Re: Interesting find while scanning AM dial (long technical response)
> WNTS, the new Spanish station here in Indianapolis, has been
> on 1590 AM forEVER...but I found a station at 690 AM that
> called WNTS - Beech Grove. I wonder if this is a sign of
> things to come, or just a funny little fluke tonight/this
> morning.
This is an "image response" - a figment of your receiver's imagination.
Long explanation:
When you turn the knob (/push the buttons/whatever) to tune your radio to 1590, what actually happens is that a "local oscillator" circuit inside the radio is tuned to 2040. It creates a "dead air" signal on that frequency.
This signal on 2040 is fed to one input of a "mixer" circuit. The other input is the AM signals from the antenna.
For any given input signal to the mixer, you get four outputs:
- The original input signal (in WNTS' case, 1590)
- The local oscillator signal (in this case, on 2040)
- The sum of the two (1590+2040 = 3630)
- The difference of the two (2040-1590 = 450)
The output of the mixer is sent to an "intermediate frequency amplifier", a circuit tuned to 450KHz which provides most of your radio's amplification. Of the four outputs from the mixer, only the last one, on 450KHz, is amplified. The rest are rejected.
Now, there are other signals going into that mixer. For example, WIBC on 1070 is going in, yielding:
- The original input signal (1070)
- The local oscillator (again, as long as your dial says 1590 this oscillator is on 2040)
- The sum of the two (1070+2040 = 3110)
- The difference of the two (2040-1070 = 970)
None of these are anywhere near 450, so they'll all be rejected, and you won't hear WIBC when you're trying to listen to WNTS.
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Now, let's say you decide to listen to WXNT 1430 instead. Your local oscillator is retuned to 1880KHz. The four outputs you get become:
- The original input signal (in WXNT's case, 1430)
- The local oscillator signal (in this case, on 1880)
- The sum of the two (1430+1880 = 3310)
- The difference of the two (1880-1430 = 450)
This signal is again passed to the intermediate frequency amplifer, which again passes only the 450KHz signal. And you get WXNT instead of WNTS.
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Now, let's try one more. Let's tune to 690KHz. The local oscillator is tuned to 1140. 1140-690 = 450, so if there was a station on 690 you'd hear it. But WNTS's 1590 signal is also still getting into the mixer. And you get:
- The original input signal (WNTS 1590)
- The local oscillator (1140)
- The sum of the two (1140+1590 = 2730)
- The difference of the two (1590-1140 = 450)
There's that magic 450 number! When tuned to 690, WNTS 1590 will still mix with the local oscillator & be amplified.
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Better AM radios have a "preselector" circuit between the antenna and the mixer. When tuned to, say, 690, the purpose of this circuit is to prevent any signal except 690 from reaching the mixer. However, this preselector is not perfect. (and cheaper radios may not have it at all!) *Some* 1590 will still get through even when tuned to 690, and especially if you're relatively close to the 1590 transmitter. 4 miles is close enough.
In general, you will receive the "image response" of any AM station on a dial setting twice the intermediate frequency below the frequency on which the station is actually broadcasting. In your case, your radio's intermediate frequency is 450KHz so you'll receive stations 450x2 = 900KHz below their actual frequency. Many radios use 455KHz as the intermediate frequency, in which case you'll hear stations 910KHz below their actual frequency.
Again, this phenomonon occurs in your receiver. It is *not* the fault of the WNTS transmitter, nor of interference from any other station.
In theory, image responses can happen in FM radios too. However, the intermediate frequency in most FM radios is 10.7MHz -- and twice this is 21.4MHz -- which is greater than the width of the FM band -- if you were tuned to 88.1 FM, your radio's image response would be on 109.5MHz and of course there aren't any stations that high!
Many older TV sets will display an image response 15 channels below the channel on which the station is actually broadcasting. (for example, picking up channel 59 on channel 44)
