Batboard

Just wondering if anybody has tried using Maxtracs/GM300/Radius mobiles as continuous duty links by "bypassing" the PA and just using the exciter? It seems to me the "exciter" power is about 200 mw out, should be enuf power from mountain to mountain.
But I wonder if something would fry?

I'm using an 800 MHz MaxTrac chassis in my Nucleus 2 transmitter to provide transmit audio/PL. I pulled the PA and the radio doesn't seem to mind one bit.

Another person is using the same radio for the same purpose and he even pulled the RF board, so all he has is the chassis, logic board, and control head, and it's STILL working just fine for the purpose we want.

When the microprocessor thinks the radio has been transmitting long enough, it will start reducing the output power by lowering the controlled B+ voltage from the logic board. As far as I can tell, that's the only thing it does. You can use a pot to manually control the output power and keep the transmitter operating - that works great on 900 MHz MaxTracs.

The radios are just not designed for more than 5-10% transmit duty cycle when used in a mobile environment. They will get hot, and will eventually burn up if they remain operating at full power.

The only real difficulty with using the exciter output from the RF board is finding a connector and coax to mate with the output jack. I have seen people snip one of the two connector/coax going to the PA and use that. A new connector and about 4 inches of coax costs about $5 from Motorola. I did find the manufacturer of the connector so you could buy just that, if you could get a small quantity.

GM300s seem to be a bit more finnicky since they also monitor the temperature of the PA with a thermistor, so if you unplug the PA, you lose that sensor input as well.

Bob M.

I too have used them as repeater exciters. And even made a 900 mhz repeater out of a pair, but they really don't get used at real continuous duty.
I sure have seen them get HOT just being used as a mobile.
Maybe I will just have to try it.
I like the various mods for keeping the PA up to power, but, again, thats not what I need to do here.
I am thinking of using a continuous remote RX link to minimize the latency on voted recievers due to the links coming up and down.

Well, it might work, but I've never tried holding the PTT button and turning on a radio to see if it will power up and begin transmitting immediately, as would happen if you had a power failure. That's the only thing I can think about that could be a problem with a continuous transmission, not to mention the wastefulness of keeping a signal on the air all the time. However, that is one way to eliminate the latency.

Bob M.

Hmmmm... maybe continuous during the drive times, and COR the other hours.....
Thanks for the replies, I guess I will have to just try it.

Set it up with a really long "hang" timer that will keep the link transmitter on for, say 15-30 minutes after the receiver drops out. At least it won't be on 24/7. The 1/2 second it will take for the link to come on, and the receiver to notice it, won't hurt most people's signals.

Bob M.

That would work, but causes another problem working around the status tone.

Four public safety repeaters in the county here use RF links with a continous signal from the satellite receivers back to the main site. The radios transmitting from the satellites back to the main sites here are four-channel M1225s, with a funky little heatsink mod, and turned down to somewhere between 250 and 500 mW. Radios stay on 24/7, 365 days a year. They are all on battery backup, so they won't turn off unless the power is out for a really long time. I'm not sure, but I think that these are 45 watt radios (or did /\/\other make a 25 watt version of the UHF M1225?), with the PA still attached and working.

You could synthesize the status tone at the link receiving (voter) site so it comes on when the radio loses its input carrier (and loses the status tone from the remote receiver through the link). I realize it's a royal pain, and then you'd need another kludge ontop of this one.

It's difficult to have an indication of something that's not present.

Bob M.