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Fall 2008 Issue |
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FM How to Crossband on VHF/UHF By Bob Witte, KØNR |
Photo A. A typical dualband radio that includes crossband repeat mode. (Photo via KØNR) |
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One handy feature that has made its way onto
the list of standard features for many dualband FM transceivers is
crossband repeat mode. This mode can be really useful for extending our
radio range, and but it does come with a few challenges to consider. In a dualband FM transceiver (photo A) crossband repeat capability takes the signal from one band and retransmits it on the other band. Typically, the two bands are 2 meters (146 MHz) and 70 cm (440 MHz), although other bands may come into play. Not all dualband transceivers are able to do crossband repeat. A key enabler is that the dualband rig must have two independent receivers that can operate simultaneously. Examples of two-receiver dualband rigs are the Alinco DR-635T, ICOM IC-2820H, Kenwood TM-V71A, and Yaesu FT-8800 and FT-8900. (The crossband repeat feature is not shown in the IC-2820H manual, so visit the ICOM website for details on how to enable it.) Examples of dualband radios that are “one frequency at a time” and don’t offer crossband repeat are the Yaesu FT-7800R and the ICOM IC-208H. As you may have experienced, a transmitter operating in close proximity to a receiver can overload the receiver, causing radio interference. In the case of 2 meters and 70 cm, the frequency spread is wide enough such that the front end of a well-built transceiver can keep the two bands from interfering. Note that these radios only repeat between different bands. They don’t have the ability to repeat one 2-meter frequency to another 2-meter frequency, as there is insufficient isolation inside the radio. (Real repeaters use very large cavity filters to accomplish this.) Inside the radio, crossband repeat is implemented via a logical connection between the squelch line of the receiver and the transmit control of the transmitter. In other words, when the squelch opens on one of the receivers, it causes other band’s transmitter to turn on. The audio is routed from the receiver to the transmitter so that the signal is repeated. This feature generally works in both directions; signals on 2 meters are repeated on 70 cm and signals on 70 cm are repeated on 2 meters. Some transceivers offer a “one way” crossband repeat that allows the retransmissions to occur in only one direction. Early implementations of crossband repeat mode had limitations such as simplex only (no repeater transmit offset) and limited use of CTCSS. These days, crossband repeat can take advantage of all of the rigs’ normal features, including transmit offset and CTCSS encode/decode. There are several reasons to use the crossband repeat feature. Probably the most common use is to extend the range of a handheld radio, as shown in figure 1. In this example, the handheld radio is set for simplex operation on the 70 cm band and the mobile transceiver is set to crossband from the same 70-cm simplex frequency to a 2-meter frequency. The distant station operates simplex on the same 2-meter frequency. I’ve arbitrarily shown the handheld radio on 70 cm and the other station on 2 meters, but there is no reason why the bands couldn’t be swapped.
Another common use for a crossband radio is
to fill in radio coverage gaps, perhaps on a temporary basis for a
special event or emergency. For example, an ARES team might have trouble
maintaining radio contact in a narrow canyon. Careful placement of a
crossband rig can help fill in the gaps. Another common example is
operating from inside a building where the radio waves don’t penetrate
very well. A crossband repeater out in the parking lot can help boost
the signal.
There are a number of things to watch out
for when using crossband repeat. You need to be careful when selecting a
frequency to avoid interfering with other radio amateurs’ operation.
There is no simple recommendation here, but understanding how the
VHF/UHF bands are used in your area is the key. Check your local band
plan to find a suitable spot and carefully listen for other activity.
You may find that the band plan has a specific frequency designated for
crossband repeat operation. Otherwise, you’ll probably need to choose a
lightly used set of simplex frequencies, one that conforms to the band
plan. |
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