Radio fan

Hi It's very subtle. At first I thought I was imagining it but after several nights its always there. It dosen't bother me at all I'm just curious about it. I do live near a large hill (Foia in portugal 902m) which is covered in transmitters. The fan is AC & has metal blades. The doors and windows are closed so I doubt that it is caused by acoustic reflection.

Thanks Andy

PS Sorry those calculations go completly over my head.

Dear Guest:

Hello and welcome to CR4. Not to worry about those calculations. They're unimportant insofar as what you can do to isolate the cause of this "radio" effect you're seeing.

Try the aluminum foil bit first and see what happens. You shouldn't hear any radio transmissions but, if you do, then try building that home-brew RF choke. Please keep us posted as to the results. We can go from there.

Kind regards,

TV

PS: The radio transmissions you hear - are they of a radio station you can hear on an ordinary AM radio?

It is possible but not likely unless the resonant frequency of the fan blades was exactly the same as the standard IF frequency between 455 and 480KHz

One of two possibilities are more likely:

Sympathetic vibrations

Acoustic Reflections

Both these require someone else in the area (say another house) to have a radio. The sound travels to your fan and is amplified, or reflected to your ear. The angle of some walls and structures will help the transmission of this sound.

I strongly doubt this hypothesis, and here's why:

Consider the wavelengths of the IF frequencies:

λ_n = c/f_n, where

c = speed of light/radio waves: 299.792458x10⁶ meters/sec (983.569x10⁶ feet/second)

f₁ = 455x10³ Hz: λ₁ = 658 meters (2162 feet)

f₂ = 480x10³ Hz: λ₂= 625 meters (2049 feet)

These wavelengths are far, far longer than fan blades which typically range from 3 to 9 inches in length. Too small even for sub-sub-...-sub-harmonics of the IF frequencies for sympathetic vibrations to be audible (assuming they were somehow converted into sound and assuming metal blades would even respond in this fashion. Plastic blades are transparent to radio waves so they're out). The energy in subharmonics drops rapidly the farther away the subharmonic is from the original frequency, furthermore.

The Faraday cage in Post #1 would eliminate this possibility at any rate.

As for acoustic wavelengths, it would make no difference if the fan were on or off. There would be reflections in either case, and the incident sound would have to be audible to produce audible reflections given that the reflections would necessarily have less energy than the incident sound. Moreover, if the fan blades were resonating, they would resonate only at certain frequencies which would dominate the perceived sound coming from the fan.

I agree, not likely to be resonant at IF frequency

I agree the bandwidth of any noise coming from the fan would be strongly attenuated / filtered for many frequencies.

As far as reflections go, it is possible. He only hears it when it is quiet, at low speed. At high speed the fan noise probably probably drowns out the weak sound. During the day other sounds will also drown out the sound. It also depends if the other person's radio is on or not.

There is also the other possibility that induced electrical noise riding on the ground is somehow transmitting into the field or armature of the fan.

Dear Andy,

If you like (just out of curiosity), remove the fan-blade assembly from the motor shaft and re-assemble the fan without the blades. Then see if you can hear the radio at the different fan speeds. Removing the blades eliminates the whirring sound they produce which, at higher speeds, would tend to mask the sound of the radio. But if you don't hear the radio even at the lowest speed, then the effect must have something to do with the blades.

Another source of the sound may be due to rectifier (diode) action in the switch. The RF choke described in Post #1 should eliminate this possibility, however.

Even the simplest AM receiver contains a diode - a device which allows current-flow in one direction only. Diodes need not be actual components, but can result from contact between dissimilar metals or metal in contact with a semi-conducting oxide. This can occur on dirty switch contacts and other places where dissimilar metals/metal oxides may come into contact with each other.

Try removing the blades and see if you can hear the radio effect at all speeds, or only at the slowest speed, or if at all. Please keep us posted as to your results, won't you? This is an interesting situation and, for my part, I'd like to see how it turns out.

Kindest regards,

TV

Hi

I have tried the foil unfortunately the wind noise increased to much. It was really whistling through the holes I tried to quieten things down a bit by adjusting it but could not hear any other sounds so I removed the foil. The sound is very subtle As I'm laying in bed I do not notice anything at first but after a short while I can make out the faint sound of music. One thing I think I should mention is the fan is about 3-4 years old and when I switch it on it's very slow and takes a few minutes to get up to speed only once it is running steady does the radio come through. It didn't used to be like this and now I think about it there were no radio sounds before either. I will try the nails (once I've got some ) and also remove the blades (tonight) Will let you know how I get on.

Thanks Andy

"...takes a few minutes to get up to speed..."

Ha! I had a car like that when I was at University. And it took about that long to stop coughing & gagging once you shut if off! It was soooo humiliating!

Andy, be sure to try the foil thing with the blades removed - but only if you hear the radio sound without them. You might still want to punch holes in the foil in the event the foil muffles the sound.

Here are some links that explain a bit about the foil, RF (radio-frequency) choke and diode/rectifier action in AM radios. Also this link re: diodes/rectifiers. My first radio was a crystal radio I built from scratch as a kid. I wound the tuning coil over a short length of broom handle. My mom was not pleased.

Elvis was quite popular at the time.

TV

Many Years ago the Ledgendary NY Radio Station WOR (50 KW Clear Channel) had it's transmitter Located in Carteret, NJ (Very close to where I lived at the time) when I would go over my friend's house (which was across the street from the XMTR) I would be able to hear WOR coming from my mouth. YES the FILLINGS in my teeth would rectify the signal, and I could hear a what was being broadcast at the time!!!

I've heard of that happening in the presence of strong AM-radio signals.

Did you feel any unusual taste in your mouth at these times? If you've ever tested a 9V battery with your tongue you know what I mean. Did you feel anything like this, but much reduced?

Funny you should ask, It was a VERY LONG TIME AGO, but I sort of remember a taste of like Aluminum Foil...

The taste of aluminum foil comes from electrolysis between the foil and the filling. Saliva is the electrolyte. It generates a small DC current. That's why biting down on a piece of foil hurts a bit. You're sending a current through the filling into the nerve in the tooth.

You hear strong radio signals because of rectifier action between the saliva and the filling (a "wet" diode, as it were). Probably the filling itself (or possibly the saliva, but I doubt it) acts as a tiny speaker. My theory, at any rate.

Ok. We have a slight problem... my wife has taken the fan away from me. I feel like a naughty schoolboy who has had his slingshot taken by a teacher. I will get it back but apparently Ive been spending too much time with it. Don't worry I've plenty of fillings and ive told her i'll be walking around with my mouth open until I pick up a tune or get my fan back.... Shouldn't take long.

Thanks Andy

Whose money bought that fan? He who gots the dough makes the rules.

Alternatively, tell her if she can make radio sounds as well as that fan, you'll spend the time with her. Then give her a tinfoil hat. Should cut down on the noise.