Tuesday, May 20, 2008

Experimental 900 MHz Ham Radio Repeater

What I like about 900 MHz is that there are no off the self radios that you can buy. That may sound crazy.  But I wanted a project where I could get back to the basics of the hobby and learn a few things in the progress.

I created a 900 MHz overview on my the projects part of my webpage.  I hope this can prove to be a great overview of getting going on this band. I have received many compliments from others already.

Below is the start of documentation to a 900 MHz repeater that I have been working on.

It uses a 800 MHz Motorola Maxtrac as a receiver that has been converted to 900 MHz. See the Repeater Builders page on this: Converting an 800 MHz MaxTrac to a 902 MHz Repeater Receiver.  These are cheap and have great receive sensitivity. There are people on the AR902 list that sell them, with the conversion to conventional firmware already done. In some cases they have the filters pre changed or jumped too. These converted receivers can usually be found for about $60. On mine I replaced the filters with 915 MHz centered Murata filters rather than shorting as this provides further rejection from out of band pagers and what not.

A receive signal COR or CAS logic had to be exported so that you can interface the radio to a controller or have some signaling to be able to make the other radio go into transmit on reception of a signal. I like the Motorola M120, Maxtrac, GTX and GM300 line of radios as picking off a COR and exporting receive audio are all fairly simple and these mods can be work universally so you can mix and match these types of radios. You don't need the expanded 16 pin logic models. All the exports can be done using un-used microphone pins.

The transmitter is a 15 watt Maxtrac. These are good fairly rugged transmitters when compared to the other options such as the GTX.

I extended the MaxTrac 900 MHz VCO Frequency Range using a conductive ink pen. Obviously these radios were not intended to operate on the ham freq's. Even the radio's that are capable of this ham band that do not necessitate hardware mods, the programming software doesn't recognize amateur frequencies. At the very least you have to hex edit the software. Even after you trick it into letting you enter those frequencies, many of the radios' have VCO locking problems and won't transmit (or receive) in various sections of the ham band with out some of these hardware mods.

I also realized that I may later need a fairly dynamic control of the output power to meet various (low) drive requirements for an amplifier. The Radio Service Software allows for small power adjustments, but generally those software controls are considered unstable beyond a 3 dB adjustment.  So manual power control mods were necessary.

The duplexers are TX/RX. Contrary to what most assume, you can tune a duplexer with nothing more than a HT as signal source and watt meter. You don't really need a fancy service monitor. It's always desirable for greater accuracy, but not necessary.

There is some good info here: http://www.seits.org/duplexer/duplexer.htm

When I need very week signal to test a receiver or antenna a police scanner can do the trick. If you can find the IF (intermediate frequency) of the scanner, you can add that (10.85 MHz in my case) to the intended freq that you need a signal on. You should have a very week "birdie" where you need it.

I'm using a very simple repeater controller. It's the Ham Gadgets ID-O-Matic Kit. It's very small and only $20. I am also still experimenting on simplex with devising my own more multi functioned Linux based controller.

I also am using a Janel Model PF849D 900 MHz 10 dB 26 volt preamp that I picked up at Grayslake for $10.

This system is On the Air on 927.5125/902.5125 with a Digital PL of 223. It's presently into a crummy Maxrad MN9155 5 dB 900 antenna that I have at 45 feet on my tower feed with LMR-400. I have to say I am rather impressed. This is pretty cool considering the antenna gain probably makes up for the coax loss at best.

For anyone who might be from Wisconsin, I want to let you know that at the WAR meeting back at the end of April, the ARCC bandplan was adopted for 900 MHz use in Wisconsin. It's works much like the SERA bandplan, which is the well known one that accommodates both 12 MHz & 25 MHz repeater splits. The advantage of the ARCC plan is that it doesn't interfere with any existing areas for other modes like SSB/week signal, etc.

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