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 at
http://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?

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 at
http://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?
Try Westlake, if they don't have it they may point you in the right
direction.
http://whwestlake.110mb.com/

A simple helical filter as fitted to some of the early Japanese ham band
only sets may be a bit more compact and still have enough rejection of
out of band signals as most of the pagers in the UK are around 153 MHz.
I use one scavenged from a scrap Icom IC22 but most of the radios from
that era used them.


Steve H

On Feb 19, 1:38*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?

This kinda agrees with what Steve posted...

First, if you want to build it, there's no reason you have to use semi-
rigid. It should work fine with any reasonably low loss coax if you
adjust for velocity factor variations from the design-specified coax.

But...if you have a way to tune an LC filter, you can make a more
compact filter using coils and either explicit capacitors or the
distributed capacitance as in a helical resonator. Using RG-59 size
coax at 150MHz, you'll get resonators with an unloaded Q about 100 --
and it takes a piece of line about 40 cm long to do it. You can get
well over twice the unloaded Q from a coil only about 6mm diameter and
6mm long. Higher unloaded Q allows you to build sharper filters and/
or filters with lower insertion loss. Coaxial stub filters make
sense--a lot of sense--at GHz and higher frequencies, but unless you
want to use really large diameter resonators for something like a
repeater duplexer that requires seriously high Qu, you're probably
better off with an LC filter at 150MHz. If you don't have access to
equipment to tune up a home-brew filter, use of a pre-tuned filter
like the helical resonator Steve suggested is a good idea.

Cheers,
Tom

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 at
http://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?

Farnell and RS if you only want a short length, or FC Lane if you want more,
but they are more used to dealing with business customers.

Chris

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

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 at
http://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?

Might have just what you need... it's somewhere in the back of the
mind...

Some years ago John Regnault G4SWX was experimenting with various kinds
of filters using coaxial stubs. Many of the most useful ones were
published in Radcom and found their way onto my 'In Practice' website:

http://tinyurl.com/g4swxfilters

One of John's ideas was a filter with notches just above and below the
2m band, specifically to knock down the strong carriers from pagers. It
only needs two pieces of coax and two small trimmers.

The idea starts with an open-circuit quarter wave stub which is produces
a notch on the pager frequency, above or below the 2m band. To make it
field tunable, the stub is cut a little short and a small trimmer
inserted in series with the hot end. The only problem is that such a
stub will produce a mismatch at 145MHz: a stub that is resonant above
the band will appear capacitive at 145MHz, while a stub resonant below
the band will appear inductive.

These reactances can be compensated by a shunt inductor or capacitor,
but G4SWX's bright idea was always to use *both* stubs - regardless of
where the pagers are - and let them compensate each other.

Some work with an optimizer was needed to produce the best design, which
proved to be quite tolerant of practical variations. We had an article
almost ready for publication since 2002, but didn't go ahead because it
seemed like "a solution waiting for a problem" - until this week.

A copy has been e-mailed to the OP, and if it works for him we will
publish it.




--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote:

snip


Some years ago John Regnault G4SWX was experimenting with various kinds
of filters using coaxial stubs. Many of the most useful ones were
published in Radcom and found their way onto my 'In Practice' website:

http://tinyurl.com/g4swxfilters

One of John's ideas was a filter with notches just above and below the
2m band, specifically to knock down the strong carriers from pagers. It
only needs two pieces of coax and two small trimmers.

The idea starts with an open-circuit quarter wave stub which is produces
a notch on the pager frequency, above or below the 2m band. To make it
field tunable, the stub is cut a little short and a small trimmer
inserted in series with the hot end. The only problem is that such a
stub will produce a mismatch at 145MHz: a stub that is resonant above
the band will appear capacitive at 145MHz, while a stub resonant below
the band will appear inductive.

These reactances can be compensated by a shunt inductor or capacitor,
but G4SWX's bright idea was always to use *both* stubs - regardless of
where the pagers are - and let them compensate each other.
================================
Tnx Ian for the very useful info.

Frank GM0CSZ / KN6WH

On Feb 20, 8:56*am, Ian White GM3SEK wrote:


Might have just what you need... it's somewhere in the back of the
mind...

Some years ago John Regnault G4SWX was experimenting with various kinds
of filters using coaxial stubs. Many of the most useful ones were
published in Radcom and found their way onto my 'In Practice' website:

http://tinyurl.com/g4swxfilters

One of John's ideas was a filter with notches just above and below the
2m band, specifically to knock down the strong carriers from pagers. It
only needs two pieces of coax and two small trimmers.

The idea starts with an open-circuit quarter wave stub which is produces
a notch on the pager frequency, above or below the 2m band. To make it
field tunable, the stub is cut a little short and a small trimmer
inserted in series with the hot end. The only problem is that such a
stub will produce a mismatch at 145MHz: a stub that is resonant above
the band will appear capacitive at 145MHz, while a stub resonant below
the band will appear inductive.

These reactances can be compensated by a shunt inductor or capacitor,
but G4SWX's bright idea was always to use *both* stubs - regardless of
where the pagers are - and let them compensate each other.

Some work with an optimizer was needed to produce the best design, which
proved to be quite tolerant of practical variations. We had an article
almost ready for publication since 2002, but didn't go ahead because it
seemed like "a solution waiting for a problem" - until this week.

A copy has been e-mailed to the OP, and if it works for him we will
publish it.

--

73 from Ian GM3SEK * * * * 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek

Thanks for the advice everyone has given.

I've decided to give Ian's solution a try when I get the relevant
lengths of coax and trimmers in a week or so.

I'll report back when I've built it.

73s de 2E0WNT