900 Mhz Technical Guide

A reference and informational page for all aspects of 900 Mhz transceivers including:
- Mobile radios
- Mobile antennas
- Control station radios & antennas
- Portable radios, chargers, batteries, etc.

The information about these radios was obtained both through personal experimentation and testing on the part of NI1U members and affiliates as well as that of the good folks on the Batlabs Forum.

Handy dBu to uV conversion table provided by Bob (WA1MIK)
dBm to uV

Mobile Radios

The Motorola GTX-900 is perhaps the simplest and cheapest way to get yourself on the air from your automobile.  This radio is capable of 900 Mhz amateur operation without any hardware modification. This makes it excellent for those who do not have the time or desire to make sometimes complicated hardware changes to the radio.  The only issue in the way is the programming software.  A very simple hex modification to the software is required to make it allow you to enter the normally “out of band” frequency. This software modification is VERY simple.  The “pre-modified” software can usually be found rather easily if you look around the net or e-mail the right person.  Of course, that would be “illegal” so act at your own risk.

The GTX is a small, dash mount radio.  There is no remote head setup for this radio, but it is so small, you almost don’t need it. The radio’s power output is about 12 Watts at the most and usually has excellent sensitivity on the receive side of things. The channel display is very similar to Maxtracs and GM300’s. It is a two digit LCD that can show 0-9 and A-D. Not as nice as a full alpha display found on the Spectra and MCS2000, but it also is about 1/2 the price. There are up/down buttons for changing channels as well as a button for scanning.  The rest of the buttons don’t do anything unless you are using a trunking system.  As far as it’s frequency capabilities, you do have a few limitations. You can not listen on a repeater input.  The receiver is deaf on 902.  It will receive and transmit on 927 so it will allow you to do direct.

The antenna connection is Mini-UHF, not a PL-259 like most ham radios. Don’t use adaptor to make it a PL259!  Solder the right connector onto the coax, it is worth it since you need to have as little loss as possible.

The accessory connector is a standard Motorola 16 pin block. Pin 10 is “Ignition Sense” and you will need +12 on this pin for the radio to turn on.  You can also jumper the ignition sense pin to the Emergency Alarm Output pin (pin 4) which has a pullup resistor to 12 volts.  You will need to jumper two pins (13 & 16) in order for the front panel speaker to function.  The point being, this radio will not function at all without the accessory connector. If you find a radio for sale that doesn’t have the connector, you will need to buy one separately.  It should also be noted that the radio will go into monitor mode unless the mic is on hook or you ground the hook pin on the accessory connector.  There are no RSS settings to bypass these functions.

In your search for a GTX mobile radio, you will find there are lots of different model numbers.  You will need a radio with a model number beginning with M11W. (H11W would be a handheld GTX by the way, more info about that down below).

Bob, WA1MIK, has been kind enough to supply NI1U with excerpts from the GTX Mobile radio service manual.  If your radio needs help, this should guide you through it.

GTX Mobile Alignment
GTX Mobile accessory plug pin-out and information
GTX Model Breakdown Chart
GTX Model Specific Page on Batlabs
GTX Info Page on Repeater-Builder

The Motorola Spectra is perhaps the grand daddy of all 900 Mhz mobile radios.  Actually, it is probably one the best mobile radios on ANY band.  However, along with its greatness comes a little more complexity.  To make a 900 Mhz Spectra function on the ham spit of 900, it needs a hardware modification. The VCO will not lock without this.  It's not impossible but unless you are handy with a soldering iron and want a good project, you might want to find one already modified or fall back to the GTX.  Of course, it also requires a software modification which again is not difficult.  Oh yeah, the Spectra will cost a little more than a GTX.

The Spectra has a few advantages over the GTX. First, you can get a remote head kit which might be a requirement if you have a small car.  You also get an alphanumeric LCD display. So, you can have it display “NI1U RPT” rather than just “01”.  The options as far as connecting accessories are much broader.  There are tons of Systems 9000 accessories for the Spectra and many of the outputs are programmable.  The Spectra can also be had in a high power 35 watt model. You won’t get that out of a GTX no matter how hard you try. Of course, you have all the goodies like MDC1200, call alerts, DTMF, etc. etc.  You might even be able to find MODAT for it!!

If you want to a get 900 Spectra but don’t want to have to do the mods and programming, you can contact Jim, KA3IDN. This gentleman from Nevada mass produces Spectras for ham radio use. It will come modified and preprogrammed by zone with every 900 amateur repeater in the country.  He is a professional Motorola tech so you will be getting something reliable. Several NI1U members have Spectras from him and are quite happy.

See the Batlabs Spectra model specific page as well as the ongoing Repeater-Builder Spectra page for lots more technical details.

The Motorola MCS2000 mobile radio is another high quality 900 Mhz unit.  The benefits vs a GTX are basically the same as the Spectra. Higher power, alpha display, multiple types of control heads, remote head ability, etc.  You can get a good 30+ watts out of the MCS2000 as opposed to the GTX’s 12 watt absolute max.  Once again, your going to want the model number with a W in it (HXXW). This is the 900 Mhz model. No independent testing has been done by NI1U however many others on the net report this radio as an excellent choice. Unfortunately, you can also expect it to be rather expensive. The Batlabs model specific page for the MCS2000, as usual, has loads of information.

If anyone has pictures of a 900 Mhz MCS2000 they would be be willing to allow on this web site, I am looking for one.  A radio installed in a vehicle would be perfect!  Contact me if you can help!

Mobile Antennas

Antennas will make or break your mobile installation.  Loss builds up quickly at 900 so you need high quality coax.  None of this Radio Shack crap.  Comtelco Microloss 900 is good stuff and can be found at www.rfwiz.com. Nothing beats an NMO mount in the roof or trunk lid. Magnetic mounts are highly discouraged.  Make sure the NMO is not the kind with the huge center contact.  The L-Bracket mount on the front or rear fender is probably a good second choice if drilling a hole in the roof or trunk lid is not your cup of tea. Your radiation pattern will be a little skewed but if your repeater has good coverage, you will probably be just fine. An advantage of this over the roof mount is the height. If you have a low clearance garage and need a gain antenna, you can fit one on this mount.

For the actual antenna, there are a few common options.  The simplest and cheapest is a 3” 1/4 wave. There is no gain but it may provide the optimum radiation pattern, especially in hilly or urban terrain. It is also really small so it won’t affect clearance in garages.  Another option is one of these upside down shot glass or hockey puck type low profile antennas. They are shorter than the 1/4 wave and actually do provide some gain.  Testing has shown they perform even better than a 3dB gain whip antenna.  If you want the look of a whip antenna, you can get gain ranging from 3dB to 5dB from the various whip designs. There are glass mount antennas for those totally opposed to attaching anything to the body of their vehicle. However, these are a compromise compared to any kind of body mounted antenna.  A glass mount should be a last resort.

Portable Radios

The Motorola GTX-900 Portable is the handheld version of the GTX mobile.  Basically, it is the same radio with less watts, and a battery. It has an 8-digit LCD display that can show number 0-9 and A-B-C-D.  It also has a DTMF keypad and speed-dial memories Just like its mobile counterpart, the GTX portable is deaf in the 902 repeater input range.  You can not use the GTX to monitor a repeater input! It will obviously TX fine in 902, but forget receiving.  It will however perform just fine in transmit and receive in the 927 range for repeater outputs.  This means you can use it on direct! This a very basic radio and has no bells and whistles. You aren’t going to find MDC1200, MODAT, zones, etc. on a GTX. However, with basic comes simple and inexpensive.  It uses the same RSS as the GTX mobile and requires the software modification just like the mobile. If you want a basic inexpensive portable that doesn’t require advanced service and modification, the GTX portable is probably the radio for you. There are boat loads of GTX portable radios on eBay. You can also find tons of batteries and chargers and all dirt cheap.  You can probably thank NexTel for that since they put so many 900 Mhz SMRs out of business, but that’s a whole other debate :). The GTX uses the same programming cable, chargers, batteries, and accessories as the Motorola GP300 which makes it even easier to find stuff.

Once again, Bob (WA1MIK) has provided excerpts from the GTX service manual in case your radio needs “help”.

The Motorola MTS2000 portable radio also comes in a 900 Mhz model. There are no modifications necessary for this radio.  A model number with a W (HXXW) is a 900 Mhz model radio  Testing by a Batlabs forum member has show favorable results. Sensitivity and power output were all very good. The only problem noted was the squelch occasionally hanging open.  If your using PL/DPL, this would be an invisible problem. The MTS2000 itself is a very high quality radio. They can take a beating are very versatile as far as options and accessories. The radio can do zones, multiple scan options, MDC1200 and other PTT-ID formats, etc. It is available with a top display, a front display, or a DTMF keypad & front display. Don’t expect to find one cheap though. Along with their appeal comes a price. Brand new, these radios can push $1,500 depending on options.

See the Batlabs model specific page for lots of general information on the MTS2000

Testing by a Batlabs forum member showed the LTS2000 and LCS2000 to not be worthwhile investments for 900 Mhz.  Both have very poor sensitivity and very low TX power output.  For that reason, we won’t be spending any more time on these models here!

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