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Antenna Basics

The TV Channels

Hertz (Hz) means cycles per second. (Heinrich Hertz was the first to build a radio transmitter and receiver while understanding what he was doing.) The radio frequency spectrum is divided into major bands:

Frequency: Wavelength (in meters):
VLF very low frequency 3 KHz 30 KHz 100 Km 10 Km
LF low frequency 30 KHz 300 KHz 10 Km 1 Km
MF medium frequency 300 KHz 3 MHz 1 Km 100 m
HF high frequency 3 MHz 30 MHz 100 m 10 m
VHF very high frequency 30 MHz 300 MHz 10 m 1 m
UHF ultra high frequency 300 MHz 3 GHz 1 m 100 mm
SHF super high frequency 3 GHz 30 GHz 100 mm 10 mm
EHF extremely high frequency 30 GHz 300 GHz 10 mm 1 mm


KHz means 1000 Hertz, MHz means 1,000,000 Hertz, and GHz means 1,000,000,000 Hertz

A TV channel in the U.S. will always occupy 6 MHz of this spectrum.

Channels 2-6 occupy consecutive spectrum from 54 MHz to 88 MHz. (with one small gap) Channels 7-13 occupy consecutive spectrum from 174 MHz to 216 MHz. Channels 14-69 occupy consecutive spectrum from 470 MHz to 806 MHz.

Channels 2-13 are the VHF channels. They are split into two groups so that antennas will work better: In general, an antenna designed for frequency N will also work well at 3N, but very poorly at 2N.

The wavelength of a radio wave is: ? = 300/F where F is the frequency in mega-Hertz and ? is the wavelength in meters. Antenna elements are usually about a half-wavelength long.

Decibels
Decibels (dB) are commonly used to describe gain or loss in circuits. The number of decibels is found from:

Gain in dB = 10*log(gain factor) or



In some situations this is more complicated than using gain or loss factors. But in many situations, decibels are simpler. For example, suppose 10 feet of cable loses 1 dB of signal. To figure the loss in a longer cable, just add 1 dB for every 10 feet. In general, decibels let you add or subtract instead of multiply or divide. There are some special numbers you might want to memorize:

20 dB = gain factor of 100 10 dB = gain factor of 10 3 dB = gain factor of 2 (actually 1.995) 0 dB = no gain or loss -1 dB = a 20% loss of signal -3 dB = a 50% loss of signal -10 dB = a 90% loss of signal

Noise
Whether a signal is receivable is determined by the signal to noise ratio. For TVs there are two main sources of noise:

Atmosphere noise. There are many types of sources for this noise. A light switch creates a radio wave every time it opens or closes. Motors in some appliances produce nasty RF noise. Receiver noise. Most of this noise comes from the first transistor the antenna is attached to. Some receivers are quieter than others.

Receiver noise dominates on the VHF and UHF bands, and atmospheric noise is usually insignificant. On an analog channel, noise looks like snow. If there were only a barely perceptible amount of snow, this would correspond to how noise-free a DTV signal must be for a DTV receiver to lock-on to it.

This article was published on Tuesday 14 December, 2004.
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