Interference with FM radio, goes away when my hand is near.

If it is in your residence, shut off the breaker to see if it stops on the portable.

Greg

There is an older aviation navigation system, VOR, that runs in the range just above the FM broadcast band (108-118 MHz). It works by broadcasting a couple of tones at a low frequency, which, when compared, gives aircraft a compass heading of their location relative to the airport that it's located at.

It could be an RF Image. The usual IF for a FM receiver is 10.7 MHz, and the local oscillator is usually above the frequency, so a super-heterodyne receiver can also be sensitive to signals 21.4 MHz above the tuned station.

It might be a nearby airport, or it could be at a farther distance than you would expect, especially now with the sunspots kicking up and strange radio propagation effects can occur.

(The interference can go the other way, too, where the radio's local oscillator will leak out and jam a VOR signal 10.7 MHz above the tuned station's frequency. This is the main reason that personal electronics are supposed to be turned off on aircraft at various times. Now, largely, an obselete problem).

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

micky wrote in news: snipped-for-privacy@4ax.com:

Your arm acts as an additional antenna, capacitively coupled. This suggests that the orientation and/or antenna size is bad for your location. Attach a wire of about 2 meter to the antenna, and play with the orientation. Even better is to find the interference source, any modern switching supply(computer, charger,GFL light,new fl light,senseo coffee) is a prime candidate, even if you think the divice is switched off, a lot of them still have an active computer chip and supply inside.

Thanks. I will try that. but it's pretty quiet here in the middle of the night**

Anyone have any idea how to duplicate the effect of my fist or hand near the radio? That gets rid of the noise. I'm lying on a sheet and mattress -- I don't think I'm grounded. A bag hung in front of the radio filled with cooked chicken? Raw chicken? Metal scraps? I can try the last one, but I'm not made of metal. Paperback books?

** only the furnace, fridge, and one outdoor IR triggered light. None of them go on for a minute then off for a minute and back on. I can tell t he furnace is not on all the time because the hot air vent is right near me when the I'm in bed.

FM is less vulnerable than AM, to impulse noise (static).

It's still vulnerable to both broadband noise, and narrowband noise, if the strength of the noise signal comes close to (or becomes greater than) the strength of the desired signal.

Most FM radios are also vulnerable to "desense" - a strong-enough signal of any sort, located anywhere in the radio's RF passband (which can be much wider than the FM band that you can tune) can saturate the first stage or two of the radio's RF section. This has the effect of making the radio less sensitive to the signal you want to hear - it's as if the desired signal "fades out".

The fact that the interference fades out when you bring your hand near the radio, makes me suspect the following:

(1) When the radio is being used normally, it's not actually getting a strong signal from the station you want to hear. (2) The interfering signal isn't actually that strong, either - but when it gets into the radio's RF section it's at least as strong as the signal you want to receive. (3) When you hold your hand near the radio, your body is acting as an auxiliary antenna - it's picking up and conducting the radio station's signal on your skin surface. The signal is then being "coupled" into the radio's antenna from your hand - capacitive coupling. (4) Your body is not acting as an efficient antenna for the interfering signal, which suggests that the interference might *not* be getting into the radio via the antenna, but via another pathway (possibly the power line).

This is consistent with the problem being one which affects only weak signals. 30 - 40 miles is a good distance for FM (it's probably "medium fringe"), and stations such as NPR are likely to be smaller (lower power) than big commercial stations.

Modern buildings often have a lot of metal in their structure, reflective metal coatings on their windows, etc. - which tends to weaken the FM signals pretty badly. I'm somewhat surprised that you can get a decent-quality FM signal indoors from 30-40 miles away.

I think there are three approaches you can take to identifying the problem:

(1) Locate the source of the interference, and either shut it down or filter it at the source. Switching off appliances and breakers, as has been suggested, is a great way to do this... probably the easiest. If the source is outside your house, you may need to go on a "T-hunt" (transmitter hunt). A local ham-radio operator might be able to help you with this... T-hunting is one aspect of the hobby, and quite a few hams have directional antennas and the skills needed to use them. If you can find the source, but can't afford to shut it down entirely, then connecting a plug-in RFI filter between the appliance and the power line would probably help a lot. (2) Filter the interference at your radio. The behavior you are seeing makes me suspect that it may be getting into the radio via the AC power cord. Adding a good plug-in RFI filter at the wall outlet might choke off the interference before it gets into the radio. (3) Try to improve the strength of the radio signal that's getting to your radio. Ideally, put up a simple outdoor antenna, and run a coax cable to the radio. If the radio doesn't have an external-antenna jack, you might be able to connect the coax to the radio with a simple clip-lead jumper (from the coax center to the radio's existing antenna), or connect two wires to the center and braid of the coax and stretch these in opposite directions near the radio.

--
Dave Platt                                    AE6EO 
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Is the interference a whine or a buzz? Does it change pitch as well as volume , when you move close to it, like a Therimin "musical" instrument