One of the most important performance parameters of a radio tag is its signal range. Unfortunately this is also the most difficult parameter to accurately predict. The variability of range in the field is due largely to the effects of topography, habitat, tag and receiver height, and receiving system performance. If a tag fitted to a large soaring bird living in open country is compared to a tag with the same power output fitted to a ground-dwelling mammal in rainforest, the range of the bird tag could be up to 50 times greater! Of course, these are two extremes of habitat and height. Most projects fall somewhere in between, but still the effects of habitat and animal behaviour can be quite dramatic.
Some of the terminology used to describe range is also rather misleading. For example, the phrase 'line-of-sight' is often used in range descriptions. This is not, as it implies, the range you would get if you could see the tag (that is, no obstacles between you and the tag). Rather, it is the 'free-space' range, achieved only if the tag and receiving system were at high altitude. The nearest radio tracking gets to true line-of-sight conditions is when a tagged bird in flight is tracked from an aircraft or a very high hill. We continue to quote line-of-sight ranges, mainly because it is the convention to do so. However, we suggest you consider them only if you plan to track high-flying birds from high points or aircraft.
A more meaningful range figure is 'ground-to-ground'. It is the signal range to be expected over open or lightly-wooded flat terrain, with the tag on the ground. The signal strength reaching the antenna is assumed to be -145 dBm (about the weakest signal you will hear in the field) and the receiving equipment is assumed to comprise a good-quality tracking receiver and 3-element Yagi antenna. The range is much lower than that achievable to an animal above ground (in flight or in a tree, for instance), but higher than if the animal was below ground or hidden in dense vegetation. In most cases the range to a tagged animal on the ground will still be better than our quoted 'ground-to-ground' range because the tag itself will be at least a few cms above ground when it is attached to an animal.
For species that spend some of their time in flight or in trees, the signal ranges given under 'above ground' conditions are meaningful. These are based on tags that are at least 3-4 metres above ground. These ranges also apply if the receiving system can be raised, such as by tracking from higher ground, or having an antenna on a mast.
There are a number of ways to improve range, the most obvious of which is to increase the power output of the radio tag. Unfortunately this has implications for battery life, and it is often impossible to have high power transmitters that also have sufficient battery life. In such cases, it is possible to improve signal range by improving the performance of the receiving system. Usually the simplest way to improve receiving system performance is to raise the height of the antenna. Even holding it above your head will help ! If that is not enough, it is possible to mount antennas on vehicle mounted masts, or - for better portability and lower cost - to mount a special light weight antenna on to a telescopic fishing rod (e.g. 'roach pole').
During fieldwork it is often necessary to track from high points (e.g. hills and trees), especially if a tag is missing. If all else fails, radio tags on dispersed animals can be located from aircraft. This is a routine technique in many projects, and is not particularly expensive in comparison to a ground search over a very large area. Signal range from the air is much better than on the ground (perhaps 10 x or more) and the speed at which an aircraft covers an area is vastly better than the speed of doing it in a vehicle on the ground. The biggest problem with aerial tracking is that you need special clamps to safely attach Yagi antennas to aircraft wing struts. These are available from some of the larger US telemetry companies. Of course, you also need a pilot who is prepared to allow you to strap the antennas to his aircraft! For a detailed description of aerial tracking techiques, see Kenward 1987.
Kenward, R. E. 2001. A Manual for Wildlife Radio Tagging. Academic Press. London. [An excellent introduction to all aspects of radio-tagging and radio-tracking, including a section on DIY tag construction.]