On the various technical issues raised here (OK, we posted the rules, we
won't discuss the legality anymore) I think there is only one main
obstacle to using FRS radios for extensions on *. They are simplex
(push-to-talk, release-to-listen). The protocol for dealing with
voice-activated-switching (VOX) has been used in ham and public safety
simplex autopatches but it's really tricky to get right. It would be
much easier if you used GMRS or business band radios programmed to
transmit on one frequency and listen on another, that way the base can
be set up with one pair of frequencies and the portables on the opposite
split. Yes, you need two antennas or a duplexer, but this would be a
better way to do it than trying VOX and carrier interruptions to make it
work simplex.
Some of the cheap FRS radios have surprisingly good RF performance, I
wouldn't be too put off by hams telling you they will self-destruct.
Yes, they are programmed to time out after a few seconds or a minute of
talking, but you will need to make your transmissions brief anyway so
you can hear what the other people on the channel are saying. They
aren't as fragile as they look.
FRS is also narrow band (3 kHz deviation, 11.25 kHz channel bandwidth).
I don't think you would be able to get more than about 1200 bps of data
over this channel. Garmin does transmit GPS data over GMRS at that rate.
The modern (last 10 years or more) commercial Motorola radios don't need
a special programmer, just an interface box and PC software (which can
be expensive). But only a few of these have the narrow band mode needed
for compatibility with FRS (and none are legal for FRS as they are not
type-accepted as such). Nearly all are OK for GMRS though. These were
very expensive radios though, and while you might get them on eBay for
$10, the batteries will be much more than that.
Only one brand of cheap GMRS radios that I've seen (Garmin) has the
duplex mode that allows use with repeaters and duplex base stations. I
think this is essential for successful integration with a phone system.
My recommendation would be to use a duplex base station radio on a low
power business band channel pair. Any of the low power UHF repeaters
would be OK for this (the repeater logic is all handled by app_rpt). You
can get a license for the itinerant frequencies that costs just a little
more than a GMRS license, and be able to use real portable and mobile
radios and real antennas. There are plenty of these available with dtmf
pads so you could have full control of your * switch.
