The aim of any good radio installation is to get the maximum possible watts out for watts in - this means keeping all losses low so that maximum power is available for radiation. Often, despite best intentions the end results do not live up to expectations, especially with marine installations where there are more factors to complicate the issue. When you are faced with a system that doesn’t perform up to expectations, it is not always easy to determine what is at fault...

Obviously fitting quality antennas, insulators, connectors and cables is essential. The antenna system may be the least expensive part of your communications set-up but unless it is working efficiently, you will get poor results, no matter how sophisticated your transceiver is - sometimes no results at all! And because antenna location has a critical influence on performance, height above ground, proximity to other objects, type of ground and surrounding terrain all come into play. Naturally, the performance of vertical antennas is more affected than other types.

Quality materials and construction are very important With antennas at sea, additional environmental protection is required if they are to stand the test of time. The sea and salt spray make short work of any product that is not designed to keep them out, while inferior types of alloy fittings can result in serious corrosion and electrolysis.

While fibreglass is frequently used as a radome for marine antennas, it should be remembered that, particularly with HF antennas, only the thin metal wires within conduct the signal rather than the whole surface area as in metal tube or rod type antennas. Moreover, the surface becomes powdery and cracks develop in the fibreglass over time. These surface cracks are prone to surface burns and also to moisture and salt absorption, which result in corrosion and subsequent deterioration in performance and eventually failure of the antenna, unless regular maintenance is carried out.

Choice of insulator material is also very important. Insulators will absorb water if the material is not designed to be moisture resistant to the correct degree. This is critical in the case of high power installations, where water ingress from saltwater spray and the like, can induce flashover and short circuits leading to severe damage.

Where maximum possible range is important, it is necessary to ensure that the correct coaxial cable is used to minimise transmission line losses. As cables vary according to specifications and deteriorate over time, it is also essential to bear this in mind. In the marine environment a cable with tinned inner and outer conductors, like that supplied by Moonraker, will survive longer.

Moonraker marine antennas are either given a high durability coating, resistant to chemical and environmental effects, or coated with semi-flexible polyeolefin and sealed with a hot melt adhesive, to ensure that the antenna is fully insulated from the environment. These coatings meet military specifications with UV protection and flame retardation. Special attention is given to choice of insulation materials and to even the smallest fitting, as the system is only as strong as the weakest link in the chain.

When you are installing VHF and UHF antennas you will hear the advice, the higher the better. Although additional height is generally better, this is only true if the cable runs you are using do not exceed those recommended for the antenna. And the higher the frequency, the more critical this becomes. You may find that all the additional benefit of mounting high up on the mast is cancelled out in cable losses. Where cable runs are too long, the choice is either to relocate the antenna or to fit lower loss, more expensive cable that can cope with the additional length.

As a general rule at VHF and higher frequencies, RG58 should not be used for outdoor runs of more than about 5-6 metres (16-20ft). RG213 is suitable for outdoors runs up to about 30 metres (100ft). For longer runs it is necessary to use Heliax type cables which have much lower losses but are not flexible.

Using cable manufacturers’ indicative attenuation charts, like the one above, can give you a good idea of what the losses would be with the different types of cable. For every 3dB down, half the radiated power is lost. Thus a 100m (350ft) run transmitting between 148 and 174 MHz using RG213 will give a 9.1dB loss (3.5 x 2.6). Using a 50W transmitter, only 6.25w would be radiated (-25w + -12.5w + -6.25w). However, using Heliax ½ in., the loss would be reduced to 2.8dB (3.5 x 0.8 and around 25w would be radiated. Your antenna manufacturer can recommend what will be the optimum cable for your installation.

Talking about antenna gain often causes confusion. Many people also think that the more gain they have the better the result will be. Whether this is true or not depends on your communications needs. High gain collinears type antennas will give you greater range which is excellent for operating in fringe areas, but this is at the expense of beamwidth. This is because the radiation beam becomes narrower and lower the more stacked elements the antenna has. In practice, this means that, if you have a high gain collinear, your signal may be blocked if you are close to a cliff, hill or building because of the low concentration of the beam, where a simple 1/2 wave dipole would not. If you are at sea on the fringe of your VHF range, as many fishermen often find themselves, the additional range offered by the Moonraker MD G3 antenna will serve you well. However, if you do not expect to be able to benefit from increased range, the 1/2 wave MD is probably more suitable.

Generally speaking, a 1/4 wave antenna will give you a beamwidth of approximately 10% of its operating frequency. However the longer the collinear antenna, the lower the angle of radiation with a resultant reduction in beamwidth. The beamwidth narrows increasingly with each additional element .

Another factor to bear in mind when choosing a high gain antenna is that in high seas, when the boat is tossed about, the angle of the antenna, may sometimes be lowered down towards the sea. In this case a normal ½ wave dipole antenna will be more forgiving than a collinear with its narrow beamwidth, as signals may not come within the beam or only at the edge of the beam, or may be transmitted at very low angles. Sometimes signals will deteriorate in quality when this occurs, sometimes signals can be lost completely. UHF collinears suffer the most, as the greater the antenna height on the boat, the more likely the antenna is to suffer from these affects.

Assuming higher gain is what you need, how do you determine what will give the best performance?

When antennas are advertised, figures are often given for the amount of gain they exhibit. Unfortunately there is no universal format for specifying performance gain. Suppliers and manufacturers often quote this in different ways which makes it difficult to make comparisons. You will find: unity gain, gain relative to a dipole, dBi. dBd or just plain dB! The trouble is you get very different figures in each case.

A real antenna in free space like the 1/2 wave dipole does not radiate uniformly in all directions. If you picture the sphere of radiation as a ball made of foam and squeeze it at centre top and bottom, you redistribute the energy from the top and bottom into the sides to form a donut shape Energy is concentrated in one particular plane producing gain in this direction and reducing radiation at the top and bottom.

The 1/2 wave dipole in free space has a donut shape with deep nulls off the ends of the wire. It is said to have a gain of 2.15 dBi in its favoured directions (ie over the isotrophic radiator). Therefore when comparing dBd with dBi, 2.15 dBi is normally added to dBd to obtain the equivalent in the dBi value (eg 2.85 dBd = 5dBi). (You will often find this rounded up to 3dBd = 5dBi.)

Whether gain is measured in dBd or dBi, the comparison is with that of its counterpart in free space. Beamwidth is often quoted to give you a general ideal of the relative gain of the major lobe or lobes of a radiation pattern.

This is normally measured at the half power or -3dB points (3 dB down) which shows the useful angles of elevation of a lobe.

Antenna gain measured in dBi (over perfect ground if the antenna needs a ground plane like ¼ wave antennas), is the most useful method for making meaningful comparisons in antenna performance providing a theoretical baseline so comparisons can be made. All Moonraker antenna gains are quoted in this way except for amplifier gains (active antennas) which relate to how much improvement is made by the amplifier on antenna performance.

VSWR or SWR (Standing Wave Ratio) is often referred to as indicative of antenna performance, the often preferred measurement being 1.5:1 or less.

This refers to how much power is delivered from the antenna to the environment compared to how much power is reflected backwards along the line creating standing waves.

Any VSWR quoted will refer to an antenna’s performance over a bandwidth, so it is important to bear this in mind.

An antenna may quote a very good VSWR but this may only be over a very limited frequency range. On the other hand the bandwidth may be much larger. For example the Moonraker MD G3 collinear antenna quotes a VSWR <1.5:1 for the entire marine band (156-162 MHz).

Of course, if an antenna does not perform up to specifications, it is time to check your installation and consult your antenna manufacturer, who should be happy to help.