Railfan Technology > Railroad Radio Bandpass Filters

So one of my radio feeds shares a tower with a couple UHF channels and two amateur radio frequencies, one analog and one digital (DMR).. while you would think there would be enough separation (145-147MHz) from the railroad band, the repeaters still like to wreak havoc with my feed sometimes, even with proper duplexers on the repeaters.. not to mention the fire paging repeater on the next tower over on site is also a PITA.. I'd like to find a bandpass or a high pass filter that would help eliminate the problem. I plan to add a preamp as well to compensate for the filter's loss, so hopefully that won't be an issue. Any recommendations that won't break the bank?

I haven't tried this one, but the AIS model from GPIO labs seems promising and inexpensive: https://www.ebay.com/itm/AIS-162-MHz-Band-Pass-Filter-Bandpass-160-MHz-161-MHz-162-MHz/282452182050?hash=item41c3764422:g:MK4AAOSw4A5YnR8B

Be aware of how the pre-amplifier handles the offending signals as well. Sometimes it creates it's own intermods, or adds to the noise floor and just adds more grief to the problem. If it has a decent noise figure and the gain simply offsets the new losses you will have done well.

The nearby transmitters may also get onto your cables any where in your system, it will likely help to look into shielding as well. Pretty tough location sometimes. you may have to upgrade your receiver to something similar to what the Fire Dept. is using, in rough comparison.

Let us know how it works out. RR Radio is very useful.

What is the freq range you want to pass(the full AAR band, or just a few hundred KHz)? What is your RX unit? I would be very careful with a preamp.

Jim

Tuned cavities work very well also to kill offending frequencies. But first you need to know which freq is causing the problem. That could be challenging with several other services nearby....

Rick2582 Wrote:
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> Tuned cavities work very well also to kill
> offending frequencies. But first you need to know
> which freq is causing the problem. That could be
> challenging with several other services nearby....

The offendending frequencies are 154.250 and 159.135. It's more of the paging transmitter (the first one) than the primary fire channel causing the issue.

TCnR Wrote:
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> Be aware of how the pre-amplifier handles the
> offending signals as well. Sometimes it creates
> it's own intermods, or adds to the noise floor and
> just adds more grief to the problem. If it has a
> decent noise figure and the gain simply offsets
> the new losses you will have done well.
>
> The nearby transmitters may also get onto your
> cables any where in your system, it will likely
> help to look into shielding as well. Pretty tough
> location sometimes. you may have to upgrade your
> receiver to something similar to what the Fire
> Dept. is using, in rough comparison.
>
> Let us know how it works out. RR Radio is very
> useful.

If I do get a preamp it will definitely be one from AR2, which is what we use on our ATCS feeds, and have been exceptional so far.

X4449 Wrote:
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> What is the freq range you want to pass(the full
> AAR band, or just a few hundred KHz)? What is your
> RX unit? I would be very careful with a preamp.
>
> Jim

It would have to be the full band. On the low side is CSX Road (160.230) and the high side is one of the CSX dispatchers (161.520) so I pretty much need the entire band. Not sure how tough that will be with a duplexer can. Currently I'm using a Kenwood TM281A but I'm gonna swap it out to a TK760HG-1 so I can put the 281A at home for my ham shack.

The Kenwood TM-281A is one of the best amateur radios out there for railfan use--it has "true" narrow band capability (it will tune the splinter frequencies) and is better than most amateur radios in overall performance. That said, strong intermod interference from nearby frequencies can overwhelm it. Nearly every other amateur radio or scanner out there will perform even worse than the TM-281A in this regard. For this type of use, I would strongly suggest getting a high quality commercial radio, such as the Kenwood TK-760 mentioned above. If one is not worried about NXDN digital capability, a very good option would be a used Kenwood TK-790 mobile. It is a 160 channel radio with excellent receive specs. These radios are being dumped on the market used like crazy as more and more radio systems are converted from analog to digital radio. The TK-790 is often available used in the $100-$125 range. They do require computer programming, and, these days, that usually requires taking the radio to a Kenwood dealer. The software for the TK-290-TK-790 series is pretty archaic--DOS or early Windows variety, that newer computers without serial ports just don't like. I have the software for this series of Kenwood radios that I bought from a Kenwood dealer many years ago, but I have not been able to successfully run it on a computer since Windows XP. The radio shops often keep an old DOS computer around just for programming these old radios. There are some newer model analog commercial mobile radios out there that are also very good, but generally they fetch considerably higher prices or are of lower performance quality.

A note about intermod interference in general: it is becoming a horrible problem, even in areas with relatively little two-way radio activity. The reason is the proliferation of electronic devices of all types capable of producing RF interference, combined with the fact that the FCC has practically no budget to adequately screen electronic manufacturer compliance with Part 15 of the FCC rules (which are supposed to ensure that electronic devices do not create RF interference). Worse yet, the FCC has almost no budget to investigate or prosecute companies or individuals that violate Part 15. The situation has gotten so bad that even many very high quality radios can not adequately filter out the flood of RF interference out there.



Edited 4 time(s). Last edit at 03/01/18 19:59 by WW.

Some things to try:

Be sure the repeater people are at the site at the same time as you.

1, Tighten all connections, screws and bolts, give special attention to those that act to tie
.....equipment together and the coaxial cable connector(s.)
.....Ask the repeater folks to do the same.

2, Remove the audio out line and listen to the output of the radio. Have the repeater people
.....key up their transmitter. Sometimes, a bad connection on the audio line will be responsible for noise.
.....Also, listen to the audio line when it is disconnected from the radio with the repeater keyed.

3, Can the radio and antenna be moved? This may be the easiest solution.

4, A more modern radio such as one used in 2-way radio service.
.....These are likely to have a helical resonator section.
.....Check with your independent 2-way radio shop, many customers are switching
.......to other bands and there may be a used radio suitable for your needs at a reasonable price.
.......The repeater people should know of someone.

5, If there is junk getting into the radio, a filter may be helpful.

6, If you opt for a filter:

.....In increasing order of cost, complexity and effectiveness:

.......The lumped element filter such as suggested by a previous poster.

.......A strip line filter

.......Helical resonators

.......Cavity filters

.......Note that all except the lumped element filter require tuning with sophisticated
.........test equipment. The same gear the repeater people used to tune their duplexer
.........setup will likely suffice.


Be Well,

Tim Colbert K3HX

Tim gives good advice. A few more notes. Pager transmitters are generally some of the "dirtiest" producers of intermod interference. Proof here is that OP's radio is picking up imtermod from a frequency 7 mHz different from the frequencies being monitored. As I noted above, the Kenwood TM-281A is a very good radio, but even it will not reject strong pager intermod. Over the years, I've found that only pretty high quality commercial radios will successfully reject pager intermod--though even they will struggle if the intermod signal is strong enough.

Another potential culprit can be electronic equipment close to the radio--for example, computers and monitors can be notorious for spurious emissions. Oftentimes, they can produce enough intermod if placed close to the radio to open the squelch, then other interference can "ride" the spurious signal into the radio. Out on the road, electronic gas pumps can be notorious for producing major intermod. I've had very good commercial mobile radios overwhelmed by that at gas stations.

It may be worth it to the OP to hire a reputable radio vendor to run a spectrum analysis at his site--there may be intermod there that is not easily identifiable except by spectrum analysis.

The only good news for those monitoring the VHF railroad radio frequencies is that amount of use of the adjacent VHF frequencies is decreasing, as more and more public agencies convert to 700 mHz or 800 mHz digital. Pager use is also rapidly declining as the cell phone has effectively replaced a lot of its functions.

One final note: NXDN digital radios are not immune from intermod interference. However, they are much better at only opening their squelch to receive desired digital transmissions--that said, the interference can sometimes garble the digital signal enough that the radio can't decipher it, particularly if the offending interference is also digital. So, in that regard, the coming use of NXDN for railroad radio communications is not a panacea when it comes to the worst RF interference, but it can better handle some of it.



Edited 1 time(s). Last edit at 03/07/18 08:11 by WW.

Have you had any luck?

Also what kind of “paging channel”?

Are you speaking of data/POSAG (text messaging) or two tone? As stated POSAG data have had a history of being overly powered - but if it’s standard tone paging, then its a standard transmitter.

How are the antennas arranged on the tower? I have see where if it’s colocated on a cell tower, the tower owners commonly mount all two way on a common platform which plays havoc with each other.

Receive only antennas should be at the best possible height with transmitting antennas below - stacked vertically - it horizontally as previously mentioned.

If this is a private/ham tower - then god knows what’s going on. I’ve seen some really high end work, and some that is just inching to catch fire.

Also as stated, use a radio with a really good receiver. The Kenwood 90 series is pretty solid and easy to work with. Motorola as well, but a tad more expensive. Stay away from
Scanners and ham radios if possible.

Several years ago I bought a cavity filter to highly attenuate the VHF pager frequencies. They are particularly problematic in metropolitan areas like Houston, TX. It worked very well. It did not however shunt the NOAA frequency (also in downtown Houston.)

PAR Paging Notch Filter VHFTN152-158

de KK5OL

Sti-Co has the market for band pass filters for locomotives and they work very well.

Posted from iPhone

MP683 Wrote:
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> Sti-Co has the market for band pass filters for
> locomotives and they work very well.
>
> Posted from iPhone


Interesting.

http://sti-co.com/freight-rail/fr-high-performance-filters/