As you drive away from your favorite radio station’s transmitter, wouldn’t one expect the volume to slowly fade? Instead, the signal remains apparently unchanged until it suddenly is out of range. But, how is this possible? Why don’t I have to keep bumping up the volume as I move away from the transmitter?
FYI: IIRC, as distance doubles, the dB drops by 6 dB. So, what gives?
I am not a radio engineer, but I believe it has something to do with the fact that your radio receiver has an amplifier built into it, and only gets its information signal from the radio waves, not its power.
If you’re talking about AM, modern AM radios have AGC (automatic gain control) circuits that boost the amplitude of weak signals. Without an AGC circuit, the volume decreases as the signal becomes weaker.
And, FM?
FM is fundamentally different than AM. The FM signal is amplified and clipped to a uniform amplitude level (limiter circuit) before it is fed into the discriminator, which produces the audio. The output level from the discriminator is dependent on the modulation level of the signal, not its amplitude. An FM carrier is modulated by varying the frequency or phase of the carrier wave.
See http://www.ycars.org/EFRA/Module%20B/fmrecvr.htm for a block diagram of an FM receiver.
AM (amplitude modulation) radio uses variations in the amplitude (the height of the wave) in order to transmit information. The amplitude of the wave falls as you get further away from the source, so you need some kind of amplifier to keep the volume at a constant level.
FM (frequency modulation) radio varies the frequency (the time/distance between successive peaks in the wave) to transmit information. The frequency characteristics of a signal do not shift as you get further from the source, like the amplitude does. At least, not nearly as much.
Also, FM waves are “line-of-sight” (they do not curve to follow the earth) so the signal remains reasonably constant until you reach a certain distance from the transmitter. Then, you lose the signal entirely.
The term for this is selectivity. FM receivers automatically tune to the strongest signal on a given channel. Once background noise power slightly exceeds that of the indended signal, your station cuts out.
Similarly, you might hear two FM stations ‘jockey for position’ if you’re between the towers. You’ll never hear both at the same time. This is in contrast with AM where signals readily mix.
That’s a function of the signal’s frequency (VHF), not the modulation used.
I once conducted this experiment on a road trip east from my ancestral hometown of Ames, IA. I found that clear-channel public broadcaster WOI-AM (640 kHz, 50 kW) in Ames was audible and more or less intelligible during daylight hours on a typical car radio (with increasingly annoying bouts of static) all the way to DeKalb, IL, 250 miles almost due east of Ames.
What’s the difference between a signal from a very distant AM station vs. a nearby AM station broadcasting a soft voice or a long silence?
Not really. Although the VHF frequencies used by FM are essentially “line of sight”, they still suffer attenuation like any other electro-magnetic wave and so the strength of the signal falls rapidly with distance.
The AGC circuit adjusts the RF gain based on the average strength of the RF carrier, not the amount of modulation by the audio. Even with no modulation (silence), an AM transmitter produces an RF carrier. When modulated at 100%, 25% of the power is in the lower sideband, 25% of the power is in the upper sideband, and 50% is in the carrier. Changing the modulation index (audio volume) affects the power level in the sidebands but it does not affect the power level in the carrier. If the transmitter produces 1 kW at 100% modulation, 500 W is being used for the carrier. If the modulation is removed, you still have a 500 W carrier, the difference is that the sidebands disappear. The result is that what the AGC circuit measures, RF signal strength, is relatively unaffected by the audio level at the transmitter.
Many radios have an ALC (automatic level control) circuit that attempts to keep the audio level at a constant average value by adjusting the audio gain. This is useful when you are listening to a weak signal that is being clobbered by lightning crashes or other sources of interference.
I apologise for the hijack but this thread seems a relevant place to ask. Would we observe a doppler shift in the signal if we were listening to the radio in a hypothetical car capable of travelling up to the speed of light? How would that sound to the person listening to the radio, in my mind if we were moving towards the signal it would sound like the chipmunks and if we were moving away it would sound like the batteries dying on a cassette player, am I right about that? Also, would this only occur to FM signals as AM signals rely on amplitude rather than frequency?
If there is a doppler shift in radio waves when high speeds are involved is this corrected for in programs like SETI? Do they somehow measure the shift in frequency that has occured and correct for this before looking for a signal?
Again, I apologise for the hijack.
Yes, there is a Doppler shift in the signal. The frequency shift in Hz. is numerically equal to the radial velocity of the receiver relative to the transmitting antenna divided by the wavelength of the signal. For a station transmitting at 1 Mhz and an auto going 60 mph directly away from or toward the transmitting antenna the frequency shift from Doppler effect is about 0.1 Hz. A higher frequency results if you are going toward and a lower frequency if you are going away from the antenna.
I don’t know how SETI does it, however they know the transmitting frequency and they also know the velocity of the spacecraft and the signal has identifying modulation. Once the signal is identified, automatic frequency control will keep the receiver tuned to the transmitted signal frequency.
As David Simmons said, yes, there is Doppler shift.
For SETI, Doppler shift due to Earth’s movement and rotation is known and easy to remove. But we don’t know how fast the other planet is moving, so we need to try all possible speeds (i.e. all speeds within a reasonable range) and for each assumed relative speed, check if anything looks like an artificial signal. It’s a very computation-intensive task. See “De-chirping data” on the SETI@home page for more info.
p.s. much thanks to mks57 for the answer to my hijack.
Oh yes, SETI. I was thinking of that spacecraft that was sent with the plaque one it. :smack: As scr4 stated, they search frequency bands looking for a signal that appears to be artificial. For right now, the exact location of the source isn’t the objective. They are simply looking for evidence of intelligent life and anywhere will do. The fact that there is a Doppler shift is immaterial for that purpose.
You’re half right. FM receivers DO lock on to the strongest signal on a given channel, but this is variously called FM signal capture, the FM capture effect and other similar terms. Selectivity is the ability of the receiver to discriminate between signals on adjacent channels. In other words, if you have a receiver with poor selectivity tuned to 98.5, you might also be able to receive a signal broadcasting on 98.7 or 98.3.
I conduct amateur radio conversations via satellites in LEO (which trip along at 17,000 mph (.0015C)) often. The carrier signal subject to doppler shift but the actual voices in the signal sound quite normal.
It’s only because your radio receiver is much more sensitive to frequency change than your ears. At 0.0015c, a 1GHz signal would shift by about 1.5 MHz, which is significant. The A-string of a violin, at 440 Hz, will shift to 440.7 Hz; this is hardly noticeable, considering the next half note (A#) is 466 Hz.