Radio Modems

Overview

Pacific Crest makes radio modems, principally for connecting a GPS rover with a fixed reference station to allow submeter accuracy.

I have 3 RDDR-96 "Blue Bricks". I got them at a garage sale along with a Magnavox GPS reference station Also included were some rubber ducky omni directional antennas, a Larsen base station antenna, a Larsen YAGI directional and a 35 watt linear amp.

The Blue Bricks can operate in two modes, transparent and packet. In transparent mode a pair of the radios acts as a replacement for a serial cable operating at 9600 bps. In packet mode, one radio can be the source of a data stream and several can be receivers, allowing for example in my case one base station and two rovers.

So far I have powered them up individually with their rubber ducky antennas attached and confirmed they transmit by tuning my Kenwood TH-6A handheld to their frequency and listening the bleeps when I type.

My radios came from the factory tuned to 420 Mhz. This means that I can operate them using my amateur radio operator's technician license. It's in the 70cm band. (See http://www.arrl.org/FandES/field/regulations/bandplan.html#70cm)

My applications

I have a couple ideas for things I want to try with this set up.

DGPS

Send out DGPS corrections from the GPS reference station.

The rover will be a Leica 9400 or a Garmin or possibly a Trimble. Probably all three but not at the same time.

Garmin Fleet Management

Connect to a server running software compatible with Garmin's Fleet Management protocol. Put a radio modem in a car that has a Garmin PND in it and see what we can do.

Antenna set up

I am not sure what part of town I will be in, so I am trying the omni antenna first.

Larsen BSA Kit = "Base station antenna"

Cut radials - for frequencies above 400 MHz they should be 7 inches.

My main element is a PO series. For 420 Mhz, the lower portion should be 10 inches.