Batboard

I am looking for a little help on repeaters.I am looking for a back up repeater that would be used very seldem.Its is only used for emergency traffic most transmissions are short.I have read alot about using 2 mobile radios and connecting them together.I have also read that if you do this with the motorola radios you take the risk of burning up the radios.I have a gm300 and m1225 I was going to trying this on but not to sure on the risk.has anyone done this and how did it work ect?

The big problem on connecting 2 mobiles back to back is that they are only made for a duty cycle
of around 5% TX. You go over that and you risk burning them up from heat. One solution is to
turn the TX power down to about 60% of the normal rated power. The problem with turning
down the power is that many of the solid state PA units become unstable and start to generate
trash and spurs all over the place. So you need to have a spectrum analyzer to look at the
output and make sure it is clean.

Next issue is to have control on the TX duration so that if a signal gets hung up on the repeater
input, it will not keep the TX keyed for ever. Normally this time out timer is set from 60 to 120
seconds. It just un-keys the repeater TX until the input signal goes away. Prevents the TX
from burning up from too long a time in TX. In some of the newer radios, you could do this
with the internal time out timer on the TX.

Another issue is to be able to control the audio level. You just can't take the speaker output and
connect it into the mic input of the TX. You have to look at the levels, the characteristics of the
audio and the level.

Normally some sort of repeater controller is used to do all these functions. It has the timers
built in, the audio level controls and all that important stuff.

Last but not the least of your problems is some form of signal rejection or interaction between
the RX and TX antenna system. Normally this is done with some sort of a duplexer. This takes
a common antenna and splits it between the RX and TX with filters to keep the signals from
causing what we call de-sense in the receiver. You need a minimum of 70 db isolation between
the TX and RX ports to make a repeater system play. This can be obtained by less TX power,
more frequency separation, more antenna separation, or a bunch of filters between the TX,
RX and antenna. The higher the frequency band, the smaller the cavities are. If your trying
to do this at VHF, good luck. The needed cavities will be about the size of a large file cabinet.

Hope this point you in the right direction.

Jim

twowayradiony wrote:I am looking for a little help on repeaters.I am looking for a back up repeater that would be used very seldem.Its is only used for emergency traffic most transmissions are short.I have read alot about using 2 mobile radios and connecting them together.I have also read that if you do this with the motorola radios you take the risk of burning up the radios.I have a gm300 and m1225 I was going to trying this on but not to sure on the risk.has anyone done this and how did it work ect?

You also need power, backup power, and control. If this is going to be "always on", than control isn't a big deal. But if this needs to be switched on and off, then you need some way to do that. DTMF over-the-air control is popular if you get a controller that supports it.

For power, you need a 12 volt power supply that will support the continuous draw of all the components. If you need backup power, you'll need batteries and some way to switch from the power supply to the batteries.

nmfire10 wrote:If you need backup power, you'll need batteries and some way to switch from the power supply to the batteries.

Actually, not. Assuming a 12VDC device, plug the device into the batteries and plug the batteries into a good 12VDC battery charger. A good charger will charge the batteries and then truly float them, while also carrying any external DC loads.

Upon loss of off-site power, the charger dies. However, since the radio is connected directly the batteries, it never sees any loss of power (until the batteries become completely discharged). When off-site power is restored, the charger comes on and both charges the batteries and carries the radio load.

Now, you will ask, what is a good charger? There have been some extended debates on the point in prior threads, which I don't wish to revisit. Suffice it to say that, if you were to ask me, I'd say your only choice is a "TruCharge" by Xantrex.