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Re: Dx-ing
On Mon, 15 Mar 1999 18:37:06 Dan Strassberg wrote:
>At 05:48 PM 3/15/99 +0000, you wrote:
>
>>There is one *major* image freq per station, = xmtr
>>freq - 5.35 (?--10.7-:-2?):
>>
>> 107.9 = 102.55
>> 106.7 = 101.35 (yes, on 'FNX xltr freq!)
>> 105.7 = 100.35
>> 104.1 = 98.75
>> 100.7 = 95.35
>> 96.9 = 91.55
>>
>I think you have that backwards. There is one major imaging frequency per
>station and it's the station frequency plus (or is it minus?) (10.7 MHz
>_times_ two). I forget whether the local oscillator (LO) on FM radios runs
>above or below the frequency to which the radio is tuned. But the LO
>frequency differs from the tuned frequency by the intermediate frequency
>(IF). As far as I know the IF on FM radios is 10.7 MHz. In the absence of a
>tuned amplifier ahead of the mixer, the radio is also sensitive to the
>frequency 10.7 MHz on the other side of the LO frequency. In other words,
>the two frequencies to which the radio is sensitive are separated by twice
>the IF, that is, by 21.4 MHz--not 10.7.
No, we're both right: These are two different animals.
What you are referring to is the *receiver* image, for FM = freq - 21.4 (e.g., 107.9-21.4=86.5, which is why you can hear the FM band on Ch.s 2-6 TV audio, which is right below FM). A receiver also creates a blocking signal 10.7 *above* the frequency a radio is tuned to (e.g., 88.9+10.7=99.6, thus creating a "local block" on 99.5 & 99.7)--this is why you're not supposed to tune a radio on a plane [107.9+10.7=118.6, in the aircraft band]).
I'm talking about something different: This is some sort of special "prime overload image" (I read something in some radio magazine, that confirmed this), that is only present near the xmtr--someone here, with a more technical background, can probably explain it better.
~Kaimbridge~
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