PAR Pager Band Filter & Intermod Question

MDScanFan

Member
Joined
Jan 23, 2010
Messages
226
Location
USA
I am getting intermod issues from 156-157 MHz and I have a question about dB reduction level needed to mitigate the issue. The primary signal is a nearby transmitter around 155 MHz that arrives at the scanner at a hefty -28 dBm. The other signals that mix with it are in pager band around 152 MHz and they arrive at around -80 to -60 dBm.

PAR offers a marine band filter that would give me ~40 dB attenuation for the 152 signals and negligible attenuation of the strong 155 GHz signal. That would make the pager band signals -120 to -100 dBm. The noise floor is around -105 dBm. Is dropping those pager band signals close to the noise floor sufficient for my situation? It seems like it may be enough but wanted to check before ordering the filter.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,978
Location
So Cal - Richardson, TX - Tewksbury, MA
You definitely want to reduce the -28dBm signal and you may have other stronger signals not yet identified that are contributing to the problem. Stuff in the -60 or -80 range should not be a problem but local FM and TV signals could be huge and pushing the receiver over the edge. -28dBm by itself is not a big deal but if you have a couple of others as strong, then all combined can pose a problem. A spectrum analyzer could help you find any additional strong signals that may need some filtering.

I worked at a company in the early 90s that was designing a military 225-400MHz SDR receiver and they were mostly computer people trying to design RF equipment and didn't understand the need for receiver preselector filters. We had a couple of Discones on the roof and I simply connected an RF power meter to the coax and showed them the resulting -10dBm of aggregate signals were going to render their receiver design useless. That would push the front end preamp well into IMD land and later a spectrum analyzer identified some of the worst our out band offenders.





I am getting intermod issues from 156-157 MHz and I have a question about dB reduction level needed to mitigate the issue. The primary signal is a nearby transmitter around 155 MHz that arrives at the scanner at a hefty -28 dBm. The other signals that mix with it are in pager band around 152 MHz and they arrive at around -80 to -60 dBm.

PAR offers a marine band filter that would give me ~40 dB attenuation for the 152 signals and negligible attenuation of the strong 155 GHz signal. That would make the pager band signals -120 to -100 dBm. The noise floor is around -105 dBm. Is dropping those pager band signals close to the noise floor sufficient for my situation? It seems like it may be enough but wanted to check before ordering the filter.
 

MDScanFan

Member
Joined
Jan 23, 2010
Messages
226
Location
USA
I have an FM band stop filter inline during these observations. I watch the spectrum from 151-157 MHz on my receiver. I do not see an issue while just the strong -28 dBm signal is transmitting or just the pager band signals are transmitting. I only note the issue when both are transmitting at the same time. That led me to think attenuation of just the pager band may be enough to help.

Would it be worth playing with something like a stub coax that is resonant at the pager band to see if it mitigates the issue as a test?

Beating down the strong signal was my first thought but given the strong signal is so close to the desired band I want to receive (<2 MHz away) I do not think it is possible to do that without incurring a lot of pass band loss (>2 dB) or $.

You definitely want to reduce the -28dBm signal and you may have other stronger signals not yet identified that are contributing to the problem. Stuff in the -60 or -80 range should not be a problem but local FM and TV signals could be huge and pushing the receiver over the edge. -28dBm by itself is not a big deal but if you have a couple of others as strong, then all combined can pose a problem. A spectrum analyzer could help you find any additional strong signals that may need some filtering.

I worked at a company in the early 90s that was designing a military 225-400MHz SDR receiver and they were mostly computer people trying to design RF equipment and didn't understand the need for receiver preselector filters. We had a couple of Discones on the roof and I simply connected an RF power meter to the coax and showed them the resulting -10dBm of aggregate signals were going to render their receiver design useless. That would push the front end preamp well into IMD land and later a spectrum analyzer identified some of the worst our out band offenders.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,978
Location
So Cal - Richardson, TX - Tewksbury, MA
A stub can give you a 10dB + null but they are not very narrow. I think 2MHz away the stub will pull your desired freq down a few dB.

I have an FM band stop filter inline during these observations. I watch the spectrum from 151-157 MHz on my receiver. I do not see an issue while just the strong -28 dBm signal is transmitting or just the pager band signals are transmitting. I only note the issue when both are transmitting at the same time. That led me to think attenuation of just the pager band may be enough to help.

Would it be worth playing with something like a stub coax that is resonant at the pager band to see if it mitigates the issue as a test?

Beating down the strong signal was my first thought but given the strong signal is so close to the desired band I want to receive (<2 MHz away) I do not think it is possible to do that without incurring a lot of pass band loss (>2 dB) or $.
 
Last edited:

vagrant

ker-muhj-uhn
Joined
Nov 19, 2005
Messages
1,947
Location
California
I have a 152 MHz paging repeater that is brutal at my location. I tried various stubs and they helped, but were quite broad as prcguy noted. I now have several or five of Dale's filters and they work very well. Some for RX only and others for TX/RX. I had him leave them adjustable, so I could fine tune, but he generally will tune them where you need it and seal them up.

The stubs just did not offer enough of a notch and were too broad.
 

Ubbe

Member
Joined
Sep 8, 2006
Messages
5,716
Location
Stockholm, Sweden
Would it be worth playing with something like a stub coax that is resonant at the pager band to see if it mitigates the issue as a test?
Absolutly. It will tell if you are on the right path. I use RG6 for 1/4 wave stub filters and they attenuate 20dB but will still attenuate 10dB at 10% off its cut frequency and will also attenuate at multiples of its frequency. So only usable as a temporary test, if you can't afford to loose that much from other frequencies in the band.

stub filter measurements

/Ubbe
 

MDScanFan

Member
Joined
Jan 23, 2010
Messages
226
Location
USA
I will reach out to PAR to discuss options. I am particularly interested to see the insertion loss trade of attenuating the pager band vs the strong 155 GHz.

I may play with a coax stub a bit when I get some free time. Thanks to everyone for their replies.
 

MDScanFan

Member
Joined
Jan 23, 2010
Messages
226
Location
USA
An update on my situation...Good news, I obtained a marine radio and that appears to have solved the issue of the strong 155 GHz mixing with the 152 pager band. When the strong signal is transmitting I hear no intermods. However...I am receiving unwanted signals on some frequencies within the 156-162 MHz range that sound completely different than before...

They sound like repeated quick breaks of the squelch. The timing is random. I may get one, wait a minute or several minutes, and then get another. Or i may go through a period where I get one every couple of seconds. They show up on some frequencies like 161.975 and 156.725.

This is observed on a receiver, a scanner, and an Icom marine radio. On both of my attic mounted vhf antennas. If I turn up the squelch very high then they are not received, but the level required will prevent me from hearing desired signals. A 110 MHz does not help. Not surprisingly, I do not get it when I use a handheld scanner antenna.

My first thought is intermod issues with the pager bands. However, unlike my first issue, I cannot correlate signals on my spectrum scope with the timing of these signals. Any ideas on the cause or ways to diagnose it? Thanks.
 

Ubbe

Member
Joined
Sep 8, 2006
Messages
5,716
Location
Stockholm, Sweden
The one on 161.975 are the AIS system that sends GPS positions from ships. They also has another frequency at 162.025MHz, at least in europe, so they can choose to transmit on both channel to avoid telegram collisions, although they are very short databurst that doesn't even are long enough to open up squelch in some receivers. It could possible be that in US waters they couldn't use the 162 frequency and instead use that 156 frequency.


/Ubbe
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,978
Location
So Cal - Richardson, TX - Tewksbury, MA
For marine band only reception, this is the ultimate thing you can do. Its a really sharp band pass filter from 156 to 162MHz. I have this on a mid grade marine radio at my house connected to a 9dBd gain 4-bay dipole array and its perfect noise free reception even when I'm transmitting with a 100w on 2m with antennas about 10ft apart.

IMG_4116.JPGIMG_4117.JPGIMG_4118.JPG
 
Top