Full duplex 800 MHz SMR telephone

The EF Johnson 8625 full-duplex mobile and Viking CX 8180 flip-phone full-duplex portable are both pleasant ways to 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, you have to select the repeater tower manually. A huge 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 enhance 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.

Handheld SMR full-duplex

The EF Johnson full-duplex Viking CX 8180 handset actually looks like a flip phone, which is pretty radical for a two-way radio device. The 1 watt transmit power is not a huge disadvantage, vs. full power 3 watt handheld two-way radio on 800 MHz. It looks like I have 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. I think 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. The thin Visar battery just doesn’t last through the workday.

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.

Advantage of bag phone

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, you get 50+ mile range from the tower with SMR interconnect.

The way 800 MHz full duplex interconnect works is:

  1. incoming caller calls common number (unless you have a DID controller), then enters your repeater-group number (5 digit)
  2. outbound you select closest repeater, enter number and press send. half-duplex get dial tone or busy.

In some ways it’s rather impractical in the rapidly converging digital cell phone age, but I feel like I have the ultimate in pre-cellular communications with the EF Johnson 8180 and 8625.

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