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

"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.

"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

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

"Surprising results" in a lab maybe. Still far short of real. An
attenuator would tell you how much you really need. I used to use an HT
with a dummy load instead of an antenna from Mt. Wilson to be able to talk
into a box on Santiago Pk. Otherwise the HT couldn't even hear 500 WERP
on-channel from the tower I could see with my own eyes. Be aware that you
might only have 30 to 60 db of bolt on attenuation before case or cable
leakage takes over.

I used an Alinco 2m HT with a two section helical resonator outboard (most
portable solution). There was 3 db of insertion loss and 20 db of rejection
outside of a 3 Mhz window. This was a packet radio and resulted in a 10db
improvement in performance on-channel, but this was a home station on a 6db
stick 20 ft in the air. Would have been far short on a mountain though. I
aslo used that combination for T-Hunting in addition to a fixed 60db pad and
a switched 20/20/10db pad with double shielded coax and 4 el. Quad. My best
solutions was to find places to listen from that were shielded from the
major mountain tops.

In article ,
Tio Pedro wrote:

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.

For what it's worth: I used to have terrible pager-intermod problems
with my Yaesu VX-5, when used with any reasonably-efficent antenna
(e.g. J-pole)... pager-transmitter intermod drove it wild. This seems
to be a common problem with most current-generation HTs, with their
wide-open "DC to daylight" front ends whose high sensitivity (for use
with lossy rubber-duck antennas) leaves them prone to being badly
blasted by strong signals.

The solution I settled upon was the PAR Electronics VHFTN152-158, a
notch filter specifically tuned to eliminate the VHF paging band,
while passing other signals. PAR claims a notch depth of 50 dB
(typical) at pager frequencies, with low loss at 2M and 440
frequencies. From the look of the filter, I believe it's probably a
set of three helical resonators shunted across the line.

Problem solved - the VX-5 suffers no pager intermod at all that I can
hear.

The same filter did *not* help, though, in curing a desense problem
with our repeater's remote-link receiver, which was being blasted by a
newly-installed paging system located in the same building. The pager
was operating up in the mid-160MHz range, outside of the PAR filter's
notch. We installed a DCI cavity-bandpass filter and the problem went
away.

In re the OP's problem - I wonder whether it might be possible to
home-brew a moderate-Q helical filter to serve as a notch? The old
ARRL VHF handbook has some diagrams of this sort of thing. As Tio
points out, one might not need all that deep a notch to result in an
acceptable reduction in intermodulation and desense.

--
Dave Platt AE6EO
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