Experiments in Crossband Repeaters

We’ve recently started experimenting with using vehicular crossband repeaters to extend access to the repeater we most often use. There are places where we can’t hit the repeater very easily, especially from inside buildings.

Crossband repeat, for those unfamiliar, is a feature offered by some dualband mobile rigs, in which one side of the radio listens to a 70cm simplex frequency and retransmits what it hears on the input of a 2-meter repeater. Audio from the 2-meter repeater’s output is retransmitted on the 70cm simplex channel. That allows the user to walk around talking and listening to the 70cm simplex frequency which is connected to the repeater via the mobile radio.

One downside: The mobile radio is not made for the long transmit times that can be associated with crossband mode. Turning down the RF power on the rig is not a total solution, so use crossband repeat carefully.

At our Saturday HamCram, held at the local hospital, Steggy, WY6E, parked his truck in the parking lot next to the building. This is an 81-bed community hospital that serves our city of 82,000 +/- recent foreclosure activity. While we have a radio in the emergency department, we are unable to hit the repeater from inside the building using a talkie. That includes the room where the HamCrams are held.

As part of our experiments, Steggy turned on the crossband repeat function on the Kenwood TM-D700 (or is it a 710?) in his truck.

We are using 441.000 MHz. as the handie-talkie side, using PL encode/decode of 100.0 Hz. The encode/decode is important for reducing the interference we hear from noisy electronics in the hospital. The other operating issue of note was overriding the default down offset on the frequency. While 446.000 is the ARRL-designated national simplex channel on 70 cm, some radios still think it is the output of a repeater pair.

Why are we on 441.000? Because the local repeater coordinator only designed two simplex 70 cm channels, so we are making the best of what is available to us.

The crossband repeater worked well from the room (well inside the hospital) were our event was held. David, AA6HO, and I walked around the hospital and checked coverage, which was quite good.

I was not impressed with the retransmitted audio through the repeater, which was uncrisp to the point of causing me to have to ask for occasional repeats when talking to David in the hospital as I drove around town.

Next step will be to test different radios to see if we can find a combination that results in better-sounding audio. Stay tuned for more.

Anyone else using the crossband repeat function on their dual-band mobile rig?

Comments

[Bob Burns]

The historical problem with crossband repeaters is proper identification. Let's say I have a two-way crossband repeater set up in my vehicle. What it hears on the 2m side is retransmitted on the 70cm side and vice versa. I'm operating from a location away from the vehicle and I'm receiving and transmitting on the 70cm frequency that the crossband repeater is set up for.

When I key up and transmit, I can identify my station (the handheld) in the normal fashion. However, the crossband repeater is also transmitting. I can cover it's identification by saying "W9RXR through the W9RXR repeater". This identifies the repeater station. However, when the crossband repeater hears something on 2m and repeats it on 70cm, how does that transmission on 70cm get identified? Typically, it doesn't.

Some radios, including the Kenwood TM-V71 and TM-D710, have the ability to transmit a repeater ID when operating in crossband mode. However, I have heard that the IDer is "rude" and will break in while the crossband repeater is transmitting.

Another solution is to use a one-way crossband repeater. Kenwood refers to this as "locked-band repeater". In this mode, the crossband repeater only repeats what it hears on one band. In this mode, I could set my handheld to listen to the output of the distant 2m repeater and transmit on the 70cm frequency that the crossband repeater is set up for. In other words, I listen to the distant repeater direct and transmit through the crossband repeater. This avoids the problem of the crossband repeater making unidentified transmissions on 70cm.