LESSON 8. ANTENNAS
Read the words:
whip antenna ()
wire antenna
dish antenna
rigid
plastic rod
to be mounted
range ,
height
to suspend
omnidirectional ,
tuner unit
maintenance
insulator
to reduce
shackle ,
assembly
to grease
glassfibre
2. Read the text and translate it:
Antenna Types
There are three basic types of antennas which are used for radio communication on board vessels.
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 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.
Wire Antenna 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.
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, ships move around, 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.
Antenna Tuning
Antennas which are not the optimum length can be made to operate on a particular band, or 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. 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.
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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 wish seized shackles.
Whip antennas too should be kept clean. They are made of glass fibre, which is transparent to the radio frequencies.
3. Answer the following questions:
Whip antennas are used only for Very High Frequency radio transmission and reception, arent they?
2) Do lower frequency signals need longer antennas?
3) How can a wire antenna be suspended?
4) What is a dipole?
5) What is the main difference between whips or wire antennas and dish antennas?
6) Could the antenna for domestic satellite television be used in the ship?
7) What makes the dish point at the satellite while the ship moves?
8) Why is the antenna encased in a plastic dome?
9) Is it important to keep the insulators on wire antennas clean?
10) How can the problem of seized shackles be solved?
4. Give the English equivalents from the text:
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