Selecting sites for fixed monitoring stations often involves making some trade-offs between effectiveness, efficiency and cost. An ideal site from a location and RF standpoint may not be available from current owners or the cost of development and creating access are prohibitive.
Chapter 2 of the ITU Spectrum Monitoring Handbook (revised 2004) suggests that three main considerations to be applied when assessing the appropriateness and suitability of a monitoring and direction site. These are:
- Location - located in places suited to the zone to be monitored (suitable geographical coverage);
- Obstacles – presence of various types of obstacles that may cause distortion; and
- Protection from electromagnetic emissions and interference
As far as possible, a site should be selected where the field strengths of the emissions to be recorded will be relatively undisturbed by the local structures or terrain features.
HF: for measurements below about 30 MHz the terrain should have relatively level terrain situated in an area of high conductivity and free of gravel or outcroppings of rock. Nearby overhead conductors, buildings, large trees, hills and other man-made and natural features may seriously distort or disturb the wave front of the emission. The degree to which these conditions limit the validity of the measurements depends on a number of factors including the frequency range and the type and orientation of the antenna used.
VHF/UHF: highly directional antennas will be used and it is important that the path in the general direction of the signal source be clear; additionally multiple-path reception due to local reflection or re-radiation of the wanted signal must be minimised.
There are additional considerations which have a bearing in determining suitability. These include:
- Availability of the site: is the site readily available for use as a monitoring site?
- Access to the site: is the site accessible throughout the year and if remote is it adequately secure? How much time is taken to get to the site by usual means of transport?
- What types of infrastructure are available? Are there buildings, power and telephone lines within the vicinity?
In developing criteria for selecting appropriate site, we are concerned first and foremost with existing and future RF conditions. The same three main considerations have been used in developing criteria.
- Suitability - Does the site suit the purpose of monitoring and DF in terms acceptable levels of emissions? There are few sites within the region which are not proximate to a broadcast station. The Handbook suggests that an ideal site will be adequately protected from noise – both transmitters and industrial emitters and be located on a clear site with between 40 000 to 160 000 m2. Finding a site which is level, large and not within close range of a broadcast transmitter is very unlikely within the Beirut region.
- Location - Is the site in an appropriate location in relation to the target area(s). In many cases, primary concerns are major population centres and international airports in the vicinity. Guidance is offered here as well. A good distance from the airport to the station is between 4-8 kilometres. Frequency scans have been performed from 20MHz to 4.0GHz to assess whether signals can be measured from candidate sites.
- Obstacles - Distance, elevation and angle are important considerations. The station should be close to the target area and be able to measure desired signals. Line of site is important. Elevation is helpful in terms of overcoming hills, buildings and other obstacles. Too great an angle can cause some problems in direction finding however. Generally, flat terrain is desirable and angles greater the 5° can cause problems.
- Emissions - The guideline for direction from strong emitter fields is:“A root-sum-square value of 30 mV/m for multiple signals within the passband of the monitoring receiver can be regarded as an acceptable field strength level corresponding to a distance of about 5 km between a 1 kW transmitter and the monitoring station depending on frequency, terrain and equipment used”. Chapter 2. p. 69. Noise levels have been measured for candidate sights.C
Criteria incorporated into the overall assessment and evaluation of the suitability of candidate sites should include:
- Suitability: freedom from emissions, obstacles and size of protection zone
- Location: proximity to the target area.
- Communications: availability of existing communications links permitting remote and manned operation.
- Infrastructure: existence of suitable buildings for housing equipment and permitting remote and manned operation.
- Power: availability of existing sources of electricity to permit continuous remote and manned operation.
- Road access: reasonable accessibility.
- Security: availability to adequate security for unmanned fixed sites.