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?


            TETRA and other Digital


Radio over IP techniques

            General Considerations

            Amateur Radio Methods

            Commercial Radio Methods

            Other Commercial Solutions


Receiver Combining Systems


Transmitter Combining Systems

Basic Considerations

Cavity Combining

Hybrid Combining

Directional Couplers

Circulators, Isolators, & Loads


Receiver-Transmitter Duplexers

Basic Considerations

Simple Notch Duplexer

Pass-Notch Duplexer

Rat-Race Close Spacing Duplexer


Analog vs. Digital Modulation

            Basic Considerations

            Digital Standards Open & Closed

            Vocoders Open & Closed

            The cost of Testing and fixing Digital


Interference Issues & Mediation

            Basic Considerations & Identification

            Co-Channel Interference

            Adjacent Channel Interference

            Broadband Noise


Inter-modulation Issues

            First, Understanding the math

            Rusty tower & Mounting parts

            Mixing in Receiver Front-ends

            Bandpass Filters Required       

            Mixing in Transmitter PAs


Test equipment

Watt Meters

Service Monitors

Spectrum Analyzers

Network Analyzers

References for test equipment


Ridiculously Cheap Radios




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This work is licensed under a Creative Commons Attribution 4.0 International License.