Ron Baker, Pluralitas! wrote:
> "isw" wrote:
>>
>> After you get done talking about modulation and sidebands, somebody
>> might want to take a stab at explaining why, if you tune a receiver to
>> the second harmonic (or any other harmonic) of a modulated carrier (AM
>> or FM; makes no difference), the audio comes out sounding exactly as it
>> does if you tune to the fundamental? That is, while the second harmonic
>> of the carrier is twice the frequency of the fundamental, the sidebands
>> of the second harmonic are *not* located at twice the frequencies of the
>> sidebands of the fundamental, but rather precisely as far from the
>> second harmonic of the carrier as they are from the fundamental.
>>
>> Isaac
>
> Whoa. I thought you were smoking something but
> my curiosity is piqued.
> I tried shortwave stations and heard no harmonics.
> But that could be blamed on propagation.
> There is an AM station here at 1.21 MHz that is s9+20dB.
> Tuned to 2.42 MHz. Nothing. Generally the lowest
> harmonics should be strongest. Then I remembered
> that many types of non-linearity favor odd harmonics.
> Tuned to 3.63 MHz. Holy harmonics, batman.
> There it was and the modulation was not multiplied!
> Voices sounded normal pitch. When music was
> played the pitch was the same on the original and
> the harmonic.
>
> One clue is that the effect comes and goes rather
> abruptly. It seems to switch in and out rather
> than fade in an out. Maybe the coming and going
> is from switching the audio material source?
>
> This is strange. If a signal is multiplied then the sidebands
> should be multiplied too.
> Maybe the carrier generator is generating a
> harmonic and the harmonic is also being modulated
> with the normal audio in the modulator.
> But then that signal would have to make it through
> the power amp and the antenna. Possible, but
> why would it come and go?
> Strange.

I've once listened to the first five harmonics of a
powerful medium wave transmitter (400 kW) at
a distance of some 300 m.
All harmonics gave normal audio; no strange
switching effects (Sony ICF-7600D).

What I'd like to know is if in such an 'experiment'
it can be excluded that (some of) these signals are
generated by the receiver itself.

gr, Hein