Ham Radio Cross Band Repeater
A crossband system has the ability to work on different frequency bands, as well as frequencies within the same band. The repeater can receive from a user, link to another repeater and transmit to a different user, all on different frequency bands or frequencies within the same band.
The basic crossband repeater utilizes frequencies with sufficient seperation, typically on two different bands, to avoid internal interference. An unattended station working in this way is a radio repeater. It re-transmits the same information that it receives.
Ham radio VHF/UHF crossband repeater operation is a common use for amateur operators. The crossband repeater translates back and forth between a 2m simplex channel and a 70cm simplex channel. The weaker 70cm transceiver, often a HT, can work through a mobile or base station to communicate with a 2m simplex net, taking full advantage of the mobile or base transceiver higher power or higher antenna elevation. Any dual-band or dedicated 70cm HT can be used to operate through a cross band repeater.
Selecting a Frequency Pair
Pick your frequency pair carefully, so as not to cause harmful interference to other users. The use of cross-band repeaters has the potential to cause serious disruption of communications circuits, and the creation of harmful interference to coordinated repeaters. If you are not sure of active repeater frequencies in your area, a safe rule is to stay off of the repeater sub-bands and use the FM simplex portion of each band.
You should consider using CTCSS on the frequency your transceiver lists on for your mobile input. This will help prevent your repeater from broadcasting unwanted transmissions coming into the input from interference or distant transmissions during periods of unusual band propagation.
How can I legally make my VHF/UHF station into a crossband repeater?
Modern dual-band or tri-band VHF/UHF rigs often have the capability to do crossband linking. When operating in this mode, the users may call them "crossband repeaters." Actually they are often remote bases, such as when they are used to allow an operator with a hand-held radio to access a repeater from a location where he or she would normally not be able to do so. For example, a hiker in a remote location might leave his car where his dual-band mobile rig can access a distant 2-meter repeater. Leaving the mobile rig on, he then takes his UHF hand held with him, and can access the 2-meter repeater via his mobile rig.
A crossband repeater (or "portable remote base") is okay as long as several conditions are met:
1) The user communicates with his crossband rig via the UHF side. Since this serves as his control and voice uplink, it is a form of auxiliary operation and must be conducted above 144-MHz. Since the operator is the control operator, that person must actually be able to control the station! That person must be able to turn it off remotely if a problem develops. If the operator can't control it, it's not legal [97.7, 97.201, 97.213].
2) If the control link fails, the remote station must shut down within three minutes which means a 3-minute timer is required [97.213].
3) The unattended station must be identified on all frequencies it transmits on. Since this is a form of remote base, the user's ID over the UHF uplink to the dualband radio also serves to ID the VHF output of the mobile rig. In the other direction, however, there is no way for the control operator to ID the UHF downlink from the mobile remote base, so some form of automatic ID must be employed [97.119]. Unfortunately, few manufacturers include the capabilities listed above in their rigs. Hence, to be fully legal, some form of add-on controller may be necessary. Another use for crossband operation is to link together two existing repeaters on different bands. This is usually done on a temporary basis during an emergency, a drill or a special event. Again, the requirements for proper station identification and control on both sides of the dualband radio's transmissions still apply. If both the VHF and UHF transmitters are not properly identified and controlled, the operation is not legal. In the examples cited above, the control requirement can be satisfied by having a control operator at the station, thus making it a locally controlled station. Although this may not always be convenient, it is a way to satisfy all of the station control required.
According to an article on KB9VBR called Cross-Band Repeater Operation, "Kenwood’s TM-V71A and TM-D710 now have the ability to identify the outgoing UHF transmission." This article also goes on to explain very well the legal issue with cross-band repeater operation. Here is a quote, "When you transmit, you identify your transmission with your callsign. The incoming UHF transmission is identified, and the outgoing vhf transmission is identified, since you are the control operator of your handheld radio and of the cross-band transceiver. On the other side of the transmission, the repeater identifies its own transmissions. That transmission is then retransmitted by the cross-band transceiver on the UHF channel. But this UHF transmission does not have your callsign attached to it, so it’s an unidentified transmission. Most cross-band repeat capable radios are designed with this fault."
Most radios aren’t designed for 100% duty cycle and constant use of the cross-band repeat function can damage the radio. The final stage transmitter amplification transistors will overheat causing them to fail. For example, the Tytera TH-9800 has excellent cross band repeat capability, however, it also gets very hot after 30 minutes of use even on the low power setting. Long term use of transceivers like this will inevitably result in premature failure of the final stage.