Antenna Systems ARRL Book Section 3.2

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)

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 Boom Radiator Reflectors Directors Signal Direction (gain from about 6 to 14 db)

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

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 1/4 th of the wavelength –For 146 megahertz (ie 2 meter) ¼ wavelength is 19 inches (T9A11) –A 1/4 th 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

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

Considerations in Choosing 1/4 th 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 5/8 th wave keeps most energy closest to the ground where it is needed –Paulism – I insist on 5/8 th 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?

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)

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