[Spooks] SK01, WYFR, and a sine wave

[Token]

When two AM carriers fall within the passband of your receiver the carriers 
will mix (heterodyne), causing an audio tone equal to the difference in 
carrier frequencies (in this case 3 kHz, the difference between 5950 and 
5947 kHz).  The power relationship of the two carriers also plays into the 
picture.  If your tuning and filter combination can not put one of the 
carriers outside the passband this is unavoidable and why notch filters 
exist.

Why you are just now hearing this instead of having been hearing it all 
along is a bit more iffy to answer.  If your filters and tuning combination 
normally puts one of the carriers outside the passband you might only hear 
the audio from the "off" freq station mixed with the audio of the station 
you are tuned to, and not hear the mixing of the two carriers.  If, for some 
reason, you are now tuning so that both carriers are inside the passband, or 
the filter width has been increased, so that both carriers are inside the 
passband, that might explain why you are now hearing it for the first time.

Without this phenomenon the BFO would not be possible.

This "problem" used to be employed to the radio operators advantage.  Before 
digital readouts or high precision mechanical frequency readouts you could 
inject a known frequency against your unknown (due to dial error) one, and 
either zero beat the known freq or determine the offset of the known to the 
unknown.  Either approach resulted in confirming the frequency of the 
unknown signal.

The BC-221 and LM series were developed to take advantage of heterodyning 
signals.  http://home.mchsi.com/~token/BC221.htm

T!
Mohave Desert, California, USA

--------------------------------------------------
From: "KC2TTK" <kc2ttk at gmail.com>
Sent: Saturday, October 02, 2010 4:20 AM
To: "Shortwave Spy Numbers Stations" <spooks at mailman.qth.net>
Subject: [Spooks] SK01, WYFR, and a sine wave

> Visit http://mailman.qth.net/mailman/listinfo/spooks to unsubscribe from 
> this list
>
> Two clarifications and a question re:
>
>>     Freq ENIGMA Day   MMDDYYYY  UTC  Mode
>> -------- ------ --- ---------- ---- -----
>>     5947   SK01 Sat   10022010 0900   AM
>
>
>> [08:59:15Z] High-pitched wail
>
> After doing a quick-n-dirty audio job, I've determined that it's a
> solid 3KHz sine wave.  After a little research, I found that WYFR
> transmits on 5950KHz, so the frequency of the tone makes some sense.
>
> That said, *why* am I hearing this in the first place?  When WWCR and
> V02a butt heads, all I hear is the overlapping crosstalk; with SK01
> and WYFR, it's as they're doing a cheap and irritating Emergency
> Broadcast System imitation.  If anyone could shed some light on this
> curiosity, I'd appreciate it.
>
>> [09:11Z, 09:16Z, 09:22Z, 09:26Z] Possible SK01 Tx
>
> I see faint traces of SK01 transmissions in the spectrogram beginning
> at 08:59:50Z, 09:04:50Z, 09:09:50Z, 09:14:50Z, 09:19:51Z, and
> 09:24:51Z.  What I probably heard was the closer/footer tone of RDFT
> instead of the header tone.
>
> ＫＣ２ＴＴＫ
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