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Old 01-21-2008, 01:33 AM
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Seldom Seen Seldom Seen is offline
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Join Date: Aug 2005
Location: Highlands Ranch
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Default T9

Notes on the T9 section - Radio Waves, Propagation, and Antennas


Feedline – wire connecting the antenna to the radio

- most common type is coaxial cable (“coax”)

Beam antenna – concentrates signals in one direction

-examples include: Yagi, Quad, Dish

Vertical antenna – single conductor ("element") perpendicular to Earth’s surface

- often car antennas are short vertical antennas, or 5/8 wavelengths long

- car antennas often have magnetic mounts for attaching to the roof

- 5/8 wavelength antennas have lower radiation angles than

Horizontal antenna – element parallel to Earth’s surface

Dipole antenna – wire antenna one half-wavelength long, usually horizontal

- the higher the frequency, the shorter the length

- equation: length (feet) = 468 / frequency (MHz)

- the wire is cut in the center; the coax is connected to the antenna at the center

- one half of the antenna is connected to the center conductor of the coax

- the other half is connected to the shield (the braided wire surrounding)


Rubber duck antenna – short rubber-coated antenna usually found on handheld radios

- not as efficient as full size antennas; especially bad inside cars

Dummy load – an “antenna” that does not put out a radio signal

- converts the radio energy to heat instead

- used to test radios without radiating a signal

Propagation – the ability of a radio wave to travel over distances

- different types of radio waves (HF, VHF, UHF) propagate differently

- HF signals can be reflected by the ionosphere (they “skip”)

- VHF & UHF signals don’t skip, so they usually only work line-of-sight

- they actually go slightly further than line-of-sight due to atmospheric refraction

- radio horizon: where radio signals are blocked by the Earth’s curvature

- sometimes VHF signals reflect from ionized patches in the E layer of the ionosphere (“sporadic E”)

- UHF signals work better inside buildings since their shorter wavelength allows them to penetrate better

- VHF & UHF signals can be reflected off of buildings (to repeaters, for example) using directional antennas

Problems on VHF & UHF

- bursts of tones or fragments of conversation on VHF or UHF: receiver overload from strong nearby signals

- suddenly weak or distorted signal: try moving a few feet, perhaps multiple reflections causing destructive

interference

- when a repeater is distant, be sure to hold your handheld’s antenna vertical, since repeater antennas are vertically

polarized. If polarizations don’t match, your signal might be up to 100 times weaker when it arrives.

Antenna testing is done by measuring SWR: standing wave ratio

- ratio of high to low impedance in a feed line

- is a measure of how well the feedline is able to deliver energy to the antenna

- a 1:1 SWR is perfect; this means 100% of the energy is going to the antenna, none is reflected

-above 2:1, a lot of power is being reflected; most modern radios have circuits that begin reducing transmitter power when the SWR is above this

- a SWR meter or a directional wattmeter can measure SWR

Coax – easy to use and doesn’t have many special installation requirements

- important to have low SWR when using coax to reduce losses and transfer power to the antenna efficiently

- power lost in a coax line is converted to heat

- weather and sunlight can increase losses in coax over time

- coax fails most often because of moisture contamination

- black plastic-coated coax can reduce ultraviolet damage

- most common type of coax for hams is 50 ohm

Last edited by Seldom Seen; 01-21-2008 at 02:22 AM.
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