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Re: [RFI] Help with broadcast interference

To: RFI <rfi@contesting.com>
Subject: Re: [RFI] Help with broadcast interference
From: Roger Parsons <ve3zi@yahoo.com>
Reply-to: ve3zi@rac.ca
Date: Fri, 13 Feb 2004 14:47:15 +0000 (GMT)
List-post: <mailto:rfi@contesting.com>
I would agree with Tom. You need to ensure that the
level of interference at the receiver input is reduced
to a level with which the receiver can cope. uV/m
measurements are very interesting, but only indirectly
related to your problem. 

So what I would do is:

(1) 'Measure' the levels of the fundamental signals at
your receiver input. (You can probably do this with
your transceiver and a calibrated attenuator if the
receiver sensitivity does not degrade too much in the
broadcast band. I say 'measure' because you will
probably not be able to get an absolute value without
a genuinely calibrated instrument - but that doesn't
matter because you are only interested in

(2) Determine an acceptable level for those signals.
S9 + 40dB is probably quite OK.

(3) Note the difference. That is the rejection you

(4) Buy or make a filter to reduce the unwanted
signals by the necessary amount. Notch filters, which
are very simple, will give you about 40dB which may
well be enough. There are several (free) filter design
programs around which would allow you to design more
complex filters if necessary. 

(5)If you really must you can buy a BC reject filter,
but that is not likely to have notches just where you
need them. But you could try it without knowing the
results of steps (1) to (3).

(6) Depending on how your receive set up is arranged
the filter may need to take the transceiver power
output or just receive power. With an FT1000 you
should be able to arrange it so that it is only in
circuit in the receive mode.

It really is quite easy to do. My remote 160m station
uses a 5/8 vertical and is about 1km from a high power
AM BC station. A 3rd order elliptic function filter
turned receive from unusable to no noticeable
degradation with an IC728, which does not have the
world's greatest receiver. Unfortunately some scumbag
thief turned the whole station to unusable, but it's
nearly back in commission.

73 GL

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