- Radio-Tech Limited User Manual Radio Modems
5
Frequency of operation
Often there is not a choice over operating frequency. In most countries frequency allocations are very
limited, by way of example in Europe there is only 433-435MHz UHF or 868-870MHz SHF. While the
UK and many others offer VHF, UHF and SHF allocations. However other frequencies may be used
subject to local government licence.
In order to generalise the choice of frequency should be based upon the distance of intended
operation, power supply constraints, data rate, duty cycle, attenuation, portability (antenna size) and
the presence of other users. The following table is produced to assist your choice:-
VHF (10mW) UHF (10mW) UHF (500mW) SHF (5mW) SHF
(500mW)*
*
Free Space Transmission
range
5-10km 3-5km 10-30km 100-200m 5 - 10km
Industrial installation
In large buildings
50-700m 50-500m 50m-1km 10-30m 30-100m
Penetration through
concrete walls
********** **** ****** * **
Ability to bend/ defract
around obstructions
********** ****** ****** ** **
Antenna size (dipole) 43cm 17cm 17cm 8cm 8cm
Potential users in adjacent
channels
Message pagers
Radio Microphones
Radio
Amateur on
433MHz
Radio
Amateurs on
433MHz
Message
Pages on
458MHz and
TETRA on
410-430MHz
CT2
Mobile
telephones
CT2
Mobile
Telephone
s
Transmission efficiency
For battery operation
********** **** **** *** ***
Relative cost *** ***** ***** ******* ******
R.F path Surveys
The only certain way of determining the suitability of a communication channel is to conduct a radio
path survey and spectrum scan.
The spectrum scan is something normally conducted prior to ordering a system. Normally this requires
the use of a good quality scanning receiver such as an ICOM 8500 and a broadband antenna. Failure
to use a quality scanner may result in signals being missed if channel resolution is too low and false
signals being detected if co-channel and image rejection is poor.
When scanning, both the desired and adjacent channels should be checked for signals. As
transmissions may be intermittent it is important to take time with the scan, stopping for as long as
possible on each channel and looking for at least 15-minutes on the final chosen band.