RTcom Radio Modems user manual Frequency of operation

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	5-10km	3-5km	10-30km	100-200m	5 - 10km
range
Industrial installation	50-700m	50-500m	50m-1km	10-30m	30-100m
In large buildings
Penetration through	**********	****	******	*	**
concrete walls
Ability to bend/ defract	**********	******	******	**	**
around obstructions
Antenna size (dipole)	43cm	17cm	17cm	8cm	8cm
Potential users in adjacent	Message pagers	Radio	Radio	CT2	CT2
channels	Radio Microphones	Amateur on	Amateurs on	Mobile	Mobile
		433MHz	433MHz	telephones	Telephone
			Message		s
			Pages on
			458MHz and
			TETRA on
			410-430MHz
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.

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