Same FM station at two nearby positions?

[mm]

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?

 

[Sjouke Burry]

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?

Intermodulation maybe? Or even something oscillating in the radio inter-
modulating the station?
Or a support transmitter in mountainous country put on some hilltop?
Or a far away second transmitter?
In my country I can find the same station on several frequencies,
because one transmitter cant cover the entire country.

 

[mm]

Hi!


If you're in the US, the low end of the FM band is used by public
radio operators. These stations may be low power or intended to serve
a large area (as is sometimes the case with public radio stations).

That's right. This is 88.5, WAMU, American University in Washington,
DC, an NPR station. I'm in Baltimore 35 miles north of DC (though I'm
not sure where the antenna is) and it comes in well almost everywhere
in Baltimore and 20 or 30 miles or more north of here.

The station may in fact operate on multiple frequencies. They may
advertise this fact during station ID.

I've been listening for 10 years. Never any reference to that. In fact
88.1 is a Balt. NPR station, and 88.3 is something else. Also not
much in the way of hills around here, until 50 or 60 miles west

 

[mm]

And if you have one of those older Delco radios, here's something for
you to try one day if you're bored:

http://greyghost.mooo.com/delco/

William

I had 3 GM cars in a row, and I'd like to get back to GM. I heard
about some GM convertible last week and googled for it, only to find
out it's a Pontiac, which afaik, they don't make any more. (And the
Pontiac convertibles I've seen were pretty stripped down). But with
the re-org, maybe Cadillac will make one other than the 2-seater.
I'll be looking for a car about 5 to 7 years old, and I'm glad to know
about this because 4 presets really isn't enough.

(Chrysler has 10 AM and 10 FM. Last I checked for the 80 to 95 cars,
Chrysler had 3 companies making them at one time or another, or maybe
even simultaneiously, even though they all looked alike.)

 

[Allodoxaphobia]

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.

Well, at 30 years, it sure ain't digitally tuned!

The variable capacitor that you tune the radio with is going to full
capacitance (lowest frequency) _before_ it reaches the physical stop.
Ergo, once you go past full capacitance and approach the physical stop,
you will be tuning back _up_ in frequency again -- all be it for just
a short range...

Too, with the age of the radio, 'things' have changed and undoubtedly
the alignment has shifted to exacerbate the effect you see.

Jonesy

 

[PeterD]

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.

NPR stations are well known for playing the same programming relayed
from station to station.

 

[William Sommerwerck]

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.

 

[William Sommerwerck]

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.

 

[Fred McKenzie]

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

 

[Sylvia Else]

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?

Almost sounds as if it's an AM/FM receiver that's running its MW
hetrodyne oscillator as well as that for VHF. You could then appear to
get two signals about 0.5 MHz apart.

Of course, that would imply two distinct oscillators. Was it ever done
that way?

Sylvia.