Transcript Antenna Systems - Supplemental Teaching Resources

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
Reflectors
Directors
Boom
Signal
Direction
Radiator
(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
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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
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
1/4th
Wavelength
½ Wavelength
5/8th Wavelength
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
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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