HB VHF low-pass filter
 

Hi gang,

And old chum of mine suffers badly from pager interference when he's
trying to listen on the 2m band. The interference is strong to him
(S9+) and takes out all his reception, despite the fact that it's quite
a way up-band at 153Mhz.
I've been looking into designing a filter for him to cut off by 153Mhz
to better than -20db but it turns out to be rather more tricky than I'd
imagined. Even with a seven element butterworth configuration, the
difference between the 146Mhz (wanted) and the 153Mhz (unwanted is an
insignificant 6db or so. I'm just not getting enough roll-off and need
something much sharper. Worse still, the roll-off I'm seeing is on a
computer simulation of the filter with ideal components and would no
doubt be even worse with real-world Ls and Cs. Any suggestions as to
how to tackle the problem, guys?
Thanks,
P.

HB VHF low-pass filter
 

HB VHF low-pass filter
 

wrote in message
ups.com...
Hi gang,

And old chum of mine suffers badly from pager interference when he's
trying to listen on the 2m band. The interference is strong to him
(S9+) and takes out all his reception, despite the fact that it's quite
a way up-band at 153Mhz.
I've been looking into designing a filter for him to cut off by 153Mhz
to better than -20db but it turns out to be rather more tricky than I'd
imagined. Even with a seven element butterworth configuration, the
difference between the 146Mhz (wanted) and the 153Mhz (unwanted is an
insignificant 6db or so. I'm just not getting enough roll-off and need
something much sharper. Worse still, the roll-off I'm seeing is on a
computer simulation of the filter with ideal components and would no
doubt be even worse with real-world Ls and Cs. Any suggestions as to
how to tackle the problem, guys?
Thanks,
P.

A coax stub filter to notch out the pager would probably work better. It's
cheap to make, but it also has some drawbacks - signals within roughly +/-
10 MHz of the notch may have as much as 10 dB of attenuation. Your friend
may be able to cope with some attenuation better than 100% overload.

-NM

HB VHF low-pass filter
 

"Norm Mann" wrote in message
news:HDV1g.10938$oQ2.4421@trnddc05...

wrote in message
ups.com...
Hi gang,

And old chum of mine suffers badly from pager interference when he's
trying to listen on the 2m band. The interference is strong to him
(S9+) and takes out all his reception, despite the fact that it's quite
a way up-band at 153Mhz.
I've been looking into designing a filter for him to cut off by 153Mhz
to better than -20db but it turns out to be rather more tricky than I'd
imagined. Even with a seven element butterworth configuration, the
difference between the 146Mhz (wanted) and the 153Mhz (unwanted is an
insignificant 6db or so. I'm just not getting enough roll-off and need
something much sharper. Worse still, the roll-off I'm seeing is on a
computer simulation of the filter with ideal components and would no
doubt be even worse with real-world Ls and Cs. Any suggestions as to
how to tackle the problem, guys?
Thanks,
P.

A coax stub filter to notch out the pager would probably work better.
It's cheap to make, but it also has some drawbacks - signals within
roughly +/- 10 MHz of the notch may have as much as 10 dB of attenuation.
Your friend may be able to cope with some attenuation better than 100%
overload.

-NM


You will not meet your goals with a real world LPF. A coaxial Stub may
yield -20dB attenuation but have terrible in band (2M) VSWR and high loss.
The previous poster is correct- a notch- particularly an asymmetrcial notch
is the proper solution.

50dB or greater attenuation at 163 MHz with in band loss of under -0.4dB is
easily achieved. See:
http://www.eham.net/reviews/detail/3716

Or:
http://www.eham.net/reviews/detail/385

Dale W4OP

HB VHF low-pass filter
 

On 20 Apr 2006 15:24:34 -0700,
wrote:

Hi gang,

And old chum of mine suffers badly from pager interference when he's
trying to listen on the 2m band. The interference is strong to him
(S9+) and takes out all his reception, despite the fact that it's quite
a way up-band at 153Mhz.
I've been looking into designing a filter for him to cut off by 153Mhz
to better than -20db but it turns out to be rather more tricky than I'd
imagined. Even with a seven element butterworth configuration, the
difference between the 146Mhz (wanted) and the 153Mhz (unwanted is an
insignificant 6db or so. I'm just not getting enough roll-off and need
something much sharper. Worse still, the roll-off I'm seeing is on a
computer simulation of the filter with ideal components and would no
doubt be even worse with real-world Ls and Cs. Any suggestions as to
how to tackle the problem, guys?
Thanks,
P.

A notch filter is the easiest thing to do. While a pass cavity will
probably work too it will be quite large and expensive.

Try a small box with connectors on either end and a wire passing
through. connect a series tuned coil / capacitor resonant at the
offending frequency. You can get up to 30 db of rejection with a
single notch. Two circuits connected side by side will improve around
another 10 db. A quarter wave length cable (and shielding) between the
two circuits will approach 60 db if needed.

The higher the Q of the circuit the narrower the notch will be and the
less loss of the wanted frequencies being passed through.

73
Gary K4FMX

HB VHF low-pass filter
 

Design a network that has a notch at 153 AND passes 146. Just the
notch is not good; it will have significant attenuation at 146. In
terms of poles and zeros, you want a transmission zero at 153 and a
transmission pole at 146.

One way to do this is with a coaxial resonator. The separation is
great enough that it shouldn't take a super-high-Q one. You tap your
input and output a short distance up from the shorted end of a
quarter-wave stub that's open on the other end. The distance from the
open to the attachment point is 1/4 wave at 153MHz, which reflects back
a short to the line at 153. But that's capacitive at 146, and
resonates with the short stub between the attachment point and the
shorted end to yield a high impedance across the line there.. You can
do the same thing with a couple coils and a trimmer cap: something
like 10nH across the line and a series-tuned tank of 100nH and about
10.8pF right at the same place would do it. Coil Q should be pretty
high to have a deep null and avoid loss at 146. You get to figure out
which will give you better performance.

Cheers,
Tom

HB VHF low-pass filter
 

Actually 6 dB may be of some help, since the problem is probably caused
by 3rd order intermodulation between two signals and the attenuation of
the intermodulation product will be greater. However, as the others
have pointed out a low-pass filter is not likely a good solution.
Either the notch filter suggested or a bandpass filter should be
better. There are some 2m bandpass filter designs in the RSGB VHF/UHF
Handbook 3rd edition, if you can borrow a copy.

73,
Steve VE3SMA

HB VHF low-pass filter
 

HB VHF low-pass filter
 

I question the bit about insertion loss 7MHz away being a non-issue
with only a single open stub across the line. If you put an open stub
of lossless 50 ohm line a quarter-wave long at 153MHz across the feed
line, it puts a short across the line at 153, but it puts 0.26-j3.58
ohms across it at 146MHz. That's a 99% reflection, or SWR close to
200:1 at 146. It's only about a 17dB loss, but that's more loss than
I'd want.

But if you also put at the same point across the line either a shorted
stub or an inductor to resonate that, it will let 146 pass much more
easily...the inductance required is only about 4nH, though, so it ends
up being a pretty short piece of wire, and a Q of 20 results in almost
3dB loss. Still, that's a lot better than 17dB loss.

Also, you can tune a stub by making it very slightly shorter than
calculated and adding capacitance to the open end--an air-dielectric 50
ohm stub cut 1cm short, with a 1pF capacitance across the open end,
tunes back to 153MHz. A little piston cap lets you tune it right on
the offending signal. (You can also make the 146MHz peak tuneable by
using a stub a bit _longer_ than 1/4 wave at 146 in place of the
inductor, and putting a tuning cap on it too...then it's easy to tweak
right in for optimum performance.)

These what-if scenarios are easy to simulate in RFSim99, which is
freeware. There are lots of variations on the theme, but at very
least, it's a quick check of a proposed solution to see if it's really
all that good at both getting rid of the offender and passing the
desired signal. Be sure to include expected loss resistance to
simulate the finite Q of real components.

Cheers,
Tom

HB VHF low-pass filter
 

K7ITM wrote:

I question the bit about insertion loss 7MHz away being a non-issue
with only a single open stub across the line. If you put an open stub
of lossless 50 ohm line a quarter-wave long at 153MHz across the feed
line, it puts a short across the line at 153, but it puts 0.26-j3.58
ohms across it at 146MHz. That's a 99% reflection, or SWR close to
200:1 at 146. It's only about a 17dB loss, but that's more loss than
I'd want.

You're correct. Someone pointed this out to me via email so I'll
retract my recommendation of using the stub.

Thanks,
Bill

HB VHF low-pass filter
 

Is it a single pager?
I once watched the local pager band with a spectrum analyser, and there
were at least a dozen pagers coming on and off at different times and on
different frequencies, the lowest of which was below 149MHz. (This is in
Australia) Several of them actually produced IM products between them
that were *not* from the spec an front end. I surmise that these are
actually produced locally. Apparently transmitter output stages can
produce these when there are antennas located close to each other
(within a few metres) on the same tower. Others have proposed a stub
configuration - you may have to consider beefing up the front end for
better strong signal handling as well if there are several signals.

Richard

wrote:
Hi gang,

And old chum of mine suffers badly from pager interference when he's
trying to listen on the 2m band. The interference is strong to him
(S9+) and takes out all his reception, despite the fact that it's quite
a way up-band at 153Mhz.
I've been looking into designing a filter for him to cut off by 153Mhz
to better than -20db but it turns out to be rather more tricky than I'd
imagined. Even with a seven element butterworth configuration, the
difference between the 146Mhz (wanted) and the 153Mhz (unwanted is an
insignificant 6db or so. I'm just not getting enough roll-off and need
something much sharper. Worse still, the roll-off I'm seeing is on a
computer simulation of the filter with ideal components and would no
doubt be even worse with real-world Ls and Cs. Any suggestions as to
how to tackle the problem, guys?
Thanks,
P.