In article ,
micky wrote:
How can the same FM station appear at two different spots on the dial?

To the person who complained recently that I was off topic, I'm sorry.
This is not about any repair it would be feasible to make. It's only
about electronics.

Where I live there are two FM radio stations, 88.1 which is only a few
miles away, and 88.5 which is 40 or 50 miles away.

Right now, only my expensive KLM radio plus any car radio gets the
second one well, but I've had some cheap radios that do almost as well.

I lose track of which radios those are, so I'll start tuning at 88.1 and
tune up very gradually. After a period of silence, when I get above
88.5 to what I'd estimate is 88.6 or .7 or .8 I get 88.1 again.

How is that happening? I know about harmonics, but that doesn't apply,
does it?

There are at least three ways in which you can end up with a strong FM station
at two locations on the dial.

(1) As somebody else suggested, it might be a "translator" - a second
transmitter carrying the same program material on a different
channel. Translators are sometimes used to "fill in" a station's
service footprint - e.g. to provide service to an area on the far
side of a mountain from the primary transmitter.

(2) Image. FM receivers are almost always superheterodyne
receiver... they have a local oscillator which is tuned either
above, or below, the station's frequency by a fixed amount (most
commonly 10.7 MHz). "Mixing" of the station frequency and the
local oscillator create an "intermediate frequency" signal at
(e.g.) 10.7 MHz which is then filtered, amplified, and decoded.

This architecture can cause a station to "reappear" on the dial,
if you're tuned away from it by twice the intermediate frequency
(e.g. by 21.4 MHz) - a second "image" of the station appears on
the dial. Good FM receivers have enough selectivity built into
their "front end" to keep this problem to a minimum - the tuner
"filters out" the station at the image frequency efficiently
enough, before mixing with the local oscillator, to keep it from
"reappearing" or interfering with a desired station (image
rejection is often 90-100 decibels, if I recall correctly).

(3) Intermodulation. If you have two strong stations nearby, their
signals can mix (either in the receiver front end, or elsewhere)
and create "spurious" signals located on either side of their true
locations on the dial. These spur signals will often be noisy and
distorted.

What you're describing doesn't sound like an image problem (#2)
because the second "copy" of 88.1 is so close to it on the dial. It
might be intermodulation, or the 88.1 station may have a translator
off in the distance.

Due to recent consolidation of radio-station ownership (both
commercial service and "noncommercial" FM), the signal at 88.7/88.8
might be a formerly-independent station in another market, which has
been "bought up" by the ownership of 88.1 and is now simply
rebroadcasting its signal.