wavelength 2 cycles wavelength 4 cycles

Transcription

1 Waves Frequency: Cycles per second But that is too simple so instead we call one cycle per second a Hertz (Hz) 1000 cycles/second = 1000 Hz = 1 kilohertz = 1 khz 1,000,000 Hz = 1 megahertz = 1 MHz 1,000,000,000 Hz = 1 gigahertz = 1 GHz wavelength 2 cycles wavelength 4 cycles low frequency means long wavelength high frequency means short wavelength 1 Basic terminology needed to talk about radio waves. Radio waves travel 300,000,000 meters per second so we could think of these figures as showing one second in time, or 300,000,000 meters in space. 1

2 Sound Waves and Radio Waves Sound waves from microphone: 20 20,000 Hz Modulator AM, FM, etc. Amplifier Boosts power level Radio wave carrier Antenna Transmitter power levels: Antenna size depends Walkie-talkie: 5 watts or less on wavelength: Ham radio: watts high frequency Broadcaster: ,000 watts = short wavelength (An ordinary light bulb uses 60 watts) = small antenna 2 The transmitter takes a pure radio carrier wave and modulates it with audio information (voice, music, etc.) Common types of modulation are shown in the handout: AM, FM, CW. Hams often use single side band (SSB) which is a modified version of AM that uses less bandwidth. Simple antennas are a half or quarter wavelength in size though there are various tricks for making them shorter (and usually less effective.) Receivers perform the reverse action: pick up signals from the antenna, amplify them, and de-modulate (or detect) them to pull out the audio information for you to listen to. 2

3 Wavelength: 1000 m 100 m 10 m 1 m 0.1 m Long Wave Medium Short Wave Wave Frequency: 300 khz 3 MHz 30 MHz 300 MHz 3 GHz LF MF HF VHF UHF AM broadcast Where is amateur radio? On many bands from the top of MF through UHF. Broadcasters, military, etc. TV channels 2-13 FM broadcast GPS Cell phones TV channels This portion of the radio spectrum is what we are interested in here. The spectrum actually includes lower and higher frequencies than shown here. There are numerous radio services not shown here see the NTIA spectrum chart for the full story. Notice again that long waves correspond to low frequencies. 3

4 Radio Waves Travel Direct waves go straight from the transmitter to your radio. Ionosphere Skywaves are reflected off the ionosphere and can be picked up hundreds or thousands of kilometers away. 4 You can always pick up a transmitted signal if it is in sight and you are standing in the direction the antenna is aimed. Some antennas have narrow beams (directional) while others send radio waves in all directions (nondirectional or omnidirectional) Some radio waves can travel far beyond what you can see from the transmitter antenna. These radio waves are reflected by the ionosphere (or sometimes other things in the lower atmosphere) and can be picked up on the other side of the earth if conditions are right. 4

5 Daytime Skywave Off to outer space F region: > 250 km E region: ~120 km D region: < 100 km Absorbed! HF can be picked up far, far away! MF HF VHF 300 khz 3 MHz 30 MHz 300 MHz 5 Sunlight ionizes the upper atmosphere (rips electrons away from the atoms) creating several regions that reflect radio waves. The D region absorbs radio waves in the MF and low HF range so there are no skywaves in the daytime. The E region reflects radio waves in the lower HF range. Sometimes a strange version called sporadic E can reflect radio waves up to the VHF range, but it is not the usual state. The F region is at the highest altitude and can reflect radio waves farthest if the waves have not already been absorbed or reflected by the D and E regions. Normally VHF and UHF signals pass right through the ionosphere and out into space, so these frequencies are used for satellite and space probe communication. 5

6 Nighttime Skywave Off to outer space F region: > 300 km No D-region absorption! Both MF and MF HF VHF HF can be heard over 300 khz 3 MHz 30 MHz 300 MHz long distances 6 At night when sunlight goes away, the D and E regions fade away. The F region drifts upward and fades slowly, but it usually survives until sunlight returns in the morning. With the D region out of the way, MF waves (AM radio stations) can be reflected over long distances, and HF waves tend to have the farthest reflections. VHF and UHF waves still head off into space.

7 VHF and UHF waves travel line-of-sight Works great over a couple of miles if nothing much is in between. Not so great if there are buildings or mountains in the way. 10 VHF and UHF do not usually have skywaves. There are somewhat unusual cases where skywave propagation occurs: reflection from auroras, strong sporadic E, and tropospheric ducting. For our purposes, we won t worry about these cases. Basic VHF and UHF communication relies on direct waves from the transmitter to the receiver, which are limited to line of sight as far as you can see. If mountains or tall buildings get in the way, you may not be able to communicate.

8 VHF and UHF use repeaters for better coverage Everyone who can see the repeater can talk to each other, even if they cannot see each other. 11 The solution is to put repeaters on tall buildings or mountains where line-ofsight is very good. The repeater listens on one frequency and transmits what it hears on a slightly different frequency. People can talk to each other through the repeater when direct (simplex) communication is impossible. Broadcast stations use a similar method to increase coverage.

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