OpenLMR Project

OpenLMR Project

This OpenLMR web site is a clearinghouse of concepts and technology serving the needs of the Land Mobile Radio (LMR) field, with a special focus at the system level, primarily for educational purposes. We have included information critical to understanding how LMR systems work and many helpful ideas for designing, installing and maintaining systems.

The site is open to radio amateurs, technicians, engineers, managers and users. The objective is to share knowledge that previously has been limited to those commercial interests desiring to exploit it for profit. We have no objection to anyone using the information on this site for personal gain, subject to the open-source licensing conditions that may apply.

Information is organized to first give the reader an overview of the particular concept or technology. Then explain the pros and cons of it, and last the how-to-do-it information. The knowledge necessary to build or configure some projects may be substantial, and may require precision test equipment. Information concerning test equipment, and procedures is also included to round-out the web site. The frequencies of interest extend from 30 to 1,000 MHz. and are generally referred to as VHF & UHF. The vast majority of the material presented relates to Repeaters or Base Stations, as this is where the greatest gains and losses in system performance occur. Initially, mobile and portable radios will only be discussed in passing, as we see decreased manufacturing costs are coming through to the buyers.

Again, the information on this site is purely for educational purposes, and may contain errors, or include information subject to patents or copyrights that may limit commercial use of the concept or technology. However, it was our goal to provide as much information as possible in the interest of the open-source community, to inspire readers to learn and create.

Pages covering topics of interest

Open-Source - Links to the world of Open-Source

LMR Disappointment - The BIG disappointment in LMR

Tiny Transceivers - Sources for some really nice VHF & UHF modules

Duplexer vs. Receiver combining - Some thoughts on the differences, and how to overcome them

Phase cancellation - Eliminate on-channel noise or interference almost by magic

Using the 72-76 MHz. Fixed Link Band - Point to Point links on VHF

Using Other References for Simulcast - Other linking and sync references

Proper Sealing of Outdoor Coax Connectors - Just what it says

Finding the Right GPS DO - Reviewing the many factors in selection the right GPS Disciplined Oscillator for your application

Future Topics To Be Expanded Upon

Radio Signal Propagation

Line-of-sight signal loss

Non-Line-of-sight signal loss

Polarization of Signal

Antennas Systems

Basic Considerations

Interaction with other antennas

Interaction with tower

Omni-directional Antennas

Broad-directional Antennas

Highly-directional Antennas

Special Purpose Antennas

Non-Polarized Antennas

Feedline Systems

Plastic Jacket coax like RG-8

Solid Copper coax like Heliax

Solid Aluminum coax used in CATV

Buying Quality Connectors

Resolving Moisture Problems in Connectors

Understanding SWR measurements

Repeaters & Base Stations

The Typical Repeater

Ideal Repeater Project (IRP)

Receiver Voter Systems

Simple audio voting systems

RoIP audio Voting Systems

Networking Concepts & Methods

Ribbon System

Wide-Area System

Simulcast Systems

Multi-channel Trunking Concepts

Johnson LTR

Motorola Astro?

MPT-1327

TETRA and other Digital

Radio over IP techniques

General Considerations

Amateur Radio Methods

Commercial Radio Methods