Tuesday 31 July, 2007, 15:26 - Radio Randomness

Anyone who really knows their stuff when it comes to radio propagation and stuff like that will tell you that any aerial (or an antenna for that matter) has a certain capture area. Its capture area defines, in effect, how much of the signal that is in the ether it can capture and thus present to an attached receiver. The bigger the aerial, the greater its capture area and the stronger the received signal. Obvious really.

For some antennas, such as satellite dishes, the capture area is relatively obvious. A satellite dish which covers an area of 1 square metre has a capture area of roughly 1 square metre (ignoring edge effects). For linear aerials, however, the capture area is less obvious. Which has the greatest gain - a 4 element yagi or a 4 wavelength long colinear? Not so easy is it. However, it seemed to me that there ought to be a simple rule-of-thumb which allowed simple comparison between antennas. I therefore hypothesise that:

* The gain of a linear aerial ought to be somehow related to the amount (length) of metal it represents. Thus if you add up the length of the elements on a yagi and stretch them out into a colinear, the two should have the same gain.

* A doubling of the amount (length) of metal should double the overall gain (i.e. increase it by 3dB).

For now, I'll leave my first hypothesis and concentrate on trying to test the second one. Taking the quoted gain figures for over 20 amateur radio colinear antennas I've plotted their length in multiples of half a wavelength against their gain (in dBi). The length is done logarithmically, so '1' represents a length of half a wavelength, '2' represents one wavelength, '3' represents 2 wavelengths, '4' represents 4 wavelengths, '5' represents 8 wavelengths and so on, doubling in length for each increase in the index. Some of the antennas are multi-band (and thus might represent a compromise of gain balanced across more than one band), some are single-band.

The dotted line represents my hypothesis - i.e. that the gain rises by 3dB for each doubling of length. The solid line represents a 'best fit' line. As the antenna gets long (in terms of wavelengths) there is a noticable drop in gain with respect to my hypothesis. It is probable that as the antennas become long, the losses in the 'metal' itself cause a fall away from the my theoretical figure. Alternatively my hypothesis is incorrect. Either way, the original premise isn't that far off!

Next job is to see whether my hypothesis about the total length of elements of a yagi and the length of 'wire' in a a colinear holds any water.

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