Choosing the
correct antenna

There are many different antenna designs but not all types suit all applications. Unfortunately many a radio guy has fallen into the trap of choosing the wrong antenna with the view to "get back the coax loss and more". Location is still the thing that counts most. Here is a list of the basic types, their radiation angles (with reference to the horizontal), and their prime suitable uses. Gains are all expressed as dBd (decibels over dipole).

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. With a unity gain (0dB) this antenna exhibits the widest radiation pattern. The radiation angle is maximum 45° but may be lower with a 'drooping groundplane'. The maximum radiation is at ½-angle of angle between radiator and groundplane. Example: if the antenna is mounted on the roof of a vehicle with the angle between the element and groundplane being 90°, then the radiation angle is 45°.

1/4 wavelength "Helical" (+45°): Also called "rubber duck" antennas, are used primarily for portable use as the spring design is exceptionally robust. These are not efficient antennas at all. The design represents a 1/4wave antenna that has been physically coiled into a spring. The fact the bottom bit is coiled up (this being the portion of an antenna that actually does the work) makes this portion of the antenna smaller in length. Having less "capture area" is what makes this antenna less efficient.

There is one advantage to the helical being the antenna tends to radiate in a more "isotropic" pattern therefore removing the need for the antenna to be in the correct orientation when transmitting - true 1/4-wave and higher need to be orientated correctly to work efficiently. As you can see, there is a trade-off (and why they still exist - ever wondered why they are found on cellphones!). Although I have always found it a little strange that by the time one reaches UHF that such antennas are still used! If one has to have the antenna shorter, then consider top loaded antennas where the bottom part (at least the first 1/8th wavelength) is uncoiled with the final portion then coiled up at the top.

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 ½ wavelength with groundplane. As the antenna is slightly longer than ½ 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.

Multi-element with groundplane (>+0°): These antennas consist of various combinations of elements "popped on top of one another". The typical combination is ½-wave over ¼-wave, the two elements being connected to each other through phasing coils. The radiation pattern is dependent on the combination as well as the number of elements but a rule of thumb is the more elements, the closer to horizontal the radiation pattern. Use of these types of antennas is typically mobile UHF and SHF (cellphone) applications where gain is desired. Care should be taken as antennas with higher gain have a thinner radiation pattern and when flapping in the wind could cause signal variations.

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. Although the feed to the radiating portion of the true collinear is high impedance, some collinears are fitted with groundplanes to tilt the radiation pattern upwards when working in a valley.

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 - all this with the added advantage of about 3dB gain from about twice the lower cut-off frequency. 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 0º 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.


As said in the opening statement, the primary reason for making a bad choice is done with a view to "regaining the loss caused by the coax".

A prime example is using a 9dB collinear for satellite work. The radiation pattern is flatter than a tea biscuit, so any satellite slightly higher than the horizon is actually out of its effective useful beamwidth and will therefore actually exhibit loss as opposed to, say, a ¼-wave with groundplane (effective max at +45°).

Another classic mistake was personally experienced when issued with a tactical discone antenna for use in military ops in a valley. If I was talking to others in the valley then all would be well (and our comms would be kept in the valley) but it was for comms to mountain units and helicopters (and all this on very limited power from military man-packs!).

First, I did rave about the discone, and I still do! As it can operate on such a broad frequency range without any adjustment, the discone really does shine through in a military setting. Knowing the discone's radiation pattern, a simple rig was constructed to mount the discone upside-down. Success!

In both the above examples it can be seen how a ¼-wave with groundplane would function far better than the antennas with gain and could quite easily be said to have "gain" over the other antennas. Visiting Antenna Gain Explained one can see that gain is also loss - the more gain an antenna has i.e. the more effective an antenna is, the less angle the antenna has to exhibit such effectiveness.

It is, therefore, highly important to not only chose the antenna with regards required gain, but also to chose the antenna that exhibits that gain in the desired direction.

| | Ask a Question |

© 23.11.05