Choosing the correct antenna


There are many different antenna designs but not all types suit all applications. Here is a list of the basic types, their radiation angles (with reference to the horizontal), and their prime suitable uses.

1/4 wavelength "whip" (+45): This antenna is suited mainly to motor vehicles for use in densely built-up areas as most of the radiation is bounced off surrounding concrete structures. This type of antenna suits deep valleys in fixed locations. In all situations this antenna is reliant on a good groundplane.

1/2 wavelength whip with groundplane (+22): Very suitable for use in fixed locations in valleys. Although ideal for motor vehicles the sensitivity of the matching circuit needed for this type of antenna makes it impractical for this use as the bending of the whip or any dirt or water usually causes such a high mismatch the radio suffers. In fixed locations the bending and dirt ingress can be kept under control.

5/8 wavelength with groundplane (+15): A favoured antenna for motor vehicle use by offering a superb radiation angle which is sufficient for built-up area work yet low enough for rural work too. Not often found in fixed location models as the low radiation angle would not offer advantage over the wavelength with groundplane. As the antenna is slightly longer than wavelength it exhibits a small gain, typ 1.5dB. Many mobile models have been mounted on a good groundplane in fixed installations to make use of this gain.

1/2 wavelength w/o groundplane (0): Typical constructions are the centre fed dipole or endfed J-Pole and Slim-Jim (often mistakenly called an "Endfed Dipole"). This antenna is favoured for fixed location work offering unity gain and with the radiation pattern being the typical "doughnut" shape they offer both good local and distant coverage. As they work without groundplanes they are not suited to mobile work and performance suffers with the effects of nearby metal.

Multi wavelength gain antennas (0): Examples of these are "collinear" antennas. These are extremely suitable when distance is required without the need or desire to veer from the horizontal. These would be suited to flat plains or mountain top to mountain top where reflections or interference from valleys needs to be minimised. Not a good choice for a repeater system on a mountain top which is meant to cater for stations in valleys.

Discone - (typ -10): It is surprising that there are not more of these antennas around, especially for use in repeater systems. They also have the wonderful characteristic of being very wide band, typically 10:1 of base frequency. A well designed discone could cater for all the typical VHF and UHF two-way radio frequencies of 66 to 480MHz and still have room to spare. They do have two undesirable characteristics being extremely tricky to set up and do not have an inherent DC short thus are susceptible to inducing static in high winds. Although these are the negative points they are, none the less, extremely effective as repeater antennas and well worth the effort.

Stacked Arrays (-5 to -10): These are truly ideal antennas for mountain top repeater systems that are communicating with stations in the valleys below them. They exhibit both gain and the correct radiation angle and also, unlike the discone, have an inherent DC short. Please note when ordering such antennas to request the "tilt" option as stacks are also constructed to have the same 0 radiation pattern as per the collinear.

Directional Yagi (as required): These antennas are suitable only in fixed locations where gain is required along a single path and/or interference from a known source needs to be minimised. Favoured for use in point-to-point links as using these help to keep the airwaves "uncluttered" by keeping the radiation to only the intended direction. Please note that not all constructions of Yagi antennas are suitable for minimising interference, thinking that all Yagis block signals from behind is a mistake. It takes a special construction to do that, usually at the expense of a little gain.

<<  Planning Healthy Comms

| | Ask a Question |