This is the class outline for week 2 of the SCARS Technician class 2013.
Week 2 Outline
Quiz on Week 1 Handout
- 3.75 MHz is the same as
- 3750 Hz
- 375 kHz
- 3750 kHz
- 0.0375 GHz
- If you QSY up 250 Hz from 7150 kHz, you’d be at
- 7150.25 kHz
- 7400 kHz
- 7150.250 kHz
- A. and C.
- An ordinary AA battery is about 1.5 volts, or
- 150 millivolts
- 1500 megavolts
- 1500 mV
- 150 mV
- For both question 1. and 2., you are operating in what part of the EM spectrum?
- Which of the following modes makes most efficient use of the RF spectrum?
- Which of the following modes is most efficient for voice communications?
- About how much bandwidth does that mode require?
- 3 kHz
- 6 kHz
- 3 MHz
- 6 MHz
- Fast-scan TV (video) requires about 6 MHz of bandwidth. Why do you think it is impractical in the HF part of the spectrum?
Chapter 2: Radio Signals and Fundamentals
Electromagnetic (EM) Waves (Radiation)
- alternating current (AC) in wire (“antenna”) creates time-varying electric (E) field around wire
- time-varying E field creates magnetic field (M)
- time-varying M field creates E field
- EM waves travel through vacuum (~air) at speed of light:
- C = 300,000,000 m/s
- different frequency EM waves have different properties:
- Radio waves, radio frequencies, or just RF:
- – approx. 3 kHz to 300 GHz
Characteristics of Waves
Frequency and Wavelength
Frequency: number of cycles (peak to peak, trough to trough…) per second = Hertz = Hz (cps)
- kHz, MHz, GHz
Wavelength: length, in meters, of one full wave in space (peak to peak, trough to trough…)
Since C is constant,
- f = C/λ or
The higher the frequency, the shorter the wavelength!
The lower the frequency, the longer the wavelength!
For frequency in Hz, distance in m, time in s (fundamental metric units), then,
- f = 300,000,000/λ or
- λ = 300,000,000/f
So, if you are operating in the 15 m band, the approximate frequency would be:
- f = 300,000,000/15 = 20,000,000 Hz
For convenience [by dividing both sides of the equation above by 1,000,000] we can use:
- f = 300/λ
where f is in MHz
- λ = 300/f
Our problem above becomes:
- f = 300/15 = 20 MHz
T3B07 What property of radio waves is often used to identify the different frequency bands?
- The approximate wavelength
- The magnetic intensity of waves
- The time it takes for waves to travel one mile
- The voltage standing wave ratio of waves
Because band implies a range of frequencies, band “names” are approximate. For example, if you are operating at 7150 kHz, what band are you in?300/7150 = 0.04196 meters Whoops!
Modulation and Bandwidth
- the simplest type of radio signal is a continuous wave,, or an unmodulated wave, or a single frequency:
- A continuous wave takes up virtually no bandwidth, or “space” in the radio spectrum:
- this is what a spectrum analyzer would see
- the simplest way to modulate a wave is to turn it on and off, such as with Morse code
- hams call this CW (although the continuous wave is interrupted)
Single-Sideband (SSB) Modulation
- Below 10 MHz: LSB
- Above 10 MHz: USB
- At 10 MHz: WWV!
Frequency Modulation (FM)
- narrow-band FM
- wide-band FM
- FM broadcast
Radio Equipment Basics
Station with Accessories
“Homework” for next week