Classified / secret organizations world wide use a variety of methods to
transmit information. Those methods are not limited to single frequency,
fixed bandwidth, standard modulation radio frequency emissions.
Frequencies can sweep up or down band, jump from freq to freq, transmit
more than one signal at the same time from the same transmitter, and
vary any of the parameters we would normally associate with radio waves.
... and those are the ones you can detect with normal receivers ....
there are others that you can not ....
They will not go away, as they are not considered interference and dont
follow 'rules'.
That was a good observation and a good question.
To study them closer, use a Software Defined Radio to record a band
segment to your hard drive for repeated study of the same 'signal' in a
variety of ways. A SDR-IQ for example, but there are many other choices.
73 from Bill - WD8ARZ
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