Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 17/12/2009 04:04:51
-
Driving around listening to one of my favourite radio stations, BBC 5 Live, which is broadcast on medium wave (MW), I have noticed that the reception is significantly poorer at night than during the day.
Why?
Chris
-
MW and SW transmissions are affected by the Heaviside Layer that forms part of the ionosphere. This lowers during the day and rises at night.
Radio signals are reflected off this.
At night it allows radio signals to travel much further, thus your radio will be being overloaded by foreign radio transmissions that are interfering with 5 Live.
Try tuning around and you can hear these radio stations. There are thousands.
-
Driving around listening to one of my favourite radio stations, BBC 5 Live, which is broadcast on medium wave (MW), I have noticed that the reception is significantly poorer at night than during the day.
Why?
Chris
Well, in some cases it actually gets better at night. Those who are old enough to remember (not myself you understand) have reported that Radio Luxembourg could only be received at night in the northern parts of the UK. We They could only hear their Clearasil commercials after dark!
I think the AM signals made it all the way from Luxembourg to the Frozen Northern Wastelands after dark because they bounced off the ionosphere, the mumblesphere, or the britneysphere, or something similar.
Anyway, I suspect that your inferior AM reception after dark is not because the Beeb had to turn down the power a bit because they ran out of shillings to put in the meter (how could they?) but more because the signals from other AM stations in far flung locations like Luxembourg are interfering with the Beeb's broadcast.
-
Lovely answers, thank you kindly!
I also gather from discussing this with Dave "kitchen science" Ansell that the solar wind compresses the ionosphere during the day, accounting for its descent. He also suggests that at night there is probably less turbulence (owing to the absence of heating) and this probably also contributes.
Consequently a higher ionosphere means more intrusive inputs from other radio stations, which means more interference.
Chris
-
As this is a scientific forum I would like to say that I recognize the ionosphere and the britneysphere, but the mumblesphere is unknown to me?
-
Although in practice AM is synonymous with Medium and Long Wave broadcasts, I think I'm correct in saying that the fact that the broadcasts are Amplitude Modulation is not actually a factor in the effect we're discussing here; it's just the wavelength that's important re ionospheric effects.
-
Actually, that raises an important point: why is this effect most pronounced with MW and not with, say, FM?
Chris
-
Lol - because of the wavelength!
-
Actually, that raises an important point: why is this effect most pronounced with MW and not with, say, FM?
Chris
Very funny LeeE!
I think Lee is saying that comparing MW and FM can't be done. Contrary to popular belief, FM is not actually an abbreviation for Flippin' Magic. It actually stands for Frequency Modulation. MW is an abbreviation for Medium Wave, which is a popular designation for a range, or band, of radio wavelengths or, if you prefer, radio frequencies.
FM radio is typically transmitted in the VHF (Very High Frequency) band which could also have been called very short wave instead, but wasn't!
However, the modulation technique employed (FM or AM) is independent of the wavelength/frequency of the radio transmission.
-
Indeed, but why should this (FM) wavelength be less vulnerable to the effects that compromise AM at night?
-
It's because the wavelength used by 'FM' radio is shorter than that used for 'MW' radio and is part of the reflection, refraction, diffraction, absorption, polarization and scattering effects you see with any form of electromagnetic radiation, as the wikipedia article on radio propagation states; have a read of:
http://en.wikipedia.org/wiki/Radio_propagation (http://en.wikipedia.org/wiki/Radio_propagation)
Ultimately, it's due to a combination of the right wavelength and the right medium; just as x-rays will pass through your body but light waves won't, and just blue light from the sun is scattered by the atmosphere but the longer wavelengths aren't.
-
AM and FM are both forms of modulation, basically the method how information is carried via a radio wave. The most basic is via morse code ( the beeping you hear on LW, MW and SW radios) where the information is carried by switching the transmitter on and off. If you vary the power output, keeping to a fixed unvarying center frequency, and do it in sympathy with the desired signal ( speech or music) and at the receiving end you feed the information about the changing power to a speaker, you get the classic AM receiver. Any interference on the same frequency will either add or subtract from what is the instantaneous power received, giving rise to the noise and static heard on the radio.
FM radio works by transmitting at a constant power output, and transferring the information by varying the frequency of the transmitter around a centre frequency. At the receiver the signal is limited ( so as to remove any changes due to varying level due to noise and changing signal levels due to moving or local interference) and then the change in frequency is recovered to form the original signal. As most of the noise is AM this reduces it considerably without affecting sound quality.
The major reason that FM is on higher frequencies is that the bandwidth required for a FM signal is a lot larger than that required for AM, which has channel spacings of either 9 or 10 kilohertz, depending on the location in the world of the transmitter, between channels to reduce the possibility of interference and around 5 kilohertz bandwidth. Most broadcast FM has a channel spacing of 200 kilohertz, with a typical transmission occupying around 150 kilohertz of that. That is why most FM broadcasts are on high frequencies above 79MHz. You can operate a FM transmitter on the AM bands, using various methods that reduce the bandwidth required, although these severely limit the quality, and require a more complex receiver to decode them. FM has the ability to offer better sound quality, and stereo sound as well, with relatively simple receivers. To do the same on AM requires more bandwidth than most operators will license, and loses compatibility with the current base of listeners equipment.
AM radio can, depending on conditions and time of day, be heard worldwide relatively easily. FM transmissions can do the same, but tend to be hard to receive due to the reuse of broadcast frequencies by many transmitters world wide. TV signals from New Zealand are often received in the UK for short periods, as those who are older and who remember 405 line TV can attest to.
-
AM is more susceptible than FM to degradation due to solar activity, metal structures such as bridges, etc, although FM does not propagate as well as AM, probably due to the atmosphere itself. That’s why, under a bridge, you’ll get static on AM stations but not on FM stations. At night, the Earth blocks solar interference, allowing AM to broadcast even farther than normal, which was how, while tuning through my lousy car radio in Massachusetts, I picked up a Chicago AM station! I listened to it thinking it was local until it gave the time as one hour earlier than what I knew it to be.
Sorry DD but that's not quite right, and that's the point some of us are trying to get clear here; the the type of modulation makes no difference at all to the propagation of radio waves.
AM is only more susceptible to interference because the effect of the interference is to change the amplitude value i.e. it changes the signal that has been encoded in the amplitude of the waves. With FM, changing the amplitude of the waves has no effect on the frequency modulation, so the correct encoding is maintained, even when there is interference.
The reason why you'll get different coverage, due to solar activity, metal structures and mountains etc. is just due to the wavelengths normally used for AM and FM transmissions; if you broadcast two signals at similar wavelengths, one that was AM and the other FM, you'd get the same propagation for both.
-
...so if FM suffers the same decrement to the amplitude, why doesn't the FM signal fluctuate i.e. become a bit quieter, despite the content remaining clear? Or does it?
Chris
-
...so if FM suffers the same decrement to the amplitude, why doesn't the FM signal fluctuate i.e. become a bit quieter, despite the content remaining clear? Or does it?
Chris
FM does not get quieter because the waveform of the audio signal is encoded by varying the frequency of the RF signal. As long as the receiver can track the variation in frequency, the waveform of the audio signal remains faithful to the original audio signal. This makes FM receivers a bit more sophisticated than AM receivers because FM receivers have to be able to lock on to a variable frequency RF signal. It also explains why a FM receiver can suddenly switch to a different station.
-
So is one of the FM bits set as amplitude i.e. loudness then? Is that how the music / audio volume is determined?
-
Well, there are no "bits" as such. An audio signal can be represented by varying a voltage over time (a waveform). With AM the amplitude of the carrier frequency is modulated so that the envelope of the carrier represents the audio waveform.
With FM, the frequency of the carrier is modulated so that it deviates from the nominal centre frequency of the station. The amount of frequency deviation is used to represent the amplitude of the audio waveform at any point in time. So, if the audio signal was a pure sine wave for example, the frequency of the deviation from centre would have the same frequency as the sine wave and the extent of the deviation from centre would represent the amplitude of the sine wave. The actual amplitude of the signal could increase and decrease over time, but this would have no effect on the actual amplitude of the received audio signal.
-
So is one of the FM bits set as amplitude i.e. loudness then? Is that how the music / audio volume is determined?
Umm... FM bit? Where did that come from?
Are you referring to 'bit' in the digital sense here?
Assuming that you are, the radio waves used in broadcasting are purely non-digital. Digital data can be encoded on to it, but the wave itself, while it is notionally travelling between the transmitter and your receiver, is a continuous wave and not a series of pulses (or bits).
Sound is all about the variation of amplitude, or in other words, loudness. Now although different pitched sounds will have different frequencies associated with them, what is actually changing at all of those different frequencies is the amplitude, or loudness, of the pressure wave. If the amplitude changes at 10kHz you'll hear a relatively high-pitched sound, and if it's changing at only 20Hz you'll hear a low frequency sound, but in the end, it's all just variations in amplitude.
The natural initial method of using radio to carry sound then, was to vary the amplitude of the radio waves, as an analogue of what actually happens with sound waves in air. However, the effect of interference and noise is to change the amplitude of the radio waves, and so as a direct consequence, the sounds encoded on them.
With FM, the radio waves are transmitted at a constant amplitude and instead, as the others have explained, the variation of amplitude in sound is converted to a variation in frequency, away from a baseline frequency. If you're listening to an FM station on 93.5MHz, the actual frequency of the signal that's being broadcast varies from that 93.5MHz baseline. When your FM radio receiver picks up those FM radio waves it converts the variations in frequency, away from the baseline frequency, back into amplitude variations, which it then sends to a loudspeaker or headphones.
Both Amplitude Modulation (AM) and Frequency Modulation (FM) are analogue systems. Both overlay an analogue of the original sound over the top of a baseline frequency and amplitude radio wave.
When you listen to Digital (DAB) radio, what happens is that instead of overlaying an analogue of the sound over the baseline frequency and amplitude radio waves, the sound is first digitised and then the series of ones and zeros are broadcast using FM, with the binary ones and zeros being represented by different variations from the baseline frequency.
One of the (few) advantages of DAB is that because you only need to broadcast two different values i.e. 0 & 1, instead of the continuous ~15kHz bandwidth for analogue radio, the variations from the baseline frequency i.e. the bandwidth, need only be great enough to distinguish between the two values, with the result that you should be able to broadcast many streams of data in the same bandwidth that would be required for a single analogue stream.
-
Terrific answers; thank you for the clearest and best explanations I've yet come across for the basis of FM.
chris