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10.6 and 10.8

This has to do with the intermediate frequency of most FM radios, which is 10.7 MHz. A superheterodyne FM receiver works by generating an internal, or local oscillator signal exactly 10.7 MHz above the received signal. These two frequencies are mixed together, producing 4 components:

The station's frequency
The local oscillator frequency
The sum of the two frequencies

- and - most importantly, the difference of the two frequencies.

The first three above vary as you tune the radio. But the difference stays fixed no matter where you are tuned, so the radio can make a very good, very selective "intermediate frequency" - of "IF".

A strong FM station differing in frequency close to 10.7 MHz above the received frequency can leak into the radio, and mix right into the radio along with the local oscillator, so you will get both stations in the IF amplifier.

What is lesser known is that there are two methods of generating an IF frequency - high injection where the local oscillator is above the received frequency, and low injection - where the local oscillator is below the received frequency. Both techniques will produce an IF frequency, and if a station is 10.6 or 10.8 MHz below the received frequency, it can leak into the radio and mix with the received frequency as well - regardless of the fact the radio's local oscillator is above. So above and below are just as bad.
 
Thanks, vc. I had an IF issue earlier this summer with WSYN (103.1) that was intermittently taking out 92.3 & 92.5 with no audio and strong reception bars. There was no RDS in that case. Cumulus' engineer from Atlanta called me shortly afterward and the issue was handled.
 
Where I used to live, WFBE 95.1 was about three miles away. WOOD-FM 105.7, with 265 kW, during some tropospheric condtions, from some 90 miles away, could be heard ALL OVER THE DIAL (10.6 MHz). Why no WFBE audio? The late great Ed Rauch, the former chief engineer at WFBE, loved Classical Music and did not use any compression. When WFBE broadcast Classical Music, there were very long passages with very low modulation. So you just heard WOOD-FM with compressed Beautiful Music. Local WWCK-FM 105.5 was a similar distance away, about 3.5 miles, (10.4 MHz) never caused a problem.

WGMZ 107.9, seven miles away, and WWJ-FM 97.1, some 43 miles away, during tropo openings, could be heard all over the dial. Both used Bonneville Beautiful Music, on reel to reel tapes. There was typically a thirty to sixty second difference in the tapes due to spot placement in the automation. The effect was a very long echo.

Usually there would be a combination of the audio of both stations, like RITOIE produces.
 
Also, a radio with a strong oscillator radiation, can cause the same effect with strong signals. Another radio in the immediate vicinity will hear the strong signal tuned on the other radio all over the dial. In this case, there is no or very low modulation on the oscillator. But sometimes the oscillator has slight modulation from the signal tuned in and you can hear it on the other radio when you tune in the oscillator frequency. If you want to try this, it is better to tune the oscillator with an analog radio unless the digital radio steps in 0.05 or 0.1 MHz steps.
 
I once figured out a way to modulate the local oscillator of one of my radios. Since the frequency of the local oscillator is controlled by the voltage input, I spliced an audio cable in series with the power supply. Then I figured out how to amplify the modulated local oscillator by touching the receiver's antenna to an amplified rabbit ears antenna. Connected a non-amplified dipole antenna to the amplifier's coaxial output, and that was how I built a pirate station. The signal went about a couple miles, the power was around a watt, and I became the coolest kid in the neighborhood. I'm sure a lot of engineers today started out by building a pirate station when they were in high school.
 
Schroedingers Cat said:
Also, a radio with a strong oscillator radiation, can cause the same effect with strong signals. Another radio in the immediate vicinity will hear the strong signal tuned on the other radio all over the dial. In this case, there is no or very low modulation on the oscillator. But sometimes the oscillator has slight modulation from the signal tuned in and you can hear it on the other radio when you tune in the oscillator frequency. If you want to try this, it is better to tune the oscillator with an analog radio unless the digital radio steps in 0.05 or 0.1 MHz steps.
Click to expand...

I knew a lot of clock radios (ahem, GE and Soundesign), that were INFAMOUS for this.....
 
As a kid, I was a fan of an FM station on 107.9. My dad had a scanner with a channel programmed to monitor 117.575 (I think). He would always fuss when the scanner would lock down on that channel, but never figured out that I was listening to the radio whenever the scanner stopped working.

Later . . . .
 
If I have one radio tuned to KMPS 94.1, I get a nasty carrier with no audio obliterating KKBW 104.9, due to IF.

That's probably why, if you are on a plane, and they do allow you to use a radio (which is getting rarer and rarer by the day!), KEEP IT BELOW 97 MHZ. Above 97, the IF is above 108, so it could cause interference with the air freqs.

-crainbebo
 
crainbebo said:
If I have one radio tuned to KMPS 94.1, I get a nasty carrier with no audio obliterating KKBW 104.9, due to IF.

That's probably why, if you are on a plane, and they do allow you to use a radio (which is getting rarer and rarer by the day!), KEEP IT BELOW 97 MHZ. Above 97, the IF is above 108, so it could cause interference with the air freqs.

-crainbebo
Click to expand...

What about with DSP radios, like my Tecsun PL-398mp, PL-606 & PL-380?

Another thing I was thinking re: FM vs air .... anyone know how to find out the air frequencies for specific airports? I'd like to find out if there's an airport somewhere that uses like 108.0 or 108.1 MHz in its navigation frequencies ... within the 120 dBµV/m contour of a licensed class C FM station running -10dBc (or is it -4dBc max now?) IBOC. ;)
 
The Insignia portable HD radio does not generate a dead carrier at 10.7 mHz above the frequency it's tuned to. In fact, it does not degrade reception of a weak signal on another radio even with the Insignia radio touching the other radio's antenna. Therefore, there should be no problem listening to that on a plane.
 
DSP doesn't do that. Testing the Grundig G5 on 94.1 (with IF around 104.8-104.9 mHz) and the Grundig G8 on same frequency, there was no carrier. But I did get a carrier using an EXTREMELY crappy clock radio on 94.1. When I am in the car, if the radio is on or off, and on 94.1, there's a dead carrier on 104.8.

The 108-110 mhz area is for VOR beacons. There are a ton on 108.2 mhz, closest to me Lewiston, ID.

-crainbebo
 
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