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DIRECTORATE OF TELECOMMUNICATIONS
PUBLICATIONS
QSAM Radio Schemes; their preparation and adjustment
Author: W.R. Harris
page 1 of 1
Introduction
This small document published in January 1985 describes the preparation and adjustment of Quasi-Synchronous Amplitude Modulated (QSAM) Multi-Station Radio Schemes used by Directorate customers for country wide radio coverage.
During the late 1960's and early 1970's, Police and Fire VHF county-wide radio communication used spaced carrier for simultaneous transmission from hilltop sites to mobiles, ie: -3KHz and +3 KHz of the allocated RF Channel for a two site scheme, or other combinations for a given number of sites within the 25KHz channel spacing available at that time. This was done to ensure that the resultant audio mix would be outside of the 300-3,400 Hz audio band and therefore not audible to the end user.
As spectrum demands grew in response to user requests for more traffic channels during the 1970's, there was increasing need to ensure effective and efficient use of the limited spectrum available to Police Forces and Fire Brigades.
The availability of cheaper high stability oscillators during the 1970's enabled main transmitters to be held within a few Hz of their nominal RF channel for long periods of time. Together with a large procurement programme by the Directorate for Pye Radio Link Assembly (RLA), high quality delay circuitry needed to correct bulk delay site distance differences and audio phase compensation correction to ensure that all sites transmitted in-phase for good speech clarity, most county-wide users were eventually changed over to QSAM, which freed up more channels for subsequent re-allocation. In addition, at some point during the 1970's and early 1980's, many of the links were modified from 25KHz to 12.5KHz to free-up spectrum in the 146/152 link band.
By the time the WARC changeover program took place during the 1980's, the Directorate had amazed a wealth of expertise associated with QSAM. However, setting up procedures needed to be revised as Police users were being migrated from the 25KHz spaced 80/100 MHz band to the 12.5KHz spaced 143/155 Mhz band and Fire Brigades were migrated to 12.5 MHz spaced 70/80 MHz band. Tolerances were now much tighter and phasing was more important to ensure that good communication existed at all times between control and mobiles.
This document outlined the preparation and adjustment of QSAM schemes and gives an insight to the methodology proposed by the Central Communications Establishment (CCE) based at Harrow in 1985.
Credits
This document was kindly provided by Mike Brain and is Crown Copyright.
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