Building A 900 MHz Repeater out of Motorola Maxtrac Radios
And other "surplus" equipment info!
NEW Surplus Info Added 6/11!! Check the bottom of the page!
The 900 MHz HAM band has become very popular in many parts of the country recently. Very much so in Northern California due to the 440 interference complaints from the USAF at Beale AFB and the dozens of UHF repeaters that have been booted off the air!.
Now that many 900 commercial radios are becoming available at reasonable costs like the Maxtrac, GTX, Spectra, LCS2000 etc, the band is becoming more affordable! Coverage is comparable to UHF or better!
This will cover information needed for the average "repeater builder" to put a 900 MHz repeater on the air using inexpensive Motorola Maxtrac radios. There are several different approaches to using these radios depending on personal preferences, controller types, audio needs etc.
The problem is that the 902-928 MHz band is full of "junk"! The primary users are ISM or "Industrial, Scientific and Medical" users. These are NON COMMUNICATION type equipment such as imaging machines, inventory control devices, industrial heating devices etc. Basically, anything that is NOT communications. The SECONDARY users are the licensed users such as HAM's and a lot of point to point commercial data transmitters. They are licensed and usually in the middle of the band away from the Amateur repeater inputs from 902-903 MHz. The biggest problem for HAM's is the non licensed users. These include wireless ISP's (Internet Service Providers), low power data transmitters like remote cameras, cordless phones, baby monitors, and tank monitors etc. These ALL fall under FCC Part 15 rules! Part 15 says "You must not cause any interference and you must accept any interference from other users". In other words, they are the "bottom of the food chain" as far as the FCC is concerned and have no real "rights" on the band. Unfortunately, in metropolitan areas, they are VERY ABUNDANT! Many of them run illegal power amplifiers and antenna gain. The FCC rules plainly state, 1W MAX TX power and 4W MAX ERP power for the Part 15 users. That means 1W into a 6DB gain antenna is the maximum legal limit! I personally know of about 40 of these systems in my area and about 60% of them are running illegal power or antenna configurations! Trying to hunt them all down and get them to comply is next to impossible. Luckily, the manufacturers of these Part 15 ISP devices are smart enough to give a little "Guard Band" at the very bottom and top of the band. The closer you can get your repeater input to the bottom of the band, the better. 902.0125 is the lowest frequency and the quietest! In metropolitan areas, it is pretty quiet below 902.100 and acceptable below 902.200. Once you get close to 902.300 and above, it begins to get very noisy! My first repeater attempt at 927.8625/902.8625 was a disaster. There was so much noise, it always sounded like "picket fencing" even on a strong signal! I moved to 927.0125 and it is stone quiet! Most of the other repeaters soon followed and now the entire bottom 200 KHz of the 900 band is full of repeaters! Click HERE for an easy method of testing for ISP/Spread Spectrum Interference
Many old repeaters in the East and Midwest use 12 MHz or other odd splits. I will not cover this topic as most areas are moving to the 25 MHz split as most of the commercial radios do the 25 MHz split easier!
Now…receiver choices. Many people prefer the 800 "talkaround" maxtrac as a receiver as they are more plentiful and cheaper than the 900 maxtrac. I personally prefer the 900 maxtrac as on 900 MHz, it is all 2.5 KHz deviation. The 800 maxtrac radio is designed for 4 KHz deviation. They do work well, but I am kind of picky and I can hear the difference. The 800 radios always sound a little "under modulated" on weak signals. Plus, 16 pin 800 talkaround maxtracs aren't that common. The 5 pin radios require some modifications to bring out the COR detect, PL detect, DISC audio, MIC IN and PTT connections that are standard on the rear 16 pin connector of ALL 900 maxtracs. So for the $10-20 more for the 900 radio, it's worth it to me.
It really doesn't matter what type of 900 maxtrac you use as you will end up converting it to a conventional radio anyway. Your only choice is power level. 12W (D27) 2ch maxtracs are easy and cheap to find! The 30W (D37) units are also abundant, but you will usually pay more. Either way, it doesn't matter which you use for an RX radio. The biggest mods you have to do is the conversion to a conventional radio (unless you have software capable of programming ONE conv channel in the radio and the radio is capable). This info is avail at http://www.repeater-builder.com/maxtrac/maxtrac-900-t2c.html . The second is modifying the VCO to receive at 902 MHz. This info is all available at http://www.repeater-builder.com/maxtrac/maxtrac-900vco-ext.html . Ignore the part about adding the 15V supply as you only need the radio to RX at that freq, not to cover the entire band. The only info you need for RX tuning is the part about the VCO tuning on the "left side" of the VCO. We'll cover the "right" side on the TX portion later. Once you have it converted and programmed on the frequency of choice, just tune the VCO using the conductive ink method described or do what I do, which is use "copper foil" and tack solder a small piece on the tuning tab until you get the lock voltage somewhere around 4V. NOTE: The tabs have a thin coating on them that must be scraped off before you can solder to it. I just use a small jewelers screwdriver. The easiest way I have found to remove the cover on the VCO is to use a small "culinary" or "pencil tip" torch and heat it at the top center just to the point where the solder melts and pop it off.
Now...the dreaded maxtrac front end filters. The 915 MHz replacements are hard to get anymore, but they weren't really good enough for a hilltop anyway. Remove the entire RF board and just remove the 938 MHz filters and place a jumper across the two points where the filters were, essentially bypassing them. Don't "short-out" the existing filters as I have found a 3-4db drop in sensitivity doing this vs. removing them completely. You will now be relying on your duplexers and any external bandpass filters to keep the radio's front end clean. Cellular multicoupler filters work excellent!
Instructions for the filter removal/replacement are HERE.
RX/RPT TX Side
SIMPLEX TX Side
Once you have your Maxtrac receiver up and running on your 902 rptr input frequency after doing all the mod listed on the repeater_builder website, now assuming you have access to the programming thru your "favorite Motorola Technician with legal software" of course, it's time to set up the audio and telemetry needed for your particular controller. There are two basic types of controllers. The type that use strictly discriminator audio like the Palomar style controller, and the type that doesn't care like Link-com and many other types. We will start with the "discriminator" type first.
The Palomar controller is very common on systems with many links and other functions and is a high quality, expensive and complicated controller. They have their own squelch circuits and COR sensing so all they really need from a receiver is discriminator audio and PL detect. The Maxtrac is perfect for this. All that needs to be done is to set JU551 to position "A". This makes pin 11 on the 16 pin connector discriminator audio. NOTE: The maxtrac radio apparently has a lot of "noise voltage" present on the receiver when there is no signal. This makes using the front panel control pad on the Palomar difficult as it cannot always overpower the noise from the receiver. This is true for ALL maxtracs, not just the 900 maxtrac. For the PL detect, either pin 8, pin 12 or pin 14 can be programmed in the RSS for this signal. The RSS will also program this signal to be active high or active low depending on your controller's preference. On the maxtrac, LOW means an open collector pulled to GND. HIGH means open collector open with TTL level 5V present on whichever pin you programmed.
Once connected to the Palomar, all you have to do is set levels like you would on any other new radio added to the system. Just remember, 900 MHz is HALF the deviation of all other bands! If you follow the directions in the Palomar manual, remember to set levels to 420 links accordingly. Example, 2.5 KHz into the 900 receiver is max deviation, so with 2.5 KHz into the 900 maxtrac receiver, you should set your line level to the 2.0V PP first, then you should be seeing 5.0 KHz deviation out on a 420 link radio. Just the opposite when going the other direction. 5 KHz into a 420 link radio = 2.5 KHZ deviation out of a 900 transmitter.
Let's start with something I am familiar with, the Link Communications controllers. These controllers range from a simple 2 port controller to exotic multi-port voice controllers. They can run either discriminator (flat) audio or de-emphasized (speaker) audio. If you are building just a simple repeater with maybe a remote base or just one link, it is fine running the de-emphasized audio out of the radio. This will mean changing JU551 to the "B" setting. This will change pin 11 to "squelched, de-emphasized" audio. You will also have to program pins 8, 12 and 14 so you have one that detects COR and the other detects PL/COR. Then connect them to the controller accordingly. Make sure the "de-emphasis" jumper on the Link-Com controller is NOT installed! Otherwise, this will result in very "mushy" audio.
Here's a little note on DE-EMPH vs. DISC audio. What you hear out of your speaker is filtered, DE-EMPH audio. What is done back at the transmitter is that the frequency response of ALL FM transmitters is raised or "Emphasized" as the audio frequency goes up from 300HZ to 3 KHz. Picture taking your equalizer on your stereo and setting the lowest frequency adjustment in the middle and the highest frequency adjustment to the top. This makes the transmitted audio sound "tinny". What happens at the receiver is just the opposite. It takes that audio and De-emphasizes it the same amount before it gets to your speaker so it sounds just like what was being spoken into the microphone. This makes the "HISS" of FM noise much less apparent! What comes from a radios discriminator is "raw" unfiltered audio. If it is being pre-emphasized at the other end, that is what will be coming out of the discriminator audio port, PL, hi frequency noise and all, plus it is usually unsquelched! What de-emphasized audio is, is the audio after it has been "rolled off" or "equalized" back to normal frequency response, cutting out all the high frequency noise and usually the RX PL tone also. Essentially, ALL transmitted FM audio is pre-emphasized. It's just how it passes thru your controller that makes a difference. There is a better explanation of FM de-emph and pre-emph audio here!
Now let's move on to the transmitter!
The TX or "right" side of the VCO will not need much tuning. Many will TX in the 927 range already, but they will be close to the end of their range, so it is a good idea to tune the VCO a little closer to 3-4V on the frequency you are going to use. When you program the radio, program it as a "SIMPLEX" or "Talkaround" frequency. In other words, if your repeater will transmit on 927.100, set it to both TX and RX on that frequency. Don't forget to program the TX PL also! NOTE: If you are using DISC audio, and something other than the repeater port of a Palomar controller, the RX PL will pass right thru to the Transmitter! With de-emph audio, the PL is stripped off before it gets to the controller so the transmitter will be able to correctly recreate the proper PL. The Palomar repeater port has a built in PL filter to remove the PL from the received audio.
Now all you have to do is connect the TX audio from the controller to either the FLAT TX AUDIO IN, pin 5 if you are running DISC receive audio, or MIC AUDIO, pin 2 if you are running de-emph RX audio. Now the PTT signal from the controller to pin 3.(GND to TX)
You just have to remember, if a repeater's RX audio = DISC audio, the TX audio MUST be "FLAT TX AUDIO" for the input! If RX audio = "DE-EMPH" audio, the TX audio input MUST be MIC input. The transmitter's microphone audio circuit will pre-emphasize the audio again before it is transmitted out the repeater's transmitter. "FLAT TX AUDIO" will NOT emphasize the transmitted audio. It is a direct connection to the modulator with no filtering or "equalizing".
Connecting DE-EMPH audio directly to FLAT TX AUDIO will result in very "MUSHY" audio. Connecting DISC audio to a MIC AUDIO input will result in very "tinny" audio.
Now get your friend with the great service monitor to help set levels. I have found that setting maximum deviation of 900 radios to around 3 KHz full deviation WITH PL makes them sound better than if it was set to 2.5 KHz max. Check your controller for input levels. Usually requires a scope and a certain P/P voltage at a certain deviation. Once that is set, inject a signal into the receiver with about 1 KHz tone at 1.5 KHz deviation and set the transmitter for the same deviation output. Now you are on the air!
NOTE!! New information on the 900 Maxtrac! The pin 11 audio out AND pin 2 Mic Input is ALWAYS FLAT!! There is NO de-emphasis or pre-emphasis on these pins, even when the jumper is in the "B" position!! This is because on the 900 Maxtrac only, the de-emphasis AND Pre-emphasis is done on the HEARCLEAR board in the head. With the jumper in the "B" position, the audio will still be PL filtered and squelch muted, just not de-emphasized as it is in all other Maxtracs. ALSO: The PIN 2 MIC input is ALSO FLAT!! There is NO Pre-emphasis on this pin! It is the same as Pin 5, for the most part. If you really need de-emphasized audio, there may be a way to pull it from the volume pot, but I haven't looked into this yet. If you want Pre-emphasized TX audio, you will have to connect to the microphone jack's Pin 5.
A NEW Option!! You can use the microphone jack for ALL RX connections! On 900 radios, PIN-1 on the microphone jack is UNUSED! (numbering is backwards from a standard RJ-45 connector). It is very simple to run a jumper from PIN-8 on the rear accy jack to PIN-1 on the MIC connector internally for your COR/PL/DPL detect. See photo above for the "PIN-8" solder point. PIN-8 on the MIC jack is line level, squelched, de-emphasized audio! It was originally intended for a "headset" but works fine if you are looking for good repeater audio! It is unaffected by the volume control. You can also do this to 800MHz and below Maxtracs, but will have to remove the BLUE wire running from the mic/vol board to the logic board and connect your COR detect to it. I have done this to many 420MHz 5 pin maxtracs with no problems. The audio is still present on pin-8. Now you can use the cheaper, easier to find 8 pin RJ45 style connectors for your RX connection system.
If using a lower band radio like 800MHz or lower with the 5-pin accy connector logic board, you will have to install a COR circuit as described in Repeater builder and be sure to ground mic pin-3 to activate the PL/DPL detection. More info and illustrations HERE.
Next we will discuss output power. Unless you only want about a 5-8W repeater, you will have to use an external amplifier. The maxtrac will NOT run at repeater duty cycles! About %30 of rated output is maximum output for a maxtrac as a repeater! So, a D27 may be OK at 5W MAX and a D37 may be OK at 10W, but the less you operate them at, the more reliable they will be. I have a D27 running as a repeater transmitter at 5W and it gets pretty warm without some airflow over it! You will probably have to do the manual power control mod found here to set the power that low and still be clean. The software power setting that low can sometimes cause spurious problems in the transmitter.
There are a lot of inexpensive options out there! Find someone that works in the cellular industry and see what they are throwing away lately! You won't believe it! Verizon uses mainly Motorola equipment and they have some great amps! Some of the Moto amps are capable of as much as 400W output!! These will run all day at 100W! AT&T uses mainly Ericsson, Lucent and Nortel equipment. The older Ericsson analog equipment used radios with removeable 50W amps and the Nortel stuff (recently shut down) uses SCALPA's...Single Channel Linear Power Amplifiers built by Powerwave and Spectrian. These are good for 50W easily and will easily do 70W with good airflow over them. Also, a lot of 900 MHz paging systems have been abandoned lately. (A good source for antennas too!) These amps are monsters! I recently aquired one only to find it is capable of 600W output continuous! There are also a lot of multi channel power amps available from these systems also. Most of these PA's are built by Powerwave or Spectrian. The Powerwave equipment is usually more easily modified whereas the Spectrian stuff is usually very well built but harder to modify due to some specialized transistors and other designs that make them more difficult.
Here are some pictures of what to look for at the swaps.
The Nortel (Powerwave) SCLPA. Capable of 50-70W @ 927 MHz continuous with a small fan. 24V @ <10A. Three stages can be fed with from <1W to 10W. There are at least THREE different versions of this type of amp. Look for an FCC ID that starts with "E67". All the other types will still work, but are sometimes harder to "dissect" into just a plain old RF amp without all the tons of control and monitoring circuitry in these things. Info on internal working and modifications of these can be found here.
The Nortel (Powerwave) MCPA. I use the PA board out of one of these and drive it with less than 1/2W and get 250W out @ 25A @ 28V on 902 MHz! More info HERE!
The big brother to the one on the left. Rated at twice the power of the unit on the left. I haven't dug into one of these yet, but guessing they are worth close to 500W! Again, 24V. HEAVY!! About 75 lbs!
The MONSTER!! 600W continuous @ 930 MHZ! It has twin 300W amps and instructions on how to separate the final PA onto a single 300W unit that would run all day at 150W as a repeater final PA. Around 1W drive!! Needs LOTS of airflow!! 28V @ ????Amps!
The Motorola 300W unit found in older Verizon 1st generation digital cell sites. Has THREE driver stages so you can feed this with anything from 1W to 25W depending on what stage you feed. The 150W unit looks very similar. A great site on these can be found HERE.
Most of the cellular equipment is designed to operate at 24-28V and up to 896 MHz. They will work fine at 927 with a little power reduction. Many have tuning points in them so you can peak them up at 927 if you want. I haven't really found it necessary yet.
I plan on eventually putting together a page with instructions on how to use all of these amps as a standalone power amplifier. The Nortel SCLPA info can be found here.
More info below on using cellular duplexers and multicoupler filters re-tuned to 900. They work great!
An Allgon multicoupler from an Ericsson cellular system. Contained in this package is a bandpass filter, preamp and power splitter. The BPF will tune to 902 with some modification and has very tight skirts!! I have also seen these with Cellwave filters in them. Those usually have type "N" connectors on them instead of the annoying TNC connectors found on most cellular equipment, and they tune easier!
These Narda units were used by AT&T Wireless back when CDPD was the hot item! There are a lot of these being thrown out! They will tune to the 900 band without modifying them, but it is a pain to tune them! IL usually is less than 1db and rejection is around 100db! They can also handle 200W. More info HERE!
Very nice antenna! 9 dbd and not too big! Excellent performance! I have been able to find 3 of them surplus for just trades and misc.
A Basic Repeater System Block Diagram.
New Find!! Filtered LNA for 900 from Two-Way Paging Systems!
A friend of mine said he found a two way shop throwing away a bunch of paging equipment, and it turns out, it was mostly 2-way paging which used 901-902 for an uplink! Below are the tower mounted downconverters and preamps that were found in the equipment. There were 8 RX antennas and downconverter/filter units pictured below on each system!
This is the unit, top and bottom. Note the L.O. input and the +5V power cable.
The filter is centered on 902.0 and has somewhat of a notch at cellular TX freqs and around 1db of loss!
See sweep below.
As you can see, these are far superior to the hard to find maxtrac "HAM" filters for repeater use!
As you can see, I don't like using the "conductive ink" method. I have found that just "dragging" a small "nipple" of solder off of the pad will bring the SIMPLEX TX ( used for a repeater TX) VCO voltage well within range. For the RX side, that will work if you are just going to use it as a standard radio, but for a the MAXTRAC to RX at 902, you will have to add a little extra material, either ink or an small "snowflake" of copper, to extend the vco down to 902.
To make these things work, you will have to get out your small tip soldering iron and a good magnifier! You will be using the preamp chip near the "INPUT" connector and taking the output of that device directly to the output connector. This will bypass the mixer and other amps in the unit. U used a very small piece of coax I happen to find in some junk satellite receivers. It is VERY small!!
Notice circled areas where I removed L15 and L8 to disconnect power to the other unused components in the box.
The chips are about 1db Nf at about 16-18db of gain! Way more gain that I need, so I put a 3db atten on the output just because I had some. I installed a total of 9db of atten and still saw no difference in sensitivity of the radio.
Now install a 7805 or 78L05 for a 5V source and you're ready to go! The specs on these amplifier chips can be found HERE.
The finished unit!
In it's original housing still attached to the amp with 7805 regulator and 3db atten on the output.
I use these in several of my 900 repeaters and the filters and amps work great! Sens is usually around 0.18uV at the antenna port of the duplexer!
Also notice I removed the "L.O. INPUT" connector.
Closeup of "bypass" coax connection to chip output.