Railfan Technology > offset frequencies

Given the fact that Altamont Press lists those frequencies in the various subdivisions, specifically California, does anyone working for a RR have access or knows where one can or could find the offset frequency needed to know the transmit frequency ?

The Altamont Press TT's do list some PBX frequencies showing both the receive and the transmit frequencies, but aside from these few listings, where else is the known offset frequency ?

A radio, whether it's the mountain top repeater or the engine or crew radio, uses two frequencies in the transmit and receive modes.

Or if the offset frequency isn't readily available or known, of say the frequency 160.875 or 160.800, what would be the transmit frequency since both are listed as being receive only in the AP TT book #20 ?

There are way too many offset combinations, so when does one start at ?

Who out there working in a RR specially in the radio repair shop (if one does exist) would know of either the offset's or the transmit frequencies for the road channels in the various subdivisions ?

Or is there a TO member who know this information or knows where to find it ?

I've looked in places like radioreference.com and only do 2 offset values show up. On other state & federal agency frequencies, there are listings for both the transmit & the receive. But then again, there are those only showing the receive frequency pretty much like what one finds inside the AP TT's.

Unlike other radio services, the pairs used for PBX and other railroad systems are not paired like in ham radio or UHF land mobile pairs with an offset. They are usually pairs picked out of the railroad portion of the spectrum (160.215 - 161.565 MHz) based on not interfering with other frequencies already in use in the area. If you scan the RR band or just program all 97 AAR channels in to a scanner, you should be able to find the other half of the pair.

Just in case you think Mojaveflyer is a nut (he's not,) I'll back him up on what he said. In the Amateur Radio service the offset is 600KHz on the 146MHz (2 meter) band. Depending on the part of the band that offset might be 600KHz up or it might be 600KHz down. On the 440MHz band the split is 5MHz. In the days of mobile phones (pre-cellular days when the phone was the size of 'a brief case or a big box in the trunk if your car), I think the phone companies used 5.25MHz split.

I went to http://www.whrc-wi.org/railfreqs.htm to look for a few combinations. FEC uses 160.770 (AAR chan 41) for train to dispatcher and 160.530 (AAR 28) for dispatcher to train. That's 240KHz, or 13 channels. Gold Coast uses 160.245 -- [AAR channel 09] -- dispatcher to train and 161.295 -- [AAR channel 79] -- train to dispatcher. That's 1.05MHz and 70 channels difference. NS uses 160.950 - [AAR 56] Road and 160.245 - [AAR 09] Dispatcher in some area and 160.830 - [AAR 48] Dispatcher in other areas.

There might be some new standard when the railroads convert to narrow band radios. They will have additional channels and they could establish a 50 channel split standard. With some sort of standard they could reduce the likely hood of interference. That's what we do on the ham bands. If the repeater transmits on 146.940MHz, the receiver uses 146.340MHz. If someone else establishes a repeater on 146.760MHz, they have also established 146.160MHz for input and not 146.340MHz.

John

Most road traffic is SIMPLEX, no offset. The base stations stations are remote base not repeaters. VHF LMR does not have a "set" offset anyway.

I'm not going to question as to why you need the TX freqs anyway, but if you have a valid reason the people you are working for will supply you with the radio you need or the info.

Jim



Edited 1 time(s). Last edit at 04/05/10 19:29 by X4449.

Relatively few railroad channels outside of PBX's are "duplex" frequencies. As already noted, most "repeaters" are actually remote base stations that use simplex. If you listen to crew conversations, you can usually pick up when they are going to a duplex frequency. The always use the AAR channel designation, with the transmit frequency being the first two numbers and the receive being the second two. So, for example, if they say, we're going to "Channel 96-96," that's a simplex channel, while "13-94" would be a duplex channel, with AAR channel 13 being the transmit channel and 94 being the receive channel.

As to the remote base operation, when a crew wants to call the dispatcher, they enter a DTMF (touch-tone) code that transmits on the road frequency. That tone, when received by the remote base transmits the "call" over land lines or microwave to the dispatcher console indicated by the tone code. It flashes as a call on his console, also showing which base station picked up the signal. He then answers the call on that remote base station, and the base transmits the answer to the train crew's radio. There are usually two philosophies in base station location. The first, used in congested areas and on heavy traffic routes, is to have numerous remote bases, not very powerful, every few miles, so that they do not interfere with one another. Sort of a "poor-man's cellular" kind of setup. The second, especially popular in the sparsely populated mountainous areas of the West, is to have very powerful remote base stations located on mountaintops, so that they get extensive area coverage. In this case, a single remote base may cover a hundred or more miles of railroad. As I noted in another post, when I lived in Cheyenne, Wyoming, I could hear the UP dispatchers using the Cheyenne Mountain "repeater"--actually a remote base located 170 air miles south of Cheyenne near Colorado Springs, Colorado--to dispatch trains operating south of Colorado Springs to Pueblo--this on my mobile amateur radio with a good "gain" antenna.

Altamont timetables list the offset frequency (or input) whenever there is one.

If only one channel is listed, consider it simplex. Everybody transmits and receives on the same frequency.

David Curlee