Cross-Band repeating has great potential for use in EMCOMM, extending our ability to utilize handhelds as well as providing gateways to more distant repeaters.

First, a quick explanation of what cross-band repeating is: the use of radio equipment that can simultaneously transmit on one frequency what is received on the other, much as a normal repeater.  The twist is that the two frequencies will be on different bands, usually 2M and 70cm. The reason for this, is the significant difference in frequencies allows the simultaneous transmit and receive with simple  circuits, rather than large and bulky duplexors required for same-band repeating.

Many in our group, who can't directly reach the repeater with an HT, utilize cross-band repeat capability.  To cross-band, merely run an HT in simplex on an appropriate 440Mhz frequency and on your mobile or base station set one VFO to the same simplex frequency as your HT and operate the other VFO as normally for the repeater.  When the radio is in cross-band mode it will transmit everything that comes in on the repeater output to the 440Mhz frequency, and repeat everything from the 440Mhz frequency to the repeater.  Your HT now has the same range as your base or mobile rig!

Cross-banding has great potential in EMCOMM, for the same reasons.  In areas where handhelds don't reach, a mobile rig in a vehicle can be set to cross-band at higher-power and extend communications in remote areas to those handhelds.  Additionally, if we should be so far out of range that even a local mobile station can't make a repeater, additional stations along the way could be daisy-chained through multiple cross-band repeaters to reach the desired conventional repeater.

Many of you may have cross-band radios and not even know it, as it is often not well advertised in the marketing data.  An easy way to find out would be to perform an internet search with your radio's model number and "cross-band". If it has the capability, you'll likely find instructions on how to enable and disable it.

While this is a great and useful capability, there are several things to consider:

1) A cross-banding radio will be transmitting whenever the handheld user or the repeater it's linked to is transmitting, meaning it will be working hard and could overheat, so use minimum power necessary and try to keep the radio well ventilated.
2) Because the radio transmits a lot, it pulls a lot of current.  Be mindful if running in your vehicle, as it could draw the battery down if the vehicle isn't running.
3) A timeout timer should be utilized, so if the squelch on one side opens or interference exceeds the squelch setting, it won't stay locked in repeat mode, constantly transmitting.
4) Be mindful of the frequencies you use, as you don't want to interfere with normal repeater operation. There are frequencies set aside for auxiliary use, mostly in the 433-435mhz range.  I typically use 433.300.
5) Finally, the legality of operating cross-band repeat with most radios is in question.
6) Use PL/CTCSS tones to minimize interference from breaking squelch and engaging the cross-band repeat function. (Thanks for the add Blake).

Legality Of Cross-Band Repeat
FCC sections 97.119a and 97.201 would seem to imply a cross-band station would need to self-identify, something most radios don't do. Remember, the cross-band repeater is a stand-alone station we communicate with via our handheld or other radio, so our identifying may not count as a station identification for the cross0band repeater. Another consideration is that when the radio is transmitting on your simplex frequency, it never identifies and is relaying your identification to the other band, not the simplex band.  While YOU may be identifying on the simplex frequency, you may be doing so at lower power than the cross-band radio, not reaching as far as it is, leaving distant stations not hearing your identification.

That being said, the wording is a little vague, and the FCC has class certified these radios, with non-identifying cross-band capabilities.  This would seem to imply use of the feature is not restricted.

In any case, this is a good reason to always operate at lowest possible power to achieve the desired report.  If it turns out non-ident cross-banding is not legal, the sin is more easily forgiven if we haven't caused harmful interference.