posted on Sep, 21 2012 @ 03:43 PM
I've been listening to this and to put my experience in context I've been listening to the shortwave HF spectrum since the early 1970s and been a
licensed radio ham since 1974 (callsign G8KNF) having also worked at both Racal and Marconi on HF kit.
What you are listening to are two separate sounds - I'll try to explain in a not too difficult manner.
You are listening to a VHF receiver. Although it tunes 88-108 Mhz it looks like an older style of receiver. This means it will also have a second
single frequency receive strip, normally fixed at a frequency of 10.7 Mhz. This is smack bang in the middle of short wave - I'll come back to this
bit later. This part is called the IF (Intermediate frequency). This is where the signal (RF) is demodulated and turned from RF to AF (audio). In
older radios this uses a whole line of discrete components, tuned coils and so forth.
Nowadays in modern synthesized sets all this demodulation, and IF processing is done at a much higher frequency. Why you ask. Well a common problem is
that under certain circumstances you can get "IF Breakthrough". This is when the radio actually hears very strong signals which are transmitting
around 10.5 - 10.8 Mhz and appears as morse code and other strange data noises.
The bandwidth at 10.7 Mhz is such that you may hear more than one signal, and some receivers are called dual conversion where they have two IF strips,
the first at 21.4 Mhz or 10.7 Mhz and a second one at 455 Khz.
455 khz is bang smack in the middle of a band used by marine users, and they all use morse code.
Now, early evenings, just after dusk is a point where there is strong signals around 9/10/12 Mhz and propagation is such that breakthrough is quite
possible. Breakthrough can also appear if and cabinet screened panels are removed from a radio or an external wire is clipped onto a VHF style
telescopic whip - this can cause overload and spurious breakthrough.
I'd be very happy to answer any further questions.
East London Communications
edit on 21-9-2012 by radioboffin because: I thought of something extra!