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From: William Sommerwerck on 15 Mar 2010 20:06
> I have an clock radio about 30 years old that, when tuning for
> 88.5 FM, for example, by turning the dial, will find that station,
> and then a little higher will be another station, and a little higher
> still will be 88.5 again, often a stronger signal than the first one.

> I'm sure it's not a second station playing the same stuff.

> I know about harmonics on AM MW and short wave, but here the
> tuned frequencies are so close, and also I didn't think there was
> stuff like this involving FM. What is going on?

Most Public Radio stations are at or near the bottom of the FM band.
Although this could be some freaky overload problem, you're most likely
hearing (say) one of the news programs on two different stations. The way to
find out is to listen for their call signs.


From: William Sommerwerck on 15 Mar 2010 20:09
> Well, at 30 years, it sure ain't digitally tuned!

I beg your pardon. I have a GE clock-radio I purchased in 1980. It has full,
real digital tuning, the first of its kind anywhere.


From: Fred McKenzie on 15 Mar 2010 20:33
In article <2l8tp55a6cu8blie6vagqh7pdetf5r464j(a)4ax.com>,
mm <NOPSAMmm2005(a)bigfoot.com> wrote:

> I have an clock radio about 30 years old that, when tuning for 88.5
> FM, for example, by turning the dial, will find that station, and then
> a little higher will be another station, and a little higher still
> will be 88.5 again, often a stronger signal than the first one.
>
> I'm sure it's not a second station playing the same stuff.
>
> I know about harmonics on AM MW and Short Wave, but here the tuned
> frequencies are so close, and also I didn't think there was stuff like
> this involving FM. What is going on?

MM-

See if you can determine the frequency of the second station that
appears to lie between the two occurrences of 88.5.

My guess is it your radio's tuner reaches its minimum frequency before
the end of rotation. Turning it further starts to increase frequency
again until it reaches its end of rotation. The second station is so
close to the minimum frequency that you can't tell if it appears twice
or not.

It is a mechanical effect that I've seen in both capacitor tuned and
inductor tuned systems. The capacitor is fully meshed before it reaches
a stop. The inductor core passes the center of the coil before it
reaches a stop.

Fred
From: tnom on 15 Mar 2010 20:51
On Mon, 15 Mar 2010 17:17:14 -0400, mm <NOPSAMmm2005(a)bigfoot.com>
wrote:

>I have an clock radio about 30 years old that, when tuning for 88.5
>FM, for example, by turning the dial, will find that station, and then
>a little higher will be another station, and a little higher still
>will be 88.5 again, often a stronger signal than the first one.
>
>I'm sure it's not a second station playing the same stuff.
>
>I know about harmonics on AM MW and Short Wave, but here the tuned
>frequencies are so close, and also I didn't think there was stuff like
>this involving FM. What is going on?

It's poor filtering that allows an image of the desired signal. That
image is the intermediate frequency away from the ordinal.
From: tnom on 15 Mar 2010 20:55
On Mon, 15 Mar 2010 20:51:52 -0400, tnom(a)mucks.net wrote:

>On Mon, 15 Mar 2010 17:17:14 -0400, mm <NOPSAMmm2005(a)bigfoot.com>
>wrote:
>
>>I have an clock radio about 30 years old that, when tuning for 88.5
>>FM, for example, by turning the dial, will find that station, and then
>>a little higher will be another station, and a little higher still
>>will be 88.5 again, often a stronger signal than the first one.
>>
>>I'm sure it's not a second station playing the same stuff.
>>
>>I know about harmonics on AM MW and Short Wave, but here the tuned
>>frequencies are so close, and also I didn't think there was stuff like
>>this involving FM. What is going on?
>
>It's poor filtering that allows an image of the desired signal. That
>image is the intermediate frequency away from the ordinal.
correction : (original)

During the mixing process secondary mixes take place that are suppose
to be filtered out.
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