[TenTec] Narrow signal on 28.027620 MHz

[Ken Brown]

>>  I found a very narrow heterodyne 
What do you mean by "very narrow heterodyne?" Is it a single CW carrier? 
Why do you say it is very narrow? Is is so super pure that you can tell 
it has less noise around the center carrier than typical, so it is 
narrower than other CW carriers?

Or does the audio note it produces change more quickly than "normal" 
signals as you adjust your local oscillator (tuning) control? If the 
audio note changes more quickly than normal, then it is a birdie. 
Birdies are products generated by undesired mixes of components, 
harmonics for instance, of the various local oscillators in your 
receiver. If one of the components mixing together to generate the 
birdie is a harmonic of the local oscillator controlled by the main 
tuning, then the audio tone it produces will change 2X, 3X, 4X, or 
whatever harmonic number it is, as fast as a normal signal. When local 
oscillators in receivers were not locked to stable reference 
oscillators, these spurious responses would warble, tweet or chirp as 
the LO frequency changed due to mechanical, thermal or power supply 
instabilities. ( I have not found a historical reference to back this 
up, but I think this is why they came to be called "birdies". With local 
oscillators nowadays locked to super stable reference oscillators, 
birdies no longer chirp, they just whistle.)
>> just one sideband
Any real signal should be heard with the receiver operating in either 
sideband mode. If a birdie or other spurious response is detected only 
in one sideband, then it would most likely have something to do with a 
spur or harmonic of the BFO in an analog product detector type of 
receiver. Use a different BFO frequency and the birdie moves. The Orion 
is a DSP radio, so it may have to do with aliasing that occurs in one 
sideband detection mode and not the other.
>>  
>> from 28,027,618 to 28,027,627 Hz. (With my step set normally at 10 Hz, 
>> it was audible only at 28,027,620 Hz.) Anyone know its source?
>>     
The first thing I would do is try to determine whether it is an 
internally generated product of the receiver, involving no outside 
sources. Disconnect the antenna and see if it goes away. Since others 
have said their identical mode receiver does not hear it, then it could 
be a spurious response of some real signal that is strong in your area 
and not where the other guy tried to hear it. If it does involve a 
signal coming from outside on the antenna port, then a lot can be 
learned by determining how the strength of the spurious response varies 
as known amounts of antenna input attenuation are added. Does adding a 
10 dB pad to the antenna input make the spurious response drop 10 dB, 20 
dB or 30 dB?

DE N6KB