MikeShannon914 said:
So I'm tuning around on my Sony ICF-2002 radio, which, for one, tunes FM starting at 76 mHz. And what do you know, Jack-FM comes in clearly at 78.9 FM. What, CBS discovered that there's actually more un-retrofitted Japanese FM radios in DFW than there are HD radios?
Jack FM is (almost certainly) only transmitting on 100.3 where it should be. But there's a good technical reason why you're hearing them on 78.9. I apologize: this is a bit lengthy:
Your FM radio contains a "local oscillator". When you tune the radio to 100.3, the local oscillator (I'll call it "LO") is tuned to 111.0. The signal from the LO is mixed with the Jack FM signal from the antenna, and you get four outputs:
- The LO, on 111.0
- Jack FM, on 100.3
- The sum of the two, 211.3
- Their difference, 10.7
The four signals are passed to a filter which rejects all but the 10.7 signal. That 10.7MHz signal is amplified, then the FM audio is demodulated and sent to the speakers.
Now, let's say you want to listen to WRR 101.1 instead. When you turn the dial to 101.1, what actually happens is that the LO is moved from 111.0 to 111.8. The four outputs you get:
- The LO, on 111.8
- WRR, on 101.1
- The sum of the two, on 212.9
- Their difference, 10.7
Again, the filter only passes the 10.7 signal, and you get WRR in the speakers.
Next, you tune the radio to 78.9. The LO is moved to 89.6. The difference between the two signals -- the LO and whatever might be out there on 78.9 - is still 10.7, and you would hear whatever was on 78.9. (actually, that would be the video of channel 5's analog signal, except channel 5's analog signal went off the air last June so now there's *nothing* on 78.9...)
But let's look at another piece of math. With the radio still tuned to 78.9 and the LO still on 89.6, let's say the Jack FM signal on 100.3 is still reaching the antenna. (and of course, it is!) We have:
- The LO, on 89.6
- Jack FM, on 100.3
- Their sum, 189.9
- Their difference ---
10.7
The filter, amplifiers, and demodulator can't tell whether this is 10.7 resulting from 100.3-89.6, or 10.7 resulting from 89.6-78.9. It'll play it out the speakers either way. Since Channel 5's analog video signal isn't there anymore to interfere... you hear Jack FM.
I would imagine if you tuned to 79.7, you'd hear WRR. The magic number is 21.4: tune to a frequency 21.4 below the dial position of a local station, and I'll bet you'll hear it. *Most* listeners never hear this because the "magic number" is greater than the width of the FM band. Tune to the lowest frequency on an ordinary radio (88.1) and the spurious response is on 109.5. Of course, there aren't any stations up there! (OK, their are aircraft stations but they're generally not strong enough to interfere with broadcasters & won't demodulate on an FM radio.)
The same thing can happen on AM. The "magic number" is usually either 900 or 910 -- tune to 790 or 800 & you may well hear AM 1700. And on AM, the "magic number" is
smaller than the width of the band -- so
most AM radios can hear this phenomonon.
