- the pitfalls
Although insulation may seem relatively insignificant in the larger picture, on it depends the quality of your performance on air and how long you will be communicating for.
HF systems need to be well insulated, especially in the marine environment and where an antenna will be exposed to wet weather. Inefficient insulation of conductors, feedthrough insulators, antenna support insulators, etc., can result in considerable RF power losses, including loss of the system itself, not to mention serious damage to the operator! In the case of wire types, the outer end or ends of the antenna are of particularly importance.
Under transmitting conditions, all HF antennas have relatively large voltages and currents appearing along their length. Voltage is always high and current low at the far end of the antenna, and values as high as 1500 volts may appear with high power transmitters.
Although this voltage will not cause an electrical shock in the generally accepted sense, it can cause intense and painful burns. Because of these high voltages, it is important to have good insulators supporting them, especially wire antennas. Current is usually high and voltage relatively low at the transmitter end of the antenna and around 15 amps can be expected with short, unloaded antennas and high power transmitters. It is important therefore that large area conductors be used for the antenna itself and for antenna and earth connections.
Lost power and poor performance are often the first evidence of inadequate insulation. Before deciding to upgrade the transmitter it is usually best to question the antenna/earth system.
Increasing transmitter power only results in improving communications in the transmit direction and does nothing to improve the received signal. It is very expensive, consumes large amounts of (battery) power and demands a generally higher standard of installation to prevent breakdowns.
Insulators should have a long leakage path. The greater the length of an insulator compared to its surface area, the more effective it will be. As a result a long thin insulator has greater insulation capability than a short thick one. The surface leakage path can also be increased by deep ribbing, eliminating the need to increase the physical length of the insulator.
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.
Glass or glazed porcelain egg types are effective, provided that they have a perfect glaze without cracks or chips, but these damage more easily than low loss synthetic types. If egg type insulators are used, it is good practice to use several in series.
Nylon is often used as an insulation material. However some grades of nylon will absorb water and are thus highly unsatisfactory. For normal power installations, Moonraker uses a very low water absorbing nylon with high mechanical properties and dimensional stability.
In the case of high power installations, polypropylene is better, due to its high chemical and moisture resistance and excellent impact and scratch resistance. The smooth ribbed surface used in high strength polypropylene feedthrough and sidemount insulators, provides a long leakage path, easily washed free of salt and dust deposits. External connections are protected from water ingress.
Polypropylene is an excellent dielectric. Its dissipation factor and dielectric constant remain low over a wide range of frequencies and temperatures. Volume resistivity is high. This, plus its extremely low water absorption, makes it an ideal material for antenna insulators.
At sea, poorly insulated leads running close to metal decks, bulkheads or other wiring may cause arc-over and/or heat losses in the insulating materials. As a result it is also wise to ensure that high quality feedthrough insulators are used where cable needs to be passed through decks and bulkheads and that voltage protection is adequate for high power installations. It is good practice to smear sealant around insulator mounting holes before fitting. However, only neutral cure silicone sealants should be used as other types may corrode metal fixtures.
Strain insulators are often used to support copper tube type feeders above deck level. In this case the insulator must be strong enough to withstand the required strain and have an adequate leakage path. The Moonraker Standoff Strain insulator has these properties. Unprotected, few antennas will last any length of time in the harsh marine environment. Preferably both whip and wire types should be protected to prevent corrosion and, as some insulation materials degenerate in sunlight causing high leakage paths through the insulation, black is the most suitable colour for durability, if not the most fashionable.
Moonraker marine whips are protected either by a high durability epoxy based coating, resistant to chemical and environmental damage, as is used on the 100 and 80 series antennas or coated with semi-flexible polyolefin and sealed with a hot melt adhesive to ensure that the antenna is fully insulated and sealed from the environment. These coatings meet military specifications with UV protection and flame retardation. Antennas 6.7m (22ft) and over are normally given the epoxy based coating, smaller antennas have suitable quality polyolefin coating. It is important to note that PVC jackets vary in specifications considerably with differing electrical properties and degree of flexibility.
The cost of a good antenna/earth system is usually only a small part of the overall equipment cost, but it is a vital link in the communications chain. Poor insulation can ruin the performance of the very best radio equipment and shorten its life span considerably.