Friday, April 27, 2018

Net Training - Cross Band Repeaters

The following information is taken from “The ARRL Operating Manual for Radio Amateurs, 9th Edition.”
I was, recently, asked if I knew anything about CROSSBAND REPEATING.  If you have a dual band, dual-display mobile radio and a handheld radio you can set up your own repeated system.

Why do so?  There are several reasons.  My personal reason is as follows.  I have two separate radios installed in my vehicle.  One radio is a dual-band dual-display Kenwood, TM-V7 and one is a Motorola, Spectra, single band UHF radio.  My Motorola has a public address system with an external speaker, so I can hear the radio outside my vehicle.  The Kenwood does not.  There have been times when I was out of my vehicle and did not have a handheld radio with me, but I wanted to keep track of what was going on on-the-air.  I set my Kenwood as a repeater from 2 meter frequency to 70 centimeter frequency.  Any call that came in on 2 meters was repeated on the UHF and came out through the external radio speaker.  I could now hear what was happening and also hear if someone called me.

Let us say you want to operate your handheld radio, but you need the extra power of your mobile radio to hit a repeater.  For example, you’re on the lower level of a shopping mall with your handheld.  You can’t reach the local repeater from down there, but you can the repeater from your vehicle in the parking lot.  Again you repeat through your vehicle to the repeater and you have coverage from your handheld.

You may be in one of the hospitals and you have your handheld, but cannot reach the local repeater, but you can reach your vehicle in the parking lot…I think you get the idea.
Cross-band repeat operation in most mobile radios is very simple.  When the cross-band function is turned on, anything the radio hears on one band is retransmitted to the other.  When the signal stops the radio goes into receive on both bands and waits for the next signal, on either band, to repeat.  So down in the mall or down in city hall or down in the hospital, you transmit on UHF.  Your mobile hears its UHF received, and repeats you to the main repeater on VHF, flips itself around and repeats the main repeater back to you using the UHF transmitter.  Simple as this is, it can take a while to wrap your mind around the concept.

There are some problems that limit the utility of cross-band repeat.  The biggest problem is hang-time on the main repeater – the time after someone stops talking, but the repeater stays on the air, beeps, and finally drops.  On many repeaters that’s several seconds, and when two hams are in conversation, the repeater never drops until they are done.  It will automatically reset the time-out timer as each one un-keys their mike so it does not time-out.  Your cross-band repeater can’t tell the difference between a ham’s transmission through the repeater and the hang time afterwards.  It’s all just one long signal being received.  So, if you, down in city hall, are listening to two hams talk, you can’t break in until they’re done.  As long as they are talking, your mobile never stops sending the signal to you, and never listens for you. (Something else to keep in mind here is that your mobile is now transmitting a lot, and it is not designed for continuous transmission.  Keep it in low power.)

A repeater can be made “cross-band repeat friendly” by having a very short hang-time, or by specially designed CTCSS system.  If the repeater sends the tone only when a signal is on the input, and turns it off during the hang-time, your cross-band repeater can use the tone to know when to transmit and w2hen to shut off, allowing you to access the repeater between transmissions normally.  Or, if you can hear the main repeater directly on your handheld but just can’t get back to it, you do one-way cross-band repeat, from your handheld through the cross-band repeat, from your handheld through the cross-band mobile, but not back to your handheld.

A few notes of caution:  first, be careful in configuring your cross-band repeater.  Choose frequencies wisely – your coordination group may have identified band segments for cross-band repeat operation, so don’t just plunk down anywhere you want.  Do some research.  Guard the “local” side of your cross-band mobile with CTCSS or DCS.  If you don’t, and the squelch opens on your mobile, it will spew noise out to the main repeater.  Cross-band operation I particularly useful for emergency and public service event work, but a noise spewing, out of control cross-band mobile can render a vital repeater useless.

Second, maintain control.  The FCC rules require you be in control of the transmitter, but are not specific about how you do that.  If you can reach the vehicle in a few minutes from the inside of city hall, that’s probably good enough.  But don’t stop paying attention to it or leave the area.

Finally, you are required to ID both your mobile transmitter with your call sign.  How do you ID the transmitter that’s sending the main repeater signal back to you?  If you ID at the end of a transmission it goes out on the handheld and is repeated on the other frequency.  I don’t know many hams that talk for ten minutes, but you are to ID every ten minutes.

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