My Scanner is transmitting :0

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My Scanner is transmitting :0

Postby Richie_asg1 » Thu Nov 16, 2017 11:26 pm

I have a Radio Shack PRO-2038 that I was listening to the Marine band today for the Aberdeen harbour.
I am using a standard FM antenna for this on the roof.

I set up a 2M FM rig that uses a separate antenna and listened to the Banchory repeater for a good while, which was fine and quiet, with no signal showing on the S meter of the Icom- 240. When the repeater identified itself I got full scale deflection.

I tuned to the repeater at Fraserburgh - and got a full scale deflection of the S meter - permanently. Turning off the scanner removed the signal, and on again I was receiving full signal again.

I have noticed in the past I was getting a small pulse on the meter when the scanner was scanning. I thought this was acceptable as it sweeps through the frequencies - but not such a strong signal to give me a full deflection on a repeater frequency. It was even drowning out the morse identifier.

Do they all do this when they should just be receivers?
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Re: My Scanner is transmitting :0

Postby m0lsx » Thu Nov 16, 2017 11:46 pm

What you are picking up is simply the internally generated IF.
Do you remember the old TV detector vans? What your scanner is doing, is what they went after.
Back in the days of analogue TV I use to often listen to what my neighbours were watching on TV.

If you look at the history of the G7 callsign you will see that originally G7's were issued to people such as welders etc. Or to be more precise to equipment which should not, but did radiate RF.
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Re: My Scanner is transmitting :0

Postby Richie_asg1 » Fri Nov 17, 2017 12:30 pm

Thanks for the reply.
I realise it is the synthesised IF that just happens to fall on this spot, but just wondered if all scanners were so leaky as mine is, or is a way to reduce this by better shielding?

Failing that, maybe I can use it as a piece of test equipment as I have another IC-240AD that I bought as a dud for £7.50. :laughs:

Interesting about the TV detector vans. Wonder if it still works with digital :rolleyes:
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Re: My Scanner is transmitting :0

Postby G4RMT » Fri Nov 17, 2017 10:37 pm

Any radio or TV set that has a local oscillator can be detected close in - most superhet designs use the mixing of an oscillator with the wanted frequency to make them work. There are other designs, older and newer that don't need to do this, and these cannot be detected in the same way. Ideally, this spurious output has dropped to virtually nothing in a very short distance, but some designs were pretty terrible. The bit about the old detector vans just shows how simple detection is. It was always a mystery to the non-technical to work out how they knew a receiver was active inside a property from outside!
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