The EF Johnson 8625 full-duplex mobile and Viking CX 8180 flip-phone full-duplex portable can stay connected in areas with poor cellular coverage. It’s possible to be as much as 50 km from the repeater tower with a good antenna system on the car and the high-power mobile radio, making full-duplex phone calls. A main difference is that like legacy IMTS systems, the repeater tower is selected manually by the user. An advantage over cell phones is that traditional group calling (and individual calling) of two-way trunked radio is also included. Since Nextel’s Jan. 1997 public relaunch as a cellular competitor outside of its Fleet Call dispatch legacy, this type of service is nationwide. Note that Nextel dispatch group/individual calls are restricted to local geographic areas. This is necessary to reduce burdens on Nextel’s backbone network most likely. Nextel telephone calls essentially work like any other TDMA cellular phone.

The EF Johnson full-duplex Viking CX 8180 handset actually looks like a flip phone with 1 watt transmit power vs. full power 3 watt handheld two-way radio on 800 MHz. It’s like a thick flip phone, but with the super power of dispatch group/individual radio chat.

Before this, the most svelte two-radio was the Motorola Visar launched in 1993, as a very small handheld two-way trunked and conventional radio. The manufacturing difficulty is not too different from the Motorola Visar, which is even thinner, such that the battery is twice as thick as the radio with the typically used battery. Putting a duplexer in handheld phones, including the Viking CX 8180 is facilitated by the wide frequency separation. For SMR, the mobile transmit band is from 806-821 MHz, and the base transmit band is from 851-866 MHz, a 45 MHz separation between transmit and receive. AMPS analog cellular in the US and Canada also uses 45 MHz separation in the 850 MHz band.

The 8625 has 12 Watts of transmit power, unlike any of the 3 Watt bag phones. Three watts are lost in the duplexer, which prohibits talkaround in this and other full-duplex models. Instead of say 10-15 mile range of a bag phone, 50+ mile range is possible from the tower with SMR interconnect.

800 MHz full duplex interconnect:

1. incoming caller calls common number (without a DID controller), then enters repeater-group number (5 digit)
2. outbound: user select closest repeater, enter number and press send. half-duplex get dial tone or busy.