The most important component!
Funny thing about radio systems is that most, sorry, that's all of them use an antenna yet so many times the antenna is not given a second thought. This is the most vital peripheral to the radio telemetry module and is almost single handedly responsible for the efficiency with which a module will communicate with other modules. Three aspects determine the type of antenna employed at a site with the most important being terrain, then the sites use, and finally the losses and margins that need to be compensated for.
Contrary to popular belief terrain, this includes the surrounding structures, is the biggest factor determining the type of antenna used for a site. As an example, it would be of no use mounting an omni-directional dipole on the side of a metal walled factory. Similarly, it would also be of no use mounting a groundplaned quarter wave on the top of a mountain (hint: it radiates at 45 degrees upwards!). Choosing the correct antenna vs terrain follows this section.
Also explained in the following section is the next determining factor of the type antenna used being the use of the site. It serves no purpose should a repeater having to store and forward commands from all directions to be fitted with a super-directional antenna. Just as bad are mountain top repeater sites fitted with high gain collinears, which have a wafer thin radiation pattern, yet have to communicate with stations in a valley below. Also discussed in the following section is the suitability of antennas to types of sites.
Using the figures obtained in the radio tests will give a good indication as to the gain of the antenna required for each site (remembering to ensure the ERP limits are not exceeded). A handy graph is supplied for when receive sensitivites are quoted in ÁV and the results of the tests are in dBm (or vice versa).
As a general rule far sites should automatically be fitted with 6dB Yagis to ensure the maximum amount of energy is being sent towards the host or repeater. Concern over legal ERP limits should be cast aside at this point as the 6dB will more than likely be swallowed up in coax, connector, and radiation losses. Further to this using Yagi antennas on the outskirt stations will reduce radio clutter by keeping the radiation toward the host only. Any energy sent outwards (away from the host) toward the blue yonder by using an omni-directional antenna serves absolutely no purpose whatsoever.
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