 16:11