"JB" wrote in message
news

"JIMMIE" wrote in message
...
On Feb 19, 4:38 am, wrote:
I recently acquired one of these cheap 2m handhelds from China, an
FDC-150 I think. It's great for the price (£30) apart from a problem
with QRM.

I use the radio with a 3 element beam from SOTA activations from hill
tops. It varies from location to location, but I often get strong
intermodulation effects (caused by pagers I think). I suspect the
radio, being wide band 136-174MHz, has insufficient filtering to
reject these strong signals.

The intermod is a real problem, as I am often unable to hear stations,
or only get half of what they are saying before they are wiped out. I
was wondering about building a 2m bandpass filter like the one
athttp://www.arrl.org/tis/info/pdf/0005054.pdf

Does this look like a good bet?
Also any ideas where I can get the semi-rigid coax (UT-141 or RG-402)
in the UK?

If you are right about it being from a paging system the problem may
have nothing to do with the quality of radio you have. I have
experienced the same thing with a cavity filter on the front end of a
rx. Often the problem is with the pager transmiter. In that case no
amount of filtering will help.

Jimmie

Sad but true. A cavity the size of a backpack might only give you 30db of
rejection, but you might need more than 60db of rejection and you could
never get that without putting the radio in a sealed di-cast box with
bypassed power and audio. It would certainly be better though to start
with
a RX module with some isolation. The fact is, that scanners and cheap
HT's
might be rated at -40 db to -60 db of alternate channel rejection and get
blasted by everything on the mountain as well as everything on every other
mountain within 20 miles too! This is only a published spec. and doesn't
really tell you how much actual signal will result in overload of your RX
deck to cause Desense, nor does it guarantee that something else won't
cause
a mix that falls right on the frequency you want to hear!

Or you could start with a top notch commercial RX deck with -90 db or
better
and hope you can work on that.

If your problem is -only- paging TX, say 1000 WERP or +60dbm and you can
stand right under the tower (for maximum vertical separation) you might
have
at least reduced the energy at your antenna to 0 dbm but more likely +20
dbm. Now you will have to notch out the offender by 140 db more to render
it truly invisible. Rotsa ruck, but if you had a radio with a 5 pole
helical resonator and Hi level mixer, you would be certainly better than
with a scanner, which would be like sending a baby in to fight the fires
in
the Twin Towers.

If the interference is the result of a third order IMD product,
each 3 dB of rejection will yield a 9 dB improvement in the
third order intercept point.

A modest filter might yield surprising results, it isn't a linear
relationship.

Pete k1zjh