Railfan Technology > Listening to U.P. Dispatcher - need help

Enjoy listening to Dispatcher #7 (Cima Sub) here in Las Vegas. But, last couple of years I only hear one side of the conversation - the dispatcher. Used to hear the locos even if they were not in VHF range (via repeaters?). Did they make some changes? Do the train crews use some kind of PL tone or sub-audible tone when replying. Do they possibly uses an offset frequency to transmit on (a la a repeater)? Any help will be appreciated!
Thanks,
Chuck/Las Vegas

Contrary to the thread on the other Board I don't think it's the move to Narrow-Band. I recently increased my operable range by moving to a Yeasu hand held, I'm picking up stuff I have never picked up with other radios. A desk top radio was operating poorly in comparison and I found a problem with the antenna cable, made a quick fix and it now matches the Yeasu.

The audio volume seems to be lower with narrowband channels, the rated transmit power should be the same with both systems, which makes the range the same, or possibly increased. Maybe somebody can explain why the transmit bandwidth would effect either the transmit EIRP or the receiver sensitivity. Digital transmissions do effect the receiver sensitivity, but we're not there yet.

I would be looking for a damaged antenna or cable. Easy thing to do is tune to a nearby Weather Service transmission, either fiddle wit the antenna cable or connectors. If no change then the antenna and cable would appear to be ok, the Radio itself may be suspect. Could be a power supply issue or similar depending on the radio design. The Dispatcher's signal just happens to be large enough and close by to get through the defect.

You should be able to make rough comparisons with the Weather Service broadcast as a reference power level. Next would be a damaged connector on the radio, but at that point you should have already found a difference.

**edit, see the linked article below, which explains a 6dB loss of coverage.



Edited 2 time(s). Last edit at 01/28/15 20:51 by TCnR.

I know UP doesn't do it around here, but could they possible using duel channels. One for dispatcher and one for the train crew. The KCS does this here. So you'll hear KCS dispatch on ARR 78 and the train crew on ARR 39

TCnR Wrote:
-------------------------------------------------------
> Contrary to the thread on the other Board I don't
> think it's the move to Narrow-Band. I recently
> increased my operable range by moving to a Yeasu
> hand held, I'm picking up stuff I have never
> picked up with other radios. A desk top radio was
> operating poorly in comparison and I found a
> problem with the antenna cable, made a quick fix
> and it now matches the Yeasu.

I second that. When switch to narrowband occurred in my territory, I had to turn up the volume on my scanner, but no major difference in reception sensitivity. I picked up all the hotbox detectors a known distance away the same, just a little quieter.

Besides the obvious (squelch settings), the antenna itself, or connections to it might be bad somehow. It could be as simple as cleaning the connections.

TCnR Wrote:
-------------------------------------------------------
> Contrary to the thread on the other Board I don't
> think it's the move to Narrow-Band. I recently
> increased my operable range by moving to a Yeasu
> hand held, I'm picking up stuff I have never
> picked up with other radios. A desk top radio was
> operating poorly in comparison and I found a
> problem with the antenna cable, made a quick fix
> and it now matches the Yeasu.
>
> The audio volume seems to be lower with narrowband
> channels, the rated transmit power should be the
> same with both systems, which makes the range the
> same, or possibly increased. Maybe somebody can
> explain why the transmit bandwidth would effect
> either the transmit EIRP or the receiver
> sensitivity. Digital transmissions do effect the
> receiver sensitivity, but we're not there yet.
>
> I would be looking for a damaged antenna or cable.
> Easy thing to do is tune to a nearby Weather
> Service transmission, either fiddle wit the
> antenna cable or connectors. If no change then the
> antenna and cable would appear to be ok, the Radio
> itself may be suspect. Could be a power supply
> issue or similar depending on the radio design.
> The Dispatcher's signal just happens to be large
> enough and close by to get through the defect.
>
> You should be able to make rough comparisons with
> the Weather Service broadcast as a reference power
> level. Next would be a damaged connector on the
> radio, but at that point you should have already
> found a difference.


Just discovered this thread after I replied to the other one. I'm in agreement with your statement. I doubt narrow FM (NFM) is the problem. I use my scanner in NFM for all analog railroad now with out any particular trouble. Regular FM was/is also working fine.
Narrow band and digital (including NXDN) are NOT the same thing. Narrow band can, and is, capable of supporting analog transmissions, not just digital. There seems to be a lot of confusion about this...people assuming narrow band means digital.
John

About the only difference between wide band and narrow band FM is how much the signal deviates from the carrier frequency when it's modulated. The louder you talk (for this discussion, ignore the fact that there are limiters in the transmitters) the more the signal deviates. In wide band transmitter it's allowed to deviate about 12 KHz. A narrow band transmitter deviates up to 6 KHz. No deviation = no sound. A small deviation = small sound. A large deviation = louder sound. Radios are designed for a particular deviation, both transmit and receive have amplifiers that are built to accommodate the deviation that is used by that radio. If you could tune a broadcast band FM radio to the railroad frequencies, you might hear a very weak voice. Those radios are designed for a 200 KHz bandwidth.

None of this has anything to do with digital radio. If the railroad goes to digital you'll hear what sounds like noise, possibly with some beeps or some kind of a pattern. Also, if the railroad is using a sub-audible tone, that won't keep you from receiving it. The sub-audible or tone squelch is used so that only radios equipped with option will be heard by a particular receiver. Radios without the tones will not be heard. It's basically a filter on the receiver to allow only some radios to be heard. If your scanner does not support that type of squelch, you're going to hear anything on the frequency, tone squelched or not.

John

Narrow-band is designed to allow more 'channels' inside the existing designated frequency spectrum assignment. Technically, the lower bandwidth should allow cleaner transmission, maybe more EIRP but that's regulated by the license and better reception, since a narrowband radio has less bandwith and less noise in that bandwidth. Perhaps there is something else that we don't know about.

For example the Digital Radio thread pointed out 'the cliff' in the sensitivity of digital receivers. Having worked with that before it's actually an extended low power performance due to the ability to differentiate noise from the obvious, repeating modulated digital information.

I've also learned that people actually read these threads and will offer criticism but not offer explanations.

I can tell you from experience that you will lose working coverage when going from 25kHz FM to 12.5kHz FM. It has to do with the S/N (signal to noise) ratio.

There are several technical articles on the subject, here is one http://urgentcomm.com/networks_and_systems/mag/narrowbanding-system-coverage-effect-201004

Jim

Going to narrow band on a VHF radio system we had where I worked before retiring (public safety agency) made a considerable difference in areas (like mountains) where the coverage wasn't as strong as out on the plains. Many of the vehicles had 1/2 wave antennas put on them in place of the 1/4 wave antennas and they guys who worked the mountains on a regular basis had a longer whip put on their portable radios. It took some 'tweaking' of the base stations in the mountains to improve the situation as well.

In the railroad world, I was able to listen to the UP through Cheyenne and west up Sherman Hill from the Denver Metro area prior to narrow banding. This was with a mobile radio and a permanent mount 1/4 wave antenna on my truck. Following the narrow band changes, I need to go about 20 miles north of Denver to hear the same conversations.

James Nelson
Thornton, CO
www.flickr.com/mojaveflyer

Ok, I got it. In my head this says they reduced the Mod Index of the Transmitted signal. There's just less there to catch.

I've been thinking of the ability of a receiver to capture a similar signal, not realizing it's not the same 'signal'.

Later in the article they discuss adding Repeater locations, which would say the Dispatcher's transmission would appear to be a similar density as during the wide-band FM, but the mobile's signal would have reduced by 6dB, at any given location. So the old edge of coverage for mobiles has reduced by 6dB. But if your location is not near edge of coverage, simply turning up the volume makes everything appear to be 'ok'.

Thanks for the article link. It's not exactly obvious though, because of using terms like "FM improvement factor" to compare a worse condition.

-------------------------------------------------------
> I can tell you from experience that you will lose
> working coverage when going from 25kHz FM to
> 12.5kHz FM. It has to do with the S/N (signal to
> noise) ratio.
>
> There are several technical articles on the
> subject, here is one
> http://urgentcomm.com/networks_and_systems/mag/nar
> rowbanding-system-coverage-effect-201004
>
> Jim



Edited 1 time(s). Last edit at 01/28/15 20:49 by TCnR.

To answer the O.P. you need to find 6 dB of Receiver improvement. Moving to a 5/8 wave antenna would give you 3dB and maintain the Omni (ie 360 degrees) reception pattern. There are antennas with less than Omni coverage that provide better than 6 dB, similar to older era TV antennas that point to one location but provide better reception. There's also stacked dipole antennas that are taller but provide more than 6dB gain and an Omni pattern.

Another approach is to move to a Receiver that provides more than a 6 dB improvement in sensitivity. I doubt any broadband scanner can do that. Moving to a HAM radio, Industrial radio or similar may provide what you need.

It may not be a good time to invest a lot of bucks due to the unknown of 'Digital' Radio. I went with a nominal HAM radio as I expect some sort of re-sale value, unless they go Digital as well.