Another Stub Filter Thread

[vagrant]

Nothing new here other than a place to post my findings using different 1/4 wave coaxial open cable stubs. I have problems with 152 MHz, which is often used as a pager frequency and is actually stronger than any of my nearby FM radio stations. I also have a strong signal from NOAA WX on 162.400 MHz. Thus, the point of this thread is the results of using different coaxial stubs to clobber 152 & 162.xx MHz. As a stub filter is typically quite broad, just one will handle those two frequencies, and more. Of course "more" is probably frequencies you may not want it to work so well.

Before things get out of hand...this is about experimenting. I know a tuned notch filter, in particular one's made by Dale Par are available for each of my problem frequencies. I actually own the 152 amateur version which really does knock it down to almost 60 dBm. I own several other of his various filters as well. I also understand about harmonics, which in my case the stub helps with other problematic UHF frequencies too.

Anyways, I had some junky BNC RG58U patch cables in the bin. I figured I would see if I could repurpose them or back they would go in the bin; they worked. I also had some LMR 400 to test with. I have some LDF4-50A as well. I will cut a properly sized piece and post that result as well. The velocity factor is different for each coax, so slightly different lengths are needed. I used this website calculator to get my 1/4 wave length and added five cm or so, cutting it down to size while looking at the graph on a vector network analyzer (VNA). I was cutting about 1.5 cm each time and stopped when it was close enough.

Velocity
LMR 400 is 85 and the RG58U is 66.

On 152 MHz
- RG58U reduced the signal about 21 dBm
- LMR 400 reduced it about 40 dBm

ON 162.xx MHz
- RG58U reduced the signal about 23 dBm
- LMR 400 reduced it about 22 dBm

The LMR 400 provides a deeper notch and I use that with receivers that monitor the air band as well as others. The RG58U clobbers things well enough and I use it inline for a receiver for monitoring 225 - 400 MHz. Unfiltered, the 152 MHz signal provides me with RFI all the way up into the UHF third harmonic of 456 MHz and more. It is interesting to see on an SDR waterfall other signals and the noise floor shift when that 152 pager TX kicks in.

 

[kruser]

I played with different stubs as well for 152 MHz paging signals.
While they did knock down the paging signals very well, they also killed my 800 MHz reception by a significant amount. Because of that, I decided stub filters were no good for my needs but would work for radios not doing anything above say 500 MHz.
I also found that my Icom receivers were mostly immune to the paging signals. I guess that's the difference between a well designed front end with proper filtering and what is used in most scanner type radios.
The stubs I built were both the shorted and open types but both caused poor reception above 500 MHz. I also tried different coax types and I think I used some old Belden 9913 but probably did not try LMR400. I may have to dig the stubs out as I know I kept them.

I finally invested in notch filters from PAR for the 152 and 158 MHz paging bands. They worked fantastic and have little or no effect on other frequency ranges.

My 152 MHz signals are generated very close to me. I live between several hospitals that are all within a few miles or less. Almost all of them have 152 MHz paging transmitters with antennas located on their tall rooftops. I forget what the ERP is when looking at some of the licenses but it was way up there. So basically, 152 MHz paging signals here will kill all but the best designed front ends.
The Unidens also do fairly well with rejecting the paging signals but not the GRE designs.

The bad thing is the state went with a VHF statewide P25 system that utilizes a ton of 152 MHz channels at many of the sites. That can make it very hard to use a notch filter and still recover enough signal from those 152 MHz channels they use!
I did find the PAR 152 filter does pretty well in knocking down the paging signals to a level most receivers will now work and still leave enough signal through for the states P25 system using the 152 MHz range. It's tricky though.

I also have the PAR 152 MHz paging filter for amateur use and it can help as well but it can also knock down other ranges that a typical scanner user may want. Trying it is the only way to test that one.
NOAA signals are also fairly strong here but usually only cause mixing products or intermod in the old single conversion scanners I have running. I feed all these old scanners from a dedicated multicoupler which I use a 162 MHz PAR notch filter at its input. That filter works very well for those old radios. I also had to install a 152 MHz paging filter on the multicoupler.

As far as the 158MHz paging band, it appears to be mostly unused here these days. It used to be widely used not all that long ago but seems mostly quiet today. I do still hear some distant signals in that range and an occasional stronger signal that only goes live maybe once per hour or less.
450 MHz paging seems to be totally dead here today. I've not detected a signal in the two known UHF paging bands in ages.

IMO, they should nuke these old high powered paging sites but I know they are still used (mostly) by hospitals in many areas to this day.

Dale at PAR also built me a custom filter for the 929 to 932 MHz paging band. It helped bunches when I was playing with preamps for the 900 MHz band. This band still has a bunch of high power signals close to me but none of them bother any of the scanner radios.

Good luck in curing it! I know I spent many hours and days, weeks working on this problem.

 

[vagrant]

The stubs for me are an experiment. I will order more filters this week. I have two systems at home and I also listen in the vehicle. Thus, my costs are multiplied.

As to the experiment, the LDF4-50A will be interesting to compare with the LMR400 regarding the depth and width of the filter. It will probably stop there unless I can find a connector and an improved piece of coax at a local swap. It really becomes impractical when looking at an improved stub material cost versus just buying a quality filter. Making an inexpensive stub is an easy way to find out if a quality filter will help, once one identifies an RFI issue.

Basically, I went straight to the proper filters and never experimented with making something.

 

[kruser]

Making an inexpensive stub is an easy way to find out if a quality filter will help, once one identifies an RFI issue.

That's pretty much what I did before investing in the PAR filters. I really wanted to see if I could knock down the worst paging signals enough for the worst receivers to handle what was left and also still receive the lower signal level state P25 sites around me.
It was still hard to tell using just stubs but tests showed it may be possible so some PAR filters were ordered. Once they arrived and I installed the 152 MHz filter, I was a happy camper. The GRE models are still a bit of trouble and also need FM broadcast band traps ahead of them.

Today I feed signal to the radios that were susceptible to pager overload or desense from a multicoupler that has a 152 filter ahead of it.
The better receivers that can handle the full power of the paging signal are running unfiltered on another multicoupler. Most of those are communications receivers though but some of the Unidens work there as well. Mainly the x36HP and older x96XT series.
I'm seeing mixed results with the SDS models from Uniden. Sometimes they work better behind a 152 filter and other times they are better without that filter. The SDS models are proving to be very temperamental for me!

 

[nanZor]

Totally cool experiment - thanks for sharing.

Yeah, stubs are the cheap quickie-solution, but certainly no match for a proper filter. I'm assuming the stubs were tee'd right after the receiver input jack right - and not way down the line?

prcguy can correct me here since I forget - the quarter-wave open stub also attenuates odd harmonics - but if that is an issue, I *think* if my memory serves, that a half-wave closed/shorted stub will attenuate even harmonics ? It's been awhile since I played with stubs.

Other than a bunch of funky coax dangling around, the half-wave shorted stubs were a lot easier to "pin-short" for tuning, and then soldering center-conductor and braid together at just the right point where the temporary pin short was...

 

[vagrant]

Yes, on the quarter-wave odd harmonics and yes, right before the preamp or receiver. I also messed with shorting half-wave too. A VNA made things easy to test.

The PAR 152 amateur filter is my preferred choice. It’s tight enough and at $20 more it handles TX through it. I’m going to order an unsealed scanner version as well. The BNC removes adapters for the scanner in the vehicle.

Leaving it unsealed allows me to adjust. I purchased some used and they were nowhere close to what I needed, but now work fine. Perhaps they were tuned that way for the original owner. Removing the seal was a pain, but most people need not adjust.

Anyways, Dale noted the were like 11 weeks out, so ordering now.

 

[vagrant]

Whoops, their lead time is currently 11 days, not 11 weeks.