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Radios and Antenna Systems

Scanners and Receivers


Currently there are only a few radios available with factory installed descriminator taps, these are: Icom PCR-1000 - about $275 - is a good data radio - much aftermarket software available AOR - AR-8200 - about $600 - requires special factory cable ($100) WinRadio External Units - Starting at $600

The new series of digital scanners for APCO 25 have been on the market for about a year. At this point in time, only a few of our members have acquired one. It was discriminator tapped successfully using instructions found on our Yahoo group.

Almost any scanner, or fixed frequency radio can have a discriminator tap added. The railroads typically use a small black box data radio. These are manufactured by companies like MDS-Microwave Data Systems and Adaptive Broadband Corp. MDS of Rochester NY is probably the most widely used by the railroads. These radios usually cost about $1200 new. If these radios present themselves, at a price you can afford, this is as good as the quality gets. Used prices range from $50 to $250. Ebay is a good source. Make sure that programming software is included. Even these radios may need some minor work to prepare them for ATCSmon. Motorola mobile Spectra 900 mhz radios (usually former public service) can be used to monitor ATCS signals. They are readily available on the auction web sites. But be advised that they are designed to receive only in the frequency range that the BCP transmits in. These radios can not be programed to listen to the MCP frequencies. These will work fine in systems where the BCP echoes everything that the MCP transmits (like most locations on CSX).

Baseband Audio

Baseband audio, simply put, is the "Raw" audio coming from the discriminator circuitry (thus the name DiscriminatorTap). "Discriminator" is the term given to the circuitry within the radio that takes the FM radio signal and converts it back into an audio signal.

The reason that it's so important to be able to access this baseband audio for monitoring data is that all radio manufacturers perform some processing on the raw audio signal to clean it up and make it sound good. This includes things like amplification, de-emphasis, filtering, noise reduction, tweaking the bass/treble, etc. All of these additional steps the audio goes through work great for voice signals However, it wreaks havoc with data signals. Lower bandwidth protocols (4800 bps or less) may actually work okay with out a Discriminator Tap, however, you'll have much more success using the baseband audio.

What is a DiscriminatorTap

Simply put, a discriminator tap is what allows you to easily access the raw audio from the discriminator circuitry. It usually consists of a piece of wire soldered on to a specific pin on the discriminator chip in the scanner, a small tantalum capacitor (+ side towards the discriminator chip), and some type connector for you to patch the scanner into your sound card. Most people choose to use 1/8" mono jacks since they are common for applications like this and easy to obtain. However, there is no set requirement on what you need to use.

If you are handy with a soldering iron, read the "Where to tap" section below for more info. If you're not comfortable digging around inside your radio, hook up with an ATCS fan in your area who is or contact a local ham radio operator. Most hams will either be able to do the mod for you or put you in touch with someone who can. Check with your local ham radio club, or if you're feeling lucky there is a partial list of ham radio operators who are active in ATCS Monitoring on the Yahoo! web site here. This list shows the where the ATCS Monitor / railroad buff / ham operators reside and if there is someone in your area, so much the better.

Where to tap

There are lots of resources on the Internet that will tell you where to tap from. A google search for "Discriminator Tap" yields lots of results. The below serves as a good reference for "older" scanners:

However, if you have an exotic, old, or really really new scanner, finding info on the internet may be difficult. If you can't find any information, the best thing to do is look for a 16 pin IC, and then probe it's inputs using an inductive tone probe. A tone probe is commonly used to trace cabling. When all else fails, send an email out to the group. Alternately, if you don't have (or know someone who has) a tone probe, you could use a small speaker or an old set of headphones. You're looking to hear constant static when you probe the pins.

Also note that if you're having a hard time finding information, not everyone calls this modification a "discriminator tap". In an effort to tap my older Yaesu FT-2400, I had some success searching the internet for "9600 packet" and "baseband audio". All of these are really getting you to the same goal.

Chances are, any radios that are tapped "out of the box" will label the port "9600 Bps Packet" or similar. For example, the PCR-1000 (and I'm assuming the later PCR-1500/2500) does this. In some cases the radio needs a soft option set for 9600 operation.

Soldering a tap

If you're not familiar with soldering, you could try to find someone to do it for you, but it's really a good skill to have, and not expensive or difficult, really. In less than an hour of practice, you should be proficient enough to attack most scanner modifications. Here are some general pointers:

  • Try it out on some old, dead phone or radio, if you want to get used to it first. In fact, here's a good place to see about how long is too long for the iron to be applied!
  • When soldering the wire to a jack, you need the heat on the jack a bit longer, since there's more metal to heat to solder-melting temperature.
  • Use a partner. It's hard to hold an iron, solder, a wire, and sometimes the board itself on your own, and will result in at least frustration, possibly damage.
  • When cutting traces, as sometimes prescribed in a mod, you'll want to use an ohm meter to verify that continuity has been lost. When finishing a solder job, you'll want to test continuity. Touch the probes to the components at both ends of that trace to be sure the circuit is open. Pocket Ohm meters are less than $20, again, Radio Shack or even auto-parts stores.
  • If nobody responds to your request for assistance in making a modification, understand that it does take some time, and nobody enjoys being responsible for someone else's gear. My standard disclaimer is that it might go back to you in worse shape than it arrived (but almost never does).

Check out soldering videos for instruction on how to solder. The Lead Free Through Hole Soldering one is probably the best summary, however, you should just use Rosin core solder rather than separate flux, and Tin/Lead solder is fine. They have some overly specific stuff in here for our purposes. Here's a YouTube video showing some basics as well.

-- Main.GaryHahn - 31 Aug 2008

I have scanner model X? Will it work?

Probably! See the BestRadios topic as well as the list below for more information and reviews on the various radios ATCS Users are using. Also, there is a database on the YahooGroups site that contains information as well.

Radios for use with ATCSMon

The information that follows is a compilation of data from the ATCSMon Yahoo! Database and individual submissions.

  • Sensitivity refers to how weak a signal the radio is capable of receiving. The lower the sensitivity, the better the receiver. Manufacturer's ratings are shown in microvolts ( uV). User ratings are shown in brackets [n]. Sensitivity will also vary depending on the frequency the radio is tuned to. For our purposes, we will only concern ourselves with the VHF (160 MHz) and the UHF (900 MHz) bands.
  • Selectivity refers to the ability of the radio to operate in the presence of nearby stronger signals. The manufacturers ratings are usually expressed as a given number minus decibels at a given frequency away from the desired signal. You want a greater value (more negative value) as close to the tuned signal as possible. For example:

Selectivity: 10 kHz / -6dB is better than 15 kHz / -6 dB, since 10 kHz away is closer than 15 kHz away. Selectivity: 10 kHz/ -10 dB is better than 10 kHz / -6 dB, since -10 dB drops the adjacent signal more than the -6 dB figure.

  • Subjective ratings for Sensitivity and Selectivity are shown in brackets [n] and are on a scale of 1 to 10 with the benchmark being the MDS data receivers rated a [9].



Very good hand-held scanner. No longer manufactured. Top end sensitivity and selectivity among scanner class radios for ATCS freqs.

VHF Sensitivity: 0.35 uV [5]
UHF Sensitivity: 0.35 uV [7]
VHF Selectivity: 12 kHz / -6 dB, 25 kHz / -60 dB [6]
UHF Selectivity: 12 kHz / -6 dB, 25 kHz / -60 dB [7]
Power: 12 V / 160 mA - Coaxial DC power plug
Computer interface: Yes
Discriminator tap:
Approximate Price: $300 used

-- Main.DanRapak - 11 Sep 2007 (from the ATCS Yahoo! Database)

AR8600 Mark II

Mobile / portable scanner. Slightly worse than the Icom IC-PCR100.

VHF Sensitivity: 0.35 uV
UHF Sensitivity: 0.50 uV [7]
VHF Selectivity: 12 kHz / -6 dB, 25 kHz / -40 dB
UHF Selectivity: 12 kHz / -6 dB, 25 kHz / -40 dB [7]
Power: 12 V / 400 mA - Coaxial DC power plug
Computer interface: Yes
Discriminator tap: Standard, DIN connector
Approximate Price: $900 new

-- Main.DanRapak - 12 Sep 2007 (from the ATCS Yahoo! Database)



No longer in production, this was Icom's original computer controlled radio. It is literally a black box with a power connection, an antenna connection, an audio connection and a serial port to connect to your computer. It has no user controls whatsoever on the radio itself. All control is done by way of computer software. Software to control the radio is available from a variety of sources, including Icom itself.

The down side of using this radio is that if power is removed, the radio does not remember its previous settings. Therefore if the radio is to be operated in an unattended server environment, you need a means of resetting the radio in the event of a power failure.

VHF Sensitivity: 0.32 uV
UHF Sensitivity: 0.4 uV
VHF Selectivity: 6 kHz / -6 dB, 15 kHz / -6 dB
UHF Selectivity: 6 kHz / -6 dB, 15 kHz / -6 dB
Power: 12 V / 700 mA - Coaxial DC power plug
Computer interface: Yes
Discriminator tap:
Approximate Price: $200 used

The manual for the PCR100 is available at:

-- Main.DanRapak - 11 Sep 2007 (from the ATCS Yahoo! Database)


Like the IC-PCR100, the PCR1000 is no longer in production. It is also a completely computer controlled radio with no user controls whatsoever on the radio itself. All control is done by way of computer software. Software to control the radio is available from a variety of sources, including Icom itself.

Manual is available at:

VHF Sensitivity: 0.32 uV
UHF Sensitivity: 0.4 uV [8]
VHF Selectivity: 6 kHz / -6 dB
UHF Selectivity: 6 kHz / -6 dB [8]
Power: 12 V / 700 mA - Coaxial DC power plug
Computer interface: Yes
Discriminator tap: Standard
Approximate Price:

-- Main.DanRapak - 12 Sep 2007 (from the ATCS Yahoo! Database)


The ICOM IC-R7000 is an excellent general purpose VHF / UHF receiver that operates on 120 VAC. While it can also operate on 12 VDC, it is a bit large for a mobile rig and is really intended as a base station type of unit where the 12 VDC would serve as a backup power source. While not as productive as the Maxtrac or MDS radios, the R7000 receiver has been used with success in ATCS Monitoring. The radio easily tunes the ATCS BCP and MCP frequencies with fairly good sensitivity.

The R7000 does have a rear "Ext Spkr" jack as well as front panel "Phones" and "Rec" jacks. But as is the case with all scanners and receivers, these jacks cannot be used for data reception as the audio filtering and processing makes data decoding impossible. Therefore, the radio does need to have a discriminator tap installed. Fortunately, there is also an unused jack labeled "Spare" on the rear panel in the form of an RCA phono jack for just such a purpose.

The radio is roomy and uses old fashioned components with leads (as opposed to surface mount components) making it easy to work on. The discriminator tap is relatively straightforward for those technically inclined. Details on how to perform the discriminator tap mod can be found at:

There are two negatives found so far in using the R7000. First, while the radio works reasonably well, it doesn't perform as well as a Maxtrac or MDS. The second issue involves frequency stability. R7000s have been around for well over a decade and haven't been manufactured for some time. That means these radios are getting on in years. As a result, the accuracy of the digital frequency display may not be quite what it once was. This is especially true in the higher frequencies like those used for ATCS. The bottom line is, you can't just dial in 897.9375 (for example) and assume that you're on channel. You might have to fine tune a bit one way or the other to get the lowest error rate in your data. Once you find the sweet spot, you can easily store that frequency in a memory channel, but it can take some playing around. The R7000 can be aligned to correct any tuning errors, but the alignment is fairly involved and takes professional grade test equipment to perform.

In summary, if you have an R7000 on hand and would like to give it a shot before you go out and purchase a new, dedicated ATCS receiver its certainly worth the effort.

VHF Sensitivity: 0.5 uV
UHF Sensitivity: 0.5 uV
VHF Selectivity: 6 kHz / -6 dB
UHF Selectivity: 6 kHz / -6 dB
Power: 117 VAC
Computer interface: Yes
Discriminator tap:
Approximate Price:

-- Main.DanRapak - 10 Sep 2007


The Icom IC-2720H is a mobile VHF/UHF amateur radio transceiver. That is, the radio is not just a receiver, it is also capable of transmitting on amateur radio frequencies. This additional transmitter circuitry adds considerably to the cost of the radio and requires an FCC license. Therefore, the IC-2720H is not recommended for purchase exclusively for ATCS monitoring.

However, if you do have an amateur radio license and already have a 2720 or are considering purchasing one, the following information might prove useful.

In addition to its use as a very good 2 meter and 70 cm ham radio, the IC-2720H can also be used for ATCS monitoring. Like most modern amateur radios, the receivers of the 2720's are general coverage, allowing reception of frequencies well outside the normal ham radio bands including railroad voice and ATCS frequencies. In addition, the 2720 is actually two radios in one package. This makes it possible to tune the VHF railroad voice frequencies on one radio while simultaneously monitoring ATCS on the other. Note however that only the right-hand receiver is capable of tuning all the way up to the 890 MHz ATCS frequencies. Therefore the normal dual mode would be VHF comms on the left receiver and ATCS data on the right.

The 2720 also has a built-in discriminator tap intended for use with packet radio. The signal appears on a 6 pin DIN connector on the rear of the radio and works just fine for ATCS data recovery. Note that there are two data signals on the DIN connector. One is for 1200 baud packet and the other is 9600 baud. It is the 9600 baud signal that is the raw, unfiltered discriminator output that we need for ATCSMon. Also, you need to program the radio as to which receiver feeds the DIN connector. Since only the right-hand radio is capable of receiving ATCS frequencies, you need to program the packet function to "PAC-R" to keep the voice communications from screwing up your data.

The one huge drawback to using the 2720 for ATCS is that the radio only has one antenna connector. You can't attach a separate antenna to each radio. This presents a couple of problems.

- First, you need a single antenna that will work on the 2 meter, the 70 cm and the 33 cm ham bands. (In addition to loading properly on the 2 meter and 70 cm ham bands, the 2 meter section will also serve the 160 MHz railroad voice frequencies and the 33 cm portion will serve the ATCS frequencies.) Unfortunately, there is only one such antenna made and it is not sold in the United States. The shipping and import duties from foreign sources approaches or exceeds the cost of the antenna. So, you need two antennas; one for the 2m and 70 cm ham bands and one for ATCS monitoring.

- Second, ATCS frequencies are practically microwave and usually low power so you really need to have a pre amp in the line to boost the signals. Also, ATCS frequencies are very near cellular telephone, SMR and other higher power mobile radio services. This requires use of a band pass filter as well. Adding these things to your single antenna line means that you can no longer use the radio as a ham rig. Neither can you receive railroad voice communications as the filter and pre amp trap those frequencies out.

The only way this can work is to manually switch between your ham antenna and an ATCS antenna with the filter and pre amp and you still won't be able to hear voice comms when in the "railroad" antenna configuration. Things could also get pretty ugly if you forget to make the switch and try to transmit into your ATCS pre amp by mistake.

In conclusion, while the IC-2720H does indeed work for ATCS experimentation ultimately there are better ways to go.

VHF Sensitivity: 0.18 uV
UHF Sensitivity: 0.18 uV
VHF Selectivity: 6 kHz / -6 dB, 20 kHz / -60 dB
UHF Selectivity: 6 kHz / -6 dB, 20 kHz / -60 dB
Power: 12 V / 1.8 A
Computer interface: Yes
Discriminator tap: Standard, DIN connector
Approximate Price:

-- Main.DanRapak - 10 Sep 2007



Excellent for VHF voice comms and ARES. Only avg for 900 MHz ATCS.

VHF Sensitivity:
UHF Sensitivity:
VHF Selectivity:
UHF Selectivity:
Power: 12 V
Computer interface: Yes
Discriminator tap: Standard
Approximate Price: $500 to $600 new

-- Main.DanRapak - 12 Sep 2007 (from the ATCS Yahoo! Database)

MDS (GE / Microwave Data Systems)

Radios produced by GE / Microwave Data Systems are purpose-built data receivers. By all accounts, they are the "gold standard" for the reception of ATCS data signals. They operate on 12 VDC and several models are in use by ATCS monitors. The radios of interest are programmable by way of software readily available on the MDS web site. You will need to register in order to download the software, but the software is free.

(alternate: [[1]] )

You will need to fabricate a special cable to interface the radio to your computer for programming. Do not try to program the radio using a standard, off-the-shelf RS-232 cable! Damage to the radio and/or your computer could result!

Note also that there is more involved than simply reprogramming the radio. The MDS radios are intended for use in the 900 MHz band. Depending on the model, your radio may not tune down to the ATCS MCP frequencies. Some modifications and / or adjustments will likely be necessary including retuning the helical resonators that are part of the input circuitry of the radio. This requires some reasonably sophisticated test equipment. Even if you are technically proficient, you may find it necessary to send the radio to someone with the proper tools for this realignment.

One very important note: The MDS radios are purpose built to relay data by radio. They do indeed squirt RS-232 data out of the 25 pin D connector. However - odd as it may sound - you cannot use the data coming out of the radio's RS-232 serial port for ATCSMon! Instead, you must use the unfiltered (discriminator) audio output to feed an iMic or sound card and simply decode the data as you would from any other radio.

The reason for this seemingly odd situation is that the method used by ATCS systems to transmit the data and the method used by the MDS radios are not compatible. There is a whole technical explanation involving transmission protocols and error correction schemes, but essentially what it boils down to is that one radio system speaks English while the other system speaks Chinese. If you only speak English and you pick up the phone to order a pizza, but you call someone who only speaks Chinese, you ain't gonna get your pie. Similarly, if you send a byte of data with a value of 139 with an "English speaking" radio, you won't get a byte with a value of 139 coming out of the "Chinese speaking" radio. What you get is jibberish.

But when we use the MDS radio's discriminator output, all of the radio's internal data processing becomes moot. The discriminator becomes our equivalent of the Star Trek universal translator, converting everything to "discriminator-talk" - a language we all know how to understand.

Since we are using the discriminator audio from the radio rather than the RS-232 data, an additional modification or adjustment is needed to force the squelch open as required by ATCSMon. (See the link to Larry Gabriel's web site, below.)

Discriminator tap: Units with a 9600 baud modem board have raw, unfiltered audio present on the radio's 25 pin D connector and require no further modification for discriminator audio. However, radios that do NOT have a 9600 baud modem board only have filtered audio on the D connector. These radios will require a modification to access the discriminator audio.


This radio is crystal controlled and cannot easily be converted to ATCS use.

MDS-1000u (Note the "u" suffix)

Fully programmable and adaptable for use with ATCSMon. It can be used on both BCP and MCP frequencies. See Larry Gabriel's excellent treatise on the MDS-1000u / MDS-9600 at: (temporarily out of service as of 1/2009, ask for info on the Yahoo forum)

The "Installation, Operation and Field Maintenance" manual for the MDS-1000u / MDS-9600 is available at:

(alternate: )

VHF Sensitivity: NA
UHF Sensitivity: -117 uV [9]
VHF Selectivity: NA
UHF Selectivity: 12.5 kHz / -60 dB, 25 kHz / -70 dB [9]
Power: 12 V / 120 mA
Computer interface: Yes
Discriminator tap: Depends on options installed
Approximate Price: $50 to $100 used


Programmable and adaptable for use with ATCSMon. However, while this radio will work well for BCP channels, and some MCP channels (928/932Mhz), it cannot be used for ATCS MCP (896/897Mhz band) reception as the tuner will not lock to frequencies in that range. The "Installation and Operation" manual for the 2310A is available at: and

VHF Sensitivity: NA
UHF Sensitivity: [9]
VHF Selectivity: NA
UHF Selectivity:
Power: 12 V / 80 mA
Computer interface: Yes
Discriminator tap: Standard, 25 pin D connector
Approximate Price: $30 to $50 used


Same radio as the MDS-1000u, but fitted with a 9600 baud internal modem. See the MDS-1000u above for more information.

VHF Sensitivity: NA
UHF Sensitivity: -117 uV [9]
VHF Selectivity: NA
UHF Selectivity: 12.5 kHz / -60 dB, 25 kHz / -70 dB [9]
Power: 12 V / 120 mA
Computer interface: Yes
Discriminator tap: Standard, 25 pin D connector
Approximate Price: $50 to $100 used

Motorola Maxtrac


The Motorola MaxTrac series radios are End Of Life by Motorola. However, they are plentful on eBay. Motorola produced these radios in the early to mid 90s, and they were extremely popular in the public safety sector. In the second hand market, they have become popular with Amateur Radio operators in mobile applications, as well as in repeaters. MaxTrac's come in several models with different frequency ranges, output power (not applicable for our application), firmware types (affects features (trunking or not) and channel count), and available channels.

General Specifications (applies to all models)

  • Input power: 12v DC
  • Dimensions: 7"W x 2"H x 10"D (depth depends on specific model)
  • DiscriminatorTap Difficulty: Easy (with 16pin connector, which is standard on all 900Mhz models)

Connectors to make your own power cables can be found in the trailer wiring section of your local auto parts stores. The bare male pin on the radio half of the connector is positive. When making your power cable, you are strongly advised to install an in-line fuse. These radios do not have an internal fuse, so more serious internal damage can occur. In receive mode these radios draw 400 milliamps, but should be fused for 1.5 amps.

Radio prices

A quick spotting feature of these radios are the 5 white buttons on the front panel and the 16 pin connector on the back, then check the model number on the sticker.

Avoid any radios that have 5 pin connectors on the rear. The models below are all known to have 16 pin connectors.

Most radios come with the 16 pin audio plug installed, but if missing, a replacement plug is Motorola part number HLN9242A. It costs about $4. Alternatively you can use a 16 pin ribbon cable type plug found at most electronics parts or electronic surplus stores. However the non-Motorola parts will not have a strain relief and with field use the audio cable may break.

800Mhz ATCS (MCP)

The 800 mhz trunked radios normally operate in the 850 mhz range. When they have their receive filters changed out for ones in the 896 range, these radios make good MCP radios. Most of the radios below can be had for about $35 on eBay, and the modifications cost about another $35.

D35 series radios have 15 watt transmitters and 16 pin data connectors

  • D35MWA5GB4AK - 1 Ch for ATCS
  • D35MWA5GB6AK - 2 Ch for ATCS
  • D35MWA5GB7AK - 10 ch for ATCS
  • D35MWA5GC3AK - 8 ch for ATCS
  • D35MWA5GC5AK - 8 ch for ATCS
  • D35MWA5GC6AK - 8 ch for ATCS
  • D35MWA5GF6AK - presumed to have 8 ch for ATCS

D45 series radios have 35 watt transmitters and 16 pin data connectors

  • D45MWA5GB4AK - 1 ch for ATCS
  • D45MWA5GB6AK - 2 ch for ATCS - has 5 pin data connector
  • D45MWA5GB7AK - 10 Ch for ATCS
  • D45MWA5GC3AK - 8 ch for ATCS
  • D45MWA5GC5AK - 8 ch for ATCS

LTR (?) Models

  • M07UJQ6FU1AN - 10 ch for ATCS

900Mhz ATCS (BCP)

These 900 mhz trunked radios are designed and built to receive the 935 mhz range. Almost directly out of the box, these radios make good BCP radios. The only changes necessary, other than reprogramming, is to move a jumper and make up the data cable. These radios can be purchased on eBay for about $75-$115 range. These radios demand a higher price because they can also be easily modified for Ham Radio use.

D27 series radios have 15 watt transmitters and 16 pin data connectors

  • D27MWA5GB4AK - 1 ch for ATCS
  • D27MWA5GB6AK - 2 ch for ATCS
  • D27MWA5GB7AK - 10 ch for ATCS*

D37 series radios have 35 watt transmitters and 16 pin data connectors

  • D37MWA5GB4AK - 1 ch for ATCS
  • D37MWA5GB6AK - 2 ch for ATCS
  • D37MWA5GB7AK - 10 ch for ATCS*

*With a firmware change (to conventional firmware P/N FVN4019A v30.03) this model is able to handle 16 channels

900Mhz Proprietary Protocols (Genisys/SCS 128/GE MCP)

The following radios can be used±:

D27 series radios have 15 watt transmitters and 16 pin data connectors

  • D27MWA5GB4AK - 1 ch for ATCS
  • D27MWA5GB6AK - 2 ch for ATCS
  • D27MWA5GB7AK - 10 ch for ATCS*

D37 series radios have 35 watt transmitters and 16 pin data connectors

  • D37MWA5GB4AK - 1 ch for ATCS
  • D37MWA5GB6AK - 2 ch for ATCS
  • D37MWA5GB7AK - 10 ch for ATCS*

*With a firmware change (to conventional firmware P/N FVN4019A v30.03) this model is able to handle 16 channels


±Some modifications required to be able to hear the 928Mhz frequencies commonly used for this purpose.

SCS 128 /Genisys

±Some modifications required to be able to hear the 932Mhz frequencies commonly used for this purpose.

900Mhz Proprietary Protocols (Genisys/SCS 128/GE BCP)

The following radios might be used±:

D27 series radios have 15 watt transmitters and 16 pin data connectors

  • D27MWA5GB4AK - 1 ch for ATCS
  • D27MWA5GB6AK - 2 ch for ATCS
  • D27MWA5GB7AK - 10 ch for ATCS*

D37 series radios have 35 watt transmitters and 16 pin data connectors

  • D37MWA5GB4AK - 1 ch for ATCS
  • D37MWA5GB6AK - 2 ch for ATCS
  • D37MWA5GB7AK - 10 ch for ATCS*

*With a firmware change (to conventional firmware P/N FVN4019A v30.03) this model is able to handle 16 channels


±Significant modifications would be required to extend the abilities of this radio to allow it to tune to the 952Mhz band, where (on the Union Pacific) some of the BCP transmitters are tuned to transmit on.

SCS 128/Genisys

±Significant modifications may be required to extend the already existing, partial ability, of this radio to allow it to fully tune to the 941Mhz band, where (on the Union Pacific) some of the BCP transmitters are tuned to transmit on.


For ARES/VHF Reception, you're looking for model numbers that start with D33 or D43. These are typically a little more expensive on eBay since they require no modifications to work in the 2m Ham Radio band. Note that there are two different frequency ranges available, and you can't tell from the model number which one it is, however, they both cover the AAR band. The frequency ranges are 136-162 Mhz, and 146-174 Mhz. The 146-174 Mhz radios seem to be much more availabe on eBay. Prices range from $100-$200.

Required Modifications


  • 16 Pin MaxTracs:

The data out pins on the 16 pin connector are Pin 11 Data Audio, Pin 7 Data Audio ground. Pins 15 & 16 are jumpered to be able to use the interior speaker for monitoring. No external filtering capacitor is needed, one is already present inside the radio. See additional steps below.

  • 5 Pin (or "no pin"):

If you don't have a 16 Pin connector on the back, you'll have to crack your radio open to add a discriminator output. Details are on BatLabs excellent site. Look in the MaxTrac section, under "9600 Packet".

A jumper needs to be moved to allow ATCS data to be passed unfiltered to the rear 16 pin plug. It is on the bottom of the radio, right behind the two large plugs connecting to the control head. The jumper that needs to be moved is JU808. When this jumper is in position B it processes the audio thru a filtering network which will prevent ATCS data from being passed. When it is moved to position A, the data audio is passed directly to the 16 pin connector on the rear of the radio. In lieu of a jumper, a 100k ohm pot can be placed across the jumpered pins to act as a volume control for those sound card applications that lack a line input but instead have a mike input which requires a significantly lower audio level. The bottom of the pot goes to ground, the top of the pot goes to the right pin, the wiper goes to the middle pin which is connected to pin 11 on the back of the radio.

Volume Modifications

From the factory, these radios have a minimum volume setting. If it desired to be able to fully turn down the volume so as not to hear the data bursts, there is a modification to handle this problem. There is a fixed value resistor accross two of the legs of the radios front panel audio volume pot. Add a wire that bridges to two ends of this fixed value resistor. Now you will be able to turn down the volume all of the way.

Frequency Modifications

In order to receive the standard ATCS BCP Frequencies, you'll need to make some small modifications to the radio. These modifications are detailed at BatLabs. Look in the MaxTrac section, under "900Mhz Amateur Radio Conversion"

For ATCS MCP frequencies, major modifications to the radio are necessary. There is a member of the ATCS Yahoo Groups that makes these modifications, and will sell you a MaxTrac MCP radio for a nominal fee. Ask the list for details.

No modifications are required to receive VHF AAR Band transmissions.



These radios are programable, however the Radio Service Software (RSS) to do the programming costs about $400 from Motorola and you must sign a licensing agreement with Motorola prior to the sale. Motorola has a serious reputation for vigoriously persuing and prosecuting persons using boot-leg copies. It is therefore strongly recommended that you have a Commercial 2-Way Radio shop reprogram these radios. This can cost from $15 to $50 depending on the shop. However, a Motorola shop will only reprogram the 900 mhz radios. The 800 mhz radios need to be programmed outside of the official Motorola frequency ranges and normal Motorola 2-Way shops will not do this. We have members that have legitimate access to the necessary RSS and do offer there services for a nominal fee. They also have the parts on hand that need to be changed out in order to get this radio to operate out of band.

On eBay, some auctions will offer programming for a small fee, for free if you use the "Buy It Now" feature, etc. Details will usually be in the auction. Note that this would apply to the ARES/VHF radios only, since they won't be able to program the ATCS radios until the proper modifications have been performed. That is, of course, unless the radio you're buying has already been modified


The 900 mhz radios are programmed to the actual desired BCP frequency. In order for most of these radios to work properly they must contain at least one trunked channel (only with trunked firmware, this restriction is not applicable for conventional firmware equipped radios), which is then followed by the ATCS channels.

The 800 mhz radios need a bit more work. The fixed tuned front end filters must be removed and/or replaced with new adjustable or fixed front end filters. The VCO needs to be modified as described in the Batlabs site, and special programming must be done using specially modified RSS software. This work must be performed by someone who is familiar with the work that needs to be done. Most likely either another hobbyist or ask the group.

If you want to listen to the ATCS MCP (896/897Mhz) and the ATCS BCP (935/936Mhz) sides of the data,

simultaneously, you must, acquire 2 radios, one 800 mhz and one 900 mhz.

900Mhz Proprietary Protocols (Genisys/SCS 128/GE MCP/BCP)

If you want to listen to the MCP (928 or 932Mhz) and the BCP (941 or 952Mhz) sides of the data,

simultaneously, you will need 2 radios, with one being heavily modified.

RCL Protocol Summary

MaxTrac radios have been used successfully with the following protocols/frequencies. Note that the ratings are subjective.

ATCS (800 Mhz)

  • Sensitivity: Excellent
  • Selectivity: Excellent

These radios work fantastically on both BCP and MCP frequencies, and compare equally to the Microwave Data Systems 1000u radios, which are considered the "gold standard" for ATCS radios. Note that these radios won't tune the MCP or BCP frequencies "out of the box", see the Modificaitons section above.


  • Sensitivity: Excellent
  • Selectivity: Excellent

These radios are among the best I have used for reception of BNSF's ARES protocol, which is transmitted/received on the standard AAR VHF band (roughly 160-162 Mhz). These radios also make great voice radios, you may find yourself purchasing another radio to replace your scanner. :)

-- Main.BrianSwan - 25 Oct 2006

Antenna Systems

ATCS signals are rather weak when compared the regular railroad VHF voice communications. Any antenna system chosen should provide at least 3 db of gain and be rated for the frequency range in question. The "dB" scale is logarithmic. Therefore, each 3 db of stated gain in effect doubles your signal strength. Note the phrase "antenna system". This is important because it includes the antenna cabling as well. The greatest antenna and the best receiver in the world won't be worth a darn if they're connected by a crummy piece of antenna cable. This is especially true at ATCS frequencies. The higher in frequency you go (and ATCS is up there) the more the signal is weakened by the antenna cable. Therefore its crucial to not only use a good antenna, but good antenna cable as well. (See the section on antenna cables below.)

There are two basic types of antennas: omni-directional and directional. Omni-directional antennas receive signals from all directions. These are always mounted vertically.

Directional antennas concentrate their receiving power in a given direction, acting kind of like using a funnel to catch as much signal as possible. There are several different types of directional antennas, the Yagi being the most popular for ATCS frequencies. Typically, a Yagi antenna looks somewhat like a drawing of a Christmas tree. They should be oriented so that the trunk of the tree is horizontal, with it's branches extending vertically, and the top of the tree pointed at the desired target.

Some antennas are available from radio surplus dealers like Fair Radio in Lima, OH. Antenna manufacturers include: Antenna Specialists, Antenex, Astron Antenna Co., Cushcraft, Celwave, Scala Electronics, Larson, Maxrad, Decibel Products, Sinclair, Telewave and others. Antennas purchased new will range from $35 to $200 depending on manufacturer and model. Of course there's eBay as well. Do a search for "800 MHz" and "antenna" and/or "900 MHz" and "antenna" and see what comes up.

There's a database of Antennas on the ATCS Monitor Yahoo group (Yahoo login required). For each antenna there's a user-submitted evaluation and rating showing receive quality, cost estimate, etc. More detailed information may be shown here on the Wiki, as people contribute.

Antenna Gain

If you want details, see Wikipedia for something a bit meatier than this overview.

What do the various Decibel (dB) specifications mean?

dB = relative gain in power over a previous measurement (a 9 dBd antenna represents 3dB of gain over a 6 dBd antenna) dBm = absolute xmit or receive power, translatable to mW (output power is 100mW or 20dBm) dBd = gain relative to a dipole (radiates roughly in a single plane (frisbee)) dBi = gain relative to an isotropic antenna (radiates in all directions (ball))

The isotropic antenna is effectively imaginary since nothing radiates in all directions equally. Just trust that it's the bottom level baseline of all antennas.

The more you direct energy, the higher the gain in a given direction. So a 6 dBd antenna has more gain than a 6 dBi antenna. The starting reference for the 6 dBd was in the horizontal plane, which is what we're after (since there aren't many MCPs above or below us).

Mobile Antennas

General considerations

Mobile antennas are always omni directional and are always mounted vertically. For this reason, clearances need to be a consideration. You don't want to pop a light bulb every time you pull into your garage. Likewise, you don't want to leave a trail of shattered glass from florescent light fixtures when you pull into that public parking garage or when you go through the McDonalds drive-thru. If clearances are a problem, there are two options available.

The first is the flip-over antenna. These antennas allow you to manually lift and tilt the antenna horizontally by way of a spring mechanism. If you're heading into that parking garage, you stop, get out of the car, tip the antenna over and continue on into the garage.

The second option is a motorized mount that will tip your antenna over at the flip of a switch, allowing it to lay flat on your roof. (See power mounts below.) Of course no matter which option you choose, you've got to remember to tip the antenna over! :)

If you don't have to worry about light bulbs, you may still be concerned about what happens when your antenna thwacks a tree branch at 40 mph. To help keep your antenna from being destroyed you may want to consider antennas that incorporate a spring as part of the antenna structure. Whether you have a fold over antenna or not, the spring will flex and hopefully keep the antenna in one piece.

Another consideration is grounding. Mobile antennas come in two basic flavors: Elevated feed and grounded base. An elevated feed antenna will work fine without being grounded to the body of your car. This means the antenna can be fastened to a magnetic mount that simply sticks to the body of your car or can mount on a luggage rack or trunk lip / hatchback lip with the appropriate mount, even though no ground is provided.

A grounded base antenna actually uses the body of the car as part of the antenna and must have a good ground connection to the body of the car. This is an important consideration when using a clamp-on mount (such as a luggage rack mount) or a motorized mount since neither type provides a particularly good ground connection. (I have two motorized mounts on my car and opted to add a flexible grounding braid made of heavy copper solder wick to each one. That way, it doesn't matter what type of antenna I use now or in the future. The ground connection will be there.)

A final consideration is the type of mount a particular antenna uses. Some antennas bolt directly through a hole you drill in the car body or bolt through a hole in a luggage rack mount, clamp-on type body mount or a magnetic mount. However, many antennas are available with an NMO base. In this case, you purchase an NMO mount and it is the NMO mount that bolts through the car body or through the clamp-on mount. The antenna cable connects to the NMO mount and the antenna with the NMO base simply screws on to the NMO base. This allows you to easily change out an antenna in the future with no tools and no more modifications to your car body.

Re-use of old cell phone antennas for ATCS will be met with limited success. Generally they aren't quite tuned to the bands we want, but they may work better than your (equally inappropriate) VHF RR band voice antenna.

Maxrad BMAX9105(S)

Frequency range: 870 to 950 MHz Gain: 5 dB Good reports from one ATCS user so far. Adding the "S" suffix includes the optional spring. The mobile mount must be ordered seperately. Ground is not needed. Therefore the antenna will work fine on a flip over mount. -- Main.DanRapak - 11 Sep 2007

Pacific Wireless MA9-7N

Frequency range: 900 to 928 MHz Gain: 7 dB Height: 20 in As of this writing, the antenna is NOT available with an NMO mount. It is only available in a male "N" connector mount. The mobile mount must be ordered seperately. This antenna is in use as a base station antenna for an MCP server. It is mounted inside an attic and it still works great! Ground is not needed. Therefore the antenna will work fine on a flip over mount.

-- Main.DanRapak - 17 Nov 2007

Power Mounts (Mobile Antenna Accessory)

General considerations

Power mounts allow you to flip a mobile antenna horizonally, allowing it to lay flat when you pull into your garage or the drive through at the hamburger joint. This keeps the antenna from being damaged and keeps you from shattering overhead light bulbs.

An important point to remember when using a power mount is that a power mount does not provide a ground connection. If you are using an antenna that doesn't use the body of the vehicle as a part of the antenna that's fine. However, if you are using an antenna that used the vehicle's body as a ground plane, you need to make a seperate connection to ground the antenna to the body.

I have two power mounts on my car. One is for amateur radio / railroad voice monitoring. The other is for ATCS. In both cases, I made a ground connection from the base of the antenna on the power mount to the body of the card using a short length of heavy guage, copper solder wick. The solder wick provides a good solid ground connection while remaining flexible when the mount changes position.

Maldol MK30H

This power mount is very popular with the amateur radio community. It comes complete with cables, fuse line, control switch and a hatch back / trunk lip mount. Good reports from the ham radio community. Those that have had problems requiring parts report excellent customer service from Maldol.


A preamp for an antenna system does what it sounds like; it amplifies the signal before it gets to the radio, usually improving reception.

A preamp amplifies signal as well as noise, so this presents a quandary when in an urban area close to other transmitters in nearby bands. Urban areas are usually crowded with RF signals, which desensitize or "desense" the receiver. If we amplify that noise, we might not make the situation any better, or possibly worse. Unless you use a spectrum analyzer to figure out how bad the noise is in your area, you might just not know until you give it a try. If it's possible to reduce the amplification, you might find a "sweet spot" where you do more good than harm.

Since there's loss in the coax feedline between the antenna and the receiver, it's best to amplify the signal at its best point, which is as close to the antenna as possible. There are some preamps which allow DC power to be injected from inside the house, near the receiver, to power the amplifier mounted near the antenna on the mast. You can certainly still benefit from a preamp if it is mounted at the receiver end, however.

Here are some user-recommended preamp suppliers for ATCS use:

Here's some advice from Barry Baines about an Advanced Receiver model he likes:

_Note that the P160VDG (as well as the P900VDG) as a stock item is not designed for an outdoor mast mount. To use it outdoors, I would recommend the following:

1. Add the 'RF Choke Option" so that you can feed 12 VDC up the coax to power the preamp. The option adds around $5.00.

2. The preamp normally comes with female BNC connectors. Most people use female N-connectors, so if you decide to change the connectors, the cost will go up another $10.00. You'll need a jumper coax between your antenna and the preamp that will match the connections on the antenna and preamp input (an adapter may be needed).

3. The P190VDG is not weatherproof, so you'll need to create an enclosure. I use a 3" PVC pipe about 7" long with a PVC end cap on top. I paint it to protect the PVC from the sun's ultraviolet light and insert some window screen inside after inserting the preamp and coax to keep the bugs out (the screen is placed beneath the preamp at the bottom of the enclosure). Use a 5" stainless steel clamp to mount the PVC enclosure to the mast. The preamp will fit inside with one connector (the output) facing 'up' and the other (the input from the antenna) facing down, so use two right angle adapters to provide 180 degrees of 'twist' to the output connection so that the cable going to the shack can be attached to the preamp output without bending the coax.

4. Feeding 12 VDC up the coax to the preamp simplifies installation, but does require an 'injector' that will insert 12 VDC into the coax from your power supply to power the preamp. ARR sells an injector (DCINJ-N) for around $40.00. Specify the connector type that you plan to use-- female BNC or female N-Connector. You don't need to use the ARR injector if you care to build your own or use another brand. Connect the coax from the preamp to the 'preamp' side of the injector and connect a coax jumper to the 'receiver' side of the injector and your receiver.

As Dave noted, these preamps are for RECEIVE ONLY and will not tolerate ANY RF coming up the coax. If you receiving device is a transceiver, then I would not use a non-switching preamp without installing a 'RF protector' that is a device that is designed to prevent RF from going up the coax. Some transceivers will send an RF burst upon power up (not placed in transmit), so it can be relatively easy to fry a preamp with RF when using a receiving device capable of transmitting.

Barry Baines Westborough, MA_

Base Antennas

Coax Cabling

Minimizing signal loss at these frequencies is extremely important. Coax with 100% shielding is a must. While 1/2" hardline, sometimes refered to as Heliax, is certainly an excellent choice (at about $1 ft), the difficulty installing connectors and routing through our houses is usually enough of a deterant to have us choose a more flexible alternative. Two notable 3/8" diameter cables are available for about $.75 ft. They are Beldon 9913 and Times Microwave LMR-400. The LMR-400 is marginably the superior of the two. In addition to being used for normal radio installations, LMR-400 is also used internally in office buildings that are set up for wireless computer networks (2.4 ghz IEEE 802.11). Most good industrial electronic stores will have the 9913 in stock. Finding LMR-400, locally, will be a challenge for most people. There are about a dozen venders on the internet that offer stock lengths, cut to order lengths, and custom cable assemblies with connectors for a reasonable cost. Both of these cables have a larger than normal center conductor and require the special N connector designed for Beldon 9913. A Superflex version of LMR-400 is also available ($1 ft) At these frequencies, type N, BNC and TNC connectors are suitable. The use of PL-259, SO-238, Motorola, & RCA connectors is not advisable. Smaller diameter cable, of the RG-58 size, is available with stranded or solid center conductor, and combination copper wire and foil shielding for installations that require flexability. This cable is usually rated up to 1 ghz. Most manufacturers of mobile antennas supply this cable with their mounts. The use of premium Cable television cable with BNC or F connectors can be used. It should be noted that this cable is not the correct impedance for the radio and some degradation of signal will occur. If you intend to install your own N connectors, I would recommend that you buy at least one extra, and practice with it. If you have never installed one, it's a learning experience.

-- Main.BrianSwan - 29 Oct 2006