ANTENNAS
	Antenna Types There are three basic types of antennas which are used for radio communication on board vessels. (1) Whip Antenna This type is the rigid, usually white, plastic rod, which is mounted vertically. Whip antennas are just about universally used for Very High Frequency radio transmission and reception. As we saw in the section on propagation, the range of a VHF signal is pretty much dependent on the height of the antenna, so any VHF antenna should be mounted as high as possible. Whip antennas are also sometimes used for Medium Frequency and High Frequency transmission and reception. We will shortly see that lower frequency signals need longer antennas, so whips for MF and for HF will be much larger than for VHF radios. (2) Wire Antenna This is exactly what it sounds like - a length of wire, suitably suspended with insulators. If it is suspended vertically, and the signal is fed at one end, then it behaves exactly like a whip. More often though, a wire antenna is suspended horizontally. The signal can be fed from one end of the wire, but sometimes it is fed from the centre, to make what is called a dipole. More on that in a moment. (3) Dish Antenna Whips and wire antennas are more or less omnidirectional. That is, they can transmit and receive equally well in all directions. Some satellite communication systems operate at very low power, and they need a very directional antenna to focus the signal into a beam. Such a system uses a dish antenna. The dish itself is not unlike the receiving dish for domestic satellite television, and indeed, if the ship was fixed firmly in one position, virtually the same antenna could be used. However, as we all know, ships move around, all too often quite wildly, so a dish antenna must be fitted with motors and sensors to keep the dish pointed at the satellite while the ship moves. To protect the mechanism and to prevent the wind moving the dish, the antenna is encased in a plastic dome - the ubiquitous mushroom that we see on large yachts and most commercial ships. For vessels which are required to comply with GMDSS, there are certain rules for antennas. A reserve antenna must be fitted (unless specifically exempt) for the main MF/HF transmitter. For any suspended wire antenna, a complete spare must be carried, which can be erected if an antenna is lost. The halyards for a wire antenna must be wire not rope, so that in the case of fire, the antenna does not come crashing down. A weak link, with a breaking strain of 1/3 of that of the antenna itself, must be fitted, together with a safety loop to restrain the antenna if the weak link breaks. Compulsory ships are required to carry complete spare units for any whip antennas that are fitted.
	Antenna Tuning Antennas which are not the optimum length can be made to operate on a particular band, or indeed on several bands, by adding various coils and capacitors to the end of the antenna. In this way the radio can be fooled into thinking that the antenna is a perfect length for the frequency on which it is being asked to transmit. A tuner unit is used to do this, and it may require manual operation, usually by means of three rotary switches, or it may be automatic, needing just a push of a button to select the best combination of coils and capacitors for a particular frequency.
	Antenna Maintenance On wire antennas, the insulators must be kept clean. A build-up of salt or dirt on the insulators will reduce their ability to insulate, and cause a significant loss of signal, so they should be cleaned on a regular basis. Great care must be taken that the transmitter cannot be activated while anybody is near the ends of the antenna. Extremely high voltages are generated at the ends, and they can cause severe injury, or possibly even death. Any shackles used in the assembly of the antenna should be kept well greased, so that the antenna can be disassembled if there is a problem without having to fight with seized shackles. Whip antennas too should be kept clean. They are made of glass fibre, which is transparent to the radio frequencies. However, a layer of salt, perhaps mixed with a bit of dirt, will act as a fairly effective shield, stopping the radio waves from reaching the wire antenna inside. Similarly, the glass fibre dome over a dish antenna must be kept clean for the same reason. The dome must not be painted, except with a special paint, because normal paint is opaque to radio signals. Again, care must be taken not to transmit while anybody is working on or near the dome. The frequencies used for satellite communication are extremely high, and the effect of being close to the dome when the set is transmitting would be akin to putting your head in a microwave cooker - an experience to be avoided.