From: Daron.Brooke_at_EmailProtected
Date: Thu Feb 20 2003 - 15:02:09 EST
Hi Guys,
Good work finding the two signals 1640-1650MHz that you have asked about.
It is unusual to find such strong signals but that makes it more
interesting!
You described these signals as interference but they may be licensed users
of the radio spectrum.
If you were a radio astronomer then they may really interfer with your
observations but these other users are usually just doing what their
licenses allow them to do.
Firstly, there is a web site with online databases that may help you learn
more about signals you find.
The Australian Communications Authority lists all the licensed radio
transmitters and receivers in one of their databases and you can search by
frequency or by postcode.
Have a look at www.aca.gov.au
Next, I would like to explain a couple of technical ideas that might give
you some more things to look for.
Have you ever looked at a Television signal spectrum?
The spectrum of a TV signal has two large spikes that are exactly 5.5MHz
apart.
The lower frequency one is the vision carrier and the higher is the sound
carrier.
You will be able to listen to the sound carrier if you tune your receiver to
this frequency and set the mode to W-FM.
There is more to it if you look closer, you may see the colour carrier
4.43MHz above the vision carrier and possibly another sound carrier at
5.74MHz above the vision carrier.
So what I hear you say!
Well I was wondering if your two signals might actually be a TV signal.
Now nobody would expect to find a TV signal on such a high frequency but
sometimes transmitters can 'leak' on frequencies that are multiples of their
fundamental frequency.
For example if a TV station is transmitting vision on CH9 then a strong
signal is expected at 196.25MHz. This is what we call the fundamental.
But it probably leaks a bit at the 3rd harmonic frequency or three times the
frequency = 588.75MHz
In your case if there was a CH31 station on 548.25MHz then you could look to
see a similar spectrum on 1644.75MHz and the second line at
(3x553.75)=1661.25MHz
So next time you get to see any strong signals then have a look at some
other related frequencies.
I would look for half, third, and maybe fifth of the frequency to see if the
same signal is present.
There are other ways to be tricked by what you see.
Sometimes if there is a very strong signal the receiver can get overloaded
and 'mix' the whole lot up.
If you have DX/LOC button on your receiver then it will allow you to test if
the receiver is overloaded.
Have a think about all this and if you need any more info please ask.
Daron
PS I am a receiver technician with the CSIRO Australia Telescope National
Facility at Narrabri.
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