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 allExamples 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 1/4th 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

Getting a Better Signal From a Non-Directional Antenna

1/4th Wavelength ½ Wavelength 5/8th Wavelength

Take Off Angel

Main Signal Direction

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)

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?

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