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Antenna Systems

Antenna Systems

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Antenna Systems

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  1. Antenna Systems ARRL Book Section 3.2

  2. Mobile Antennas • Isotropic radiator sends radio energy out equally in all directions – may not always be what you want • Mobile antennas are commonly made of one element perpendicular (pointing straight up from) the earths surface • These are called Vertical Antennas (T9A02) • Energy from these antennas goes out in all directions except straight up and straight down • As a result you get “gain” relative to an isotropic radiator • About 2.8 db gain • The apparent power of your radio doubles for every 3 db of gain • But you loose energy in some directions • Since most mobiles aren’t talking to space ships loosing straight up isn’t an issue. • A Magnetic Mount Vertical Antenna provides good efficiency for a mobile and can be easily removed and installed. (T9A09) • Avoid having the antenna inside car with you. Metal will just bounce signal around inside with you. • The signal from a handheld inside a car with a rubber duck antenna is 10 to 20 times weaker outside the car than inside (T9A10)

  3. Other Ways to Boost Power • Concentrate the signal in just one direction • A Beam Antenna Concentrates the signal in just one direction (T9A01) • TV antenna with a boom and elements out to the side is an example of a Yagi antenna Reflectors Directors Boom Signal Direction Radiator (gain from about 6 to 14 db)

  4. More Antennas Quad Dish The Yagi, Quad, and Dish Antennas are all Examples of Directional Antennas (T9A08)

  5. Antennas are Frequency Sensitive • How do you decide what note or frequency a vibrating string will make – its length • Length of antenna also works best on radio waves with the right length. • Minimum length for good performance is 1/4th of the wavelength • For 146 megahertz (ie 2 meter) ¼ wavelength is 19 inches (T9A11) • A 1/4th wavelength needs a good plane of ground (creates imaginary reflection of the antenna like a mirror) • The top of your car is a good ground plane • Another basic design is called a dipole • It is ½ wavelength long • It has two ¼ wavelength pieces – one for positive and one for negative (ie – it is its own reflection) • A 6 meter ½ wave dipole is about 112 inches long (T9A12) • Note that as the length of a ½ wave dipole becomes shorter as the frequency increases (T9A05) • One of the weaknesses of a CB – a ½ wavelength is nearly 25 feet • That kind of antenna would flip your car • If you have to make it shorter you have to coil up the antenna and coils waste a lot of energy

  6. Getting a Better Signal From a Non-Directional Antenna The main advantage of a 5/8th wave antenna over a 1/4th Wave is that it concentrates radio waves at a lower angle (T9A06) Main Signal Direction Take Off Angel 5/8th Wavelength ½ Wavelength 1/4th Wavelength

  7. Considerations in Choosing • 1/4th wave is cheaper, won’t catch on things as easily, or can be kept in a “go kit” box in the car (can do that with 19 inches, but not 31) • ½ wave gives maximum efficiency in converting radio power to radio signals • But launches at a higher angle – remember VHS is line of sight and most of the time your target is closer to the ground • 5/8th wave keeps most energy closest to the ground where it is needed • Paulism – I insist on 5/8th wave for my mobiles • Handling such a long old thing • Some antenna designs will fold over at a hinge • Mine don’t – they are mag mounts – how hard is it for me to lay one down?

  8. Height is Important • Doubling your height increases range about as good as 4 times the power • For base station consider a tower • For mobile consider a portable tower (like Lee’s) • Go for high ground • Tennis ball and string to put antenna up in a tree • Bow and arrow to shoot string into a tree and pull up an antenna • I have 50 foot tower – how eager do you think I am for weather to mess up my antenna (remember I have to get up there to fix it) • Stainless Steel is often used for fasteners since it does not corrode as easily (T0B10)

  9. Weathering of Your Coax Cable • Weather and sunlight will make loss of power in the cable go up drastically(T9C09) • Main way Mother nature attacks • The most common reason for coax to fail is that moisture gets inside (T9C07) • UV sunlight isn’t just bad news for your skin (or curtains) – it tries to take apart the plastic shell of coax • Most Coax Cable is Black because it better resists the effects of UV sunlight (T9C10) • Usually have to figure that your coax will have to be replaced every 5 years or sometimes less • An easily neglected maintenance item