Piece of wire as FM antenna

Question

OK, this is really annoying me.

I have an old bedside radio/alarm clock that's been fine for a couple of decades, and which still works perfectly well at night. Its crude FM antenna is a simple strand of PVC-insulated wire dangling from the back of it, with a plastic lump on the end about 1cm long and 4mm diameter which might or might not contain any components moulded in. Recently in the mornings (only), the audio has been seriously compromised by crackly interference.

It seems to make no difference how I arrange the wire antenna, but I get perfect reception whenever I hold the lump on the end of it, and back to crackles when I let go.

Any suggestions as to what's going on, and/or what I might clip on the end of that piece of wire to approximate an adult human being and suppress the interference?

Answer 1

Have you recently plugged in a phone charger close to the radio?

@andy_aka You were spot on. Moving the USB phone charger to the other side of the room has eliminated the interference.

I think the problem as to why it interferes in the morning is due to the completed state of charge of the phone's battery. I can envisage the flyback circuit in the charger generating more EMI when it is supplying a light (or non-existant) load. Fly back circuits are somewhat known for not regulating properly on light loads and they can go into "burst" mode where the whole switching regime is somewhat altered.

There is another possibility that on normal load (charging the depleted battery), the interference peaks in the spectrum don't coincide with the radio channel you are listening to and, a lighter load (battery fully charged) causes the switching (still present) to hit the band you want to listen to. The EMI produced by switching converters extends all the way through the FM radio band and can still cause problems at 1 GHz.

It's worth experimenting to see if removing the phone (when the radio is being interfered with) makes the problem go away i.e. is it the wire from the charger to the phone that is emitting.

Answer 2

The lump at the end of the wire is probably just a weight intended to keep it hanging straighter than it would on its own.

When you grab the end of it, you're coupling your own body capacitance to the end of the antenna, increasing its effective length/area and therefore its sensitivity, producing better "quieting" in the receiver.

One way to replicate this effect would be to take one or more alligator clip jumper leads and connect them to the end of the existing antenna wire (just before it enters the lump) in order to lengthen it. Try to squeeze the jaws so that they cut through the insulation and make contact with the conductor inside.

You can clip the other end of the jumper to something convenient on the wall (e.g., curtains or blinds) in order to raise the antenna up in the air and further improve its sensitivity.

Answer 3

Andy aka (comments to the question) was spot on. The interference source seems to have been a USB phone charger plugged in next to the radio. The charger, not the phone. Moving the charger to the other side of the room has cured the problem.

Which leaves one small puzzle: why interference mornings, not evenings / night? The only thing I can think is that this is the only time that the phone was connected to the charger with its battery fully charged. Evenings it is not connected, late evenings / early night it would be recharging.