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.
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). The station may in fact operate on multiple frequencies. They may advertise this fact during station ID.
But sometimes strange things do happen. The other day I was listening to the Delco AM/FM/Cassette radio in a 1988 Buick. This is a digitally tuned radio or "ETR" in GM parlance. I bumped the tuning knob, advancing the frequency from 99.1 to 99.7. Imagine my surprise when the same station--with the same call sign and frequency announcement-- was found to be playing in perfect fidelity!
The surest thing to try is another radio, situated in another area (another room ought to do).
And if you have one of those older Delco radios, here's something for you to try one day if you're bored:
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.
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
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.)
I'm not so sure. Get the call letters for each station when they ID. My guess(tm) is that it's an FM translator or booster:
We have a local religious station that can be heard on multiple FM band frequencies. All of them are translators.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Old radio? I had that happen once when the tuning control was nonlinear. Actually passed over the same tuning points multiple time. Did happen only at the ends of the band, too.
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
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
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Pontiac is winding down through 2010. Their cars will be available for a while yet.
By that point, the GM radios didn't do that any more. I reckon the last ones to support it would been sold sometime in the mid-90s, when that particular style (did you visit the page?) finally faded away. It would have been a very old design at that time!
These weren't bad radios though. The tuner is pretty good (although it's quite sensitive to antenna placement), FM stereo separation is *excellent* and the audio section doesn't lack power output. With the bass slider all the way up, these things sound like they are driving a subwoofer, and they will shake the rearview mirror. (!!!) Delco Electronics could build a good radio when they wanted to.
The two sources I've seen radios from were Mitsubishi and Chrysler themselves. It seems that Chrysler had some electronics production facilities of their own, mostly used for the simpler (AM, AM/FM and AM/FM/Tape) radios.
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.
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.
If the receiver has a 10.7MHz IF and low side LO injection at: 88.5 - 10.7 = 77.8 MHz the image frequency will be: 77.8MHz - 10.7 = 67.1 MHz
If the receiver uses high side LO injection, the LO will be at: 88.5 + 10.7 = 99.2 MHz with an image frequency of: 99.2 + 10.7 = 109.9 MHz
Neither 67.1 or 109.9 can be heard on the FM dial.
There are also spurious responses caused by harmonics of the local oscillator which I can calculate if you want.
I also like the theory that the LO tuning capacitor is at the end of its range and that dial cord and "slide rule dial" are mis-adjusted.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Aw, it sure could be! I'd be in agreement to say that digital tuners weren't commonplace, but things did have them thirty+ years ago.
My dad's '79 Cadillac has a digitally tuned radio. It's very primitive. Each segment is a single element, and the colons/decimal point in each were eschewed in favor of LEDs to indicate those things. It's also got a casssette player, something that I'm sure was a bit of a novelty then as well (my guess is that most people had AM/FM in those, possibly with an
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?
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