Railroad Scanner & NOAA Interference Filter

[prcguy]

Hi Dale,
I have a number of your products and they are well designed and manufactured, I hope your not taking my previous post as PAR bashing. My comment was more about the type of filter and its limitations due to the size, Q and cost vs the very closed spaced offending signal the OP has.

A new full size 1/4 wave notch cavity in a 5"-6" dia size would cost several hundred $$ minimum for a cheap one so if comparing new filters the PAR is a real bargain if the problem your trying to fix is within the specs of the filter. However, I'm a bit of a scrounge and am usually able to come up with surplus cavity filters cheap for many of my needs.

Hell to the Group,
I followed Tom's thread and have a couple of thoughts. Yes, a full sized cavity configured as a notch will have superior Q over our small filters. With that said, there is no other small filter on the market that achieves the selectivity of our filters. In Tom's case, we opted to make the filter response asymmetrical so it recovered from the notch more quickly on the RR side of the spectrum.
Likely the notch depth can be reduced somewhat and yield even less attenuation in the high side of the RR spectrum. We custom design each filter based on the client's requirements at no additional cost and include a Siglent VNA plot of that filter. The filter housings are small brass cavities with sealed piston caps and high end connectors-your choice all teflon dielectric. Our main business is milcom and aerospace. Their quantities allow us to price our amateur and scanner filters quite reaonably- that's my opinion knowing the amount of work that goes into machining and building the filters.
Regards,
Dale Parfitt
for PAR Electronics, Inc.

 

[W4OP]

Hi Dale,
I have a number of your products and they are well designed and manufactured, I hope your not taking my previous post as PAR bashing. My comment was more about the type of filter and its limitations due to the size, Q and cost vs the very closed spaced offending signal the OP has.

A new full size 1/4 wave notch cavity in a 5"-6" dia size would cost several hundred $$ minimum for a cheap one so if comparing new filters the PAR is a real bargain if the problem your trying to fix is within the specs of the filter. However, I'm a bit of a scrounge and am usually able to come up with surplus cavity filters cheap for many of my needs.

No issues at all- and I agree 100%. At some point we will run out of piston caps. I bought 13,000 years ago and we are nearing the end. New ones are outrageously expensive. At this point in my life. I do this for fun, not dollars. Always looking for clients who have new requirements. We have filters in the middle East, under the oceans and in space. It has been a very satisfying career. Thank you to all who have purchased from us in the past.
Dale W4OP

 

[vagrant]

There is an important point to note about filters and that is size. The three different PAR filters inline on my scanner and the one on my amateur radio are easily accommodated inside my vehicle. I use additional one’s in the house as well due to my filtering needs now vs. finding a sweet deal on cavities. (No pun intended)

I purchased plenty of late as I feared what Dale noted might come too soon. What also made that happen is am still kicking myself for not purchasing an Angle 225-400 MHz preamp while still in production.

 

[Ubbe]

From my friend in Michigan who provides & maintains the equipment

Then I understand the difficulty in testing different solutions. A quick test is to move the PAR filter to between preamp and splitter. If the interference comes back it is the preamp that are not of the best quality. If the interference are still gone then its the radios that seem to be bad and overloads easily, but seems unlikely when they are expensive professional radios.

It's probably not a NOAA transmitter that are the problem, too low signal level by its own, but another new transmitter at another VHF frequency or FM broadcast that takes the preamp over its limits.

/Ubbe

 

[AK9R]

I can only comment on our experiences with the 160.230 MHz, 160.320 & 160.635 MHz channels.

It's notable in the context of this discussion that the three frequencies you are monitoring are at least 1.7 MHz from the NOAA Weather Radio channels.

 

[tom49801]

Then I understand the difficulty in testing different solutions. A quick test is to move the PAR filter to between preamp and splitter. If the interference comes back it is the preamp that are not of the best quality. If the interference are still gone then its the radios that seem to be bad and overloads easily, but seems unlikely when they are expensive professional radios.

It's probably not a NOAA transmitter that are the problem, too low signal level by its own, but another new transmitter at another VHF frequency or FM broadcast that takes the preamp over its limits.

/Ubbe

Hi, from what I was told, the NOAA transmitter's output is 300 Watts and pretty much in line with our antenna's height (490 feet above ground) & approx 14 or so miles away. We do know it's NOAA interference because we could hear it's 24 hour weather reporting clearly when it would intermittingly (many times a day) break onto the railroad frequencies we monitor. Another note, this interference was mostly heard on 160.230 MHz (CSX's road channel 0808 in Grand Rapids, Mi).

 

[tom49801]

Tom mentions that he has several scanners connected to a single antenna, so probably some sort of multicoupler are in use and it could be that one that got overloaded and not the scanners. Maybe a simple FM trap filter would have helped as it is the sum of all signals that makes a device overload. It's not much signal strength left if a transmitter are 14 miles away. Free air attenuation are more than 100dB and transmit power are less than 1000W, 60dBm and results in less than -40dBm at the antenna. A FM broadcast transmitter can have 100,000W or more.

/Ubbe

I was in error about the distance from the NOAA's antenna site, it's approx 40 miles away (not 14) from our antenna.

 

[Ubbe]

Hi, from what I was told, the NOAA transmitter's output is 300 Watts and pretty much in line with our antenna's height (490 feet above ground) & approx 14 or so miles away. We do know it's NOAA interference because we could hear it's 24 hour weather reporting clearly when it would intermittingly (many times a day) break onto the railroad frequencies we monitor. Another note, this interference was mostly heard on 160.230 MHz (CSX's road channel 0808 in Grand Rapids, Mi).

300W are 55dBm and attenuation thru air for 40 miles are 113dB so resulting signal strength using unity gain antennas are -58dBm, a non-critical signal level for any quality of receiver.

If you do not hear the interference constantly it indicates that it needs another transmitter to be active to create the intermodulation. It is that other transmitter that have a much higher signal level that are the cause of the problem, and most probably it is the preamps fault. You could very well start to get interferences again when the frequencies and signal strength are the right ones to create mixing product at the railroad frequencies, and the hunt for another filter starts. It's usually high power pager transmitters in the 155MHz range that seems to cause the most problem, and PAR has a suitable filter for those frequencies. It takes a spectrum analyzer like SDR# to find that offending frequency, or maybe the CloseCall function in a scanner can work as it has to be a strong signal to create intermod.

/Ubbe

 

[vagrant]

Anyone know if there is a standard antenna NOAA uses in order to calculate the gain and get an idea of the ERP?

 

[RRR]

" It's probably not a NOAA transmitter that are the problem, too low signal level by its own "

/Ubbe

Good grief, those are hundreds of watts, just above the RR band

 

[kruser]

Good grief, those are hundreds of watts, just above the RR band

The NOAA transmitter is 40 miles away. 300 watts is not going to be a problem at that distance. Not even for a GRE model with their over sensitive front ends.

 

[RRR]

Apparently it was noticeable to the OP.

We have had to install commercial grade fillters at some base stations near NOAA transmitters.

And then you get the brilliant coordinators that will put a Fire Dept. right in the middle of the Railroad band

 

[kruser]

I think it was noticeable due to an inter-modulation problem that happened to mix with the 24x7 NOAA broadcast.
Most likely a nearby powerful transmitter comes on the air and then mixes in the OPs equipment with the NOAA signal. Find and notch the local transmitter and problem will likely be solved.

 

[CcSkyEye]

I have the same issue with a state owned tower that has an NOAA broadcast antenna on it, basically in my backyard. I have had the "bursts" of NOAA coming through on my BC895XLT, BC355C and BCD996P2. However on my FT7900 it has never happened. I'm not sure if the amatuer radio is just tighter on the rx or something.

Just thought I'd throw my experience with the NOAA interference out there.

 

[Ubbe]

However on my FT7900 it has never happened. I'm not sure if the amatuer radio is just tighter on the rx or something.

It's tighter as it has a varactor tuned 100-200MHz filter that follows the monitored frequency. Transmitters that close to you will always be a problem and needs really good receivers and/or notch filters.

/Ubbe

 

[CcSkyEye]

It's tighter as it has a varactor tuned 100-200MHz filter that follows the monitored frequency.

/Ubbe

That makes sense.

 

[WPXS472]

As someone who has worked in the industry for a company that made filters, among other things, I would like to comment. I had never heard of PAR before reading this. Looking at their website, they seem to be a legit filter company. My hat is off to anyone who can make a living at making filters. Modifying a filter to have an asymmetrical response, one off, for the quoted price is nearly a miracle. He knows his stuff, for sure. Since the filter solved the problem to the OP's satisfaction, case closed as far as I am concerned. Sure, there are other means to the same end, but this seems the best in this case. As to exactly what was cauuing the problem to begin with, we will probably never know.

 

[cbehr91]

So which radio feed is this for?

 

[Rawkee1]

Where can a notch filter be purchased to rid the Noaa weather interferance?

 

[AK9R]

PAR Electronics has filters that are designed to notch out NOAA weather radio signals (in the 162 MHz range). But, they will also attenuate railroad frequencies (in the 160-161 MHz range).

PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

PAR Electronics Inc. Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave

www.parelectronics.com