View Full Version : Fluctuating Radio Frequency
06-08-2001, 03:21 AM
This is just something that has been bugging me for a while. I listen regularly to one FM radio station at home. The transmission frequency is 101.90 MHz. However it is very rarely that. It seems to fluctuate between 101.75 and 102.00 MHz. I am wondering why this is, what would affect the radio frequency.
Now for a bit of geography - We are only 5 km from the transmitter in a straight line, however there is a huge block of granite (extinct volcano) inbetween so we generally have to bounce the signal of a nearby mountain. The antenna is underneath the veranda, so is somewhat wind protected.
Because of bouncing the signal, I would expect the frequency to change somwhat, but I think that the signal should be consistantly on one side of the main frequency or the other, not oscillating. DOes anyone have any ideas?
Thank you for any posts.
06-08-2001, 06:08 AM
You haven't mentioned what type of receiver you are using. In your position, I would supect the receiver more than the transmitter. Normal home receivers are not reliable as a frequency standard. The oscillators can wander, and generally do. If you have one with a mechanical dial, then temperature and humidity variations can change the tuning components enough that the mechanical scale is incorrect. On receivers with a digital indicator, there are several possibilities. One is that the tuning knob directly controls the receiver frequency and the display is driven by a frequency counter. In this case, temperature changes can cause the counter to be inaccurate. The other possibility is that the display is driven by a digital controller that also generates a control voltage for the oscillator. In this case, the controller shows the frequency and for some reason (heat, humidity, etc,) the oscillator doesn't hit the correct frequency, so that you have to tell the controller to generate a different frequency in order for the oscillator to hit the right one. The final possibility is that you have a PLL controlled receiver, and the reference oscillator frequency is not stable (again, heat or maybe vibration.)
Forget about the reflection causing the frequency changes. That requires either a moving source or a moving reflector, or a moving receiver. Broadcast antennas are normally pretty stable, and I can't see your mountain moving (or vibrating) to such an extreme that it could change the frequency as much as you indicated. You may also have noticed that when driving your car you don't have to retune your radio when you stop or when you speed up again.
BTW: This is how police radar guns work - they shoot radio waves at you and measure the frequency change of the reflected signal. The change is so small that they have to use some very sophisticated methods to measure it. The frequency changes you give would equate to very high speeds indeed. Thousands of miles an hour or more, I would think. My physics books are at home (posting from work) so I don't have the doppeler equations handy to figure it.
06-10-2001, 01:33 AM
Thanks for that Mort Furd,
I know that it is this particular reciever. We have several radios in the house, and it is only the most expensive and dedicated one that this happens to. It also has the largest antenna. Our house is climate controlled (or at least in winter like now) and so that mainly leaves the humidity affecting the circuits. It is a digital reciever and tuner with heaps of options (most of which I don't use).
So again, thanks. The reason I was asking if was the radio frequencies was because this is the most sophisticated radio we have and we can tune it to a much higher degree, so maybe the other radios aren't as sensitive. Also, it only seems to happen to the one station.
06-10-2001, 02:10 AM
The effect of speed is so slight in this situation thatit wouldn't be a problem. I would think it the fault of the feed line u are using. If u have a semi expensive setup, check the coax cable leading out, that can cause some strange problems.
From ham radio user:
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