Gary, you have that reversed somewhat. The mobiles transmit on the 468 range while the hospital transmits in the 463 range.They aren't simplex, they are duplex, or at least were designed that way so that phone like full duplex conversations could be held between field personnel and hospital personnel.
The base (hospital) side is on the 468Mhz frequencies, the ambulance side is on the 463Mhz frequencies. Med 4 is repeated, at least in MA where it is the calling channel. In other parts of the country Med 10 is the calling channel.
Some of the Metro-Boston channels are repeated, but for the most part they are not.
Not very few, I'd say none. During the 1970s Motorola and GE made full duplex radios for CMED use. In Motorola's case, they used UHF Micors with duplexers built in and the audio modified for full duplex. They also made a radio called the APCOR, which was essentially two MX300 radios with a duplexer, a high powered PA, and other features. Heavy as all hell, but I digress. GE made version of their Mastr II (I think) in UHF. The Motorola radios had dual control heads, one for the front and one for the rear of the ambulance. There was even a transmit inhibit feature so the two control heads could communicate with each other as an intercom.Very few radios are full duplex anymore as they are quite expensive. I would hazard a guess that CMED removed that requirement from the radio specs a long time ago.
Pretty accurate. The Metro Boston CMED uses a computer controlled switching network to patch hospitals to base stations, select channels for multi channel bases (most are four channel bases) and the system allows more than one hospital to be patched on the same base at the same time. I don't know what other CMEDs do, but I think Bristol County uses permanently assigned channels which are dialed up by CMED when a patch is needed.In CT (and I suspect MA as well), many of the hospitals do not use radios, instead they have dedicated phone lines from the CMED center to each hospital. The call to the hospital is really a really a radio call to CMEDs tower and then a radio-phone patch to the ED. During major incidents they do bed counts and those are not heard over the air. The CMED operator can also shut off the repeater portion of the call so you would have to monitor both sides to hear a complete patch. The mobiles are obviously weaker than the CMED portion so you have to be up high or nearby. Even more so if the EMS worker is using a portable. If you are programming a scanner, I would suggest putting the mobile freq first (468.xxxx) and then the base (463.xxxx). If they are not using a repeater, you may get an open carrier through the base side. It would lock up the scanner and you would not hear the patch.
Putting aside terminology:" Duplex " might mean 3 things in this thread -
1. receive and transmit on different freqs (maybe "half duplex" to some people) (maybe also called "three way" by some people) (like taxi cabs)
2. repeater or non repeaterized ops
3. listen and talk at the same time ("full duplex" or "duplex duplex" as I like to call it)
Boston EMS seems to have the ability to use 462.95 in duplex, simplex, and half duplex modes. I am not sure if the BEMS dispatchers can receive and transmit at the same time on 462.95. I also dont know if the BEMS dispatchers can hear any field units that might be using 462.95 in direct/simplex mode.
Maybe the simplest way to sort this all out is to determine
- what freqs are the field units transmitting and receiving on ?
- what freqs are the dispatchers transmitting and receiving on ?
- are comms being "repeated" ?
- can anyone receive and transmit when someone else is receiving or transmitting ?
- can anyone receive and transmit at the same time ?
Peabody FD is the only nearby agency that I can think of where the dispatcher can hear the field units if they are on "direct" (they call it Channel 2)