1 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance Chelmsford Amateur Radio Society Intermediate Course (3) Technical Basics - 2 AC & Impedance

2 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance AC - Alternating Current AC is easier to generate and transform Mains is 50Hz AC. Radio Frequencies (RF) use High Frequency AC Simple items such as Filament Light Bulbs work with AC and DC, but many electronic components are sensitive to the direction of current DC - Direct Current Cells/Batteries provide a source of DC power Direct Current flows in a single direction DC & AC  

3 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance AC Frequency & Period The Foundation course just described the shape of a sine wave whilst Intermediate requires a deeper understanding The Period, T of one cycle, in seconds is equal to 1/f, where f is in Hertz Frequency, f = 1 / T or Period, T = 1 / f Amplitude Time One Cycle

4 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance An AC Cycle One way of looking at the sine wave is as a cycle of 360 degrees The voltage or current has a complete rotation (like an alternator) The use of degrees indicates the time or ‘phase’ within one cycle Unlike a constant DC source, volts/current vary from zero to a +/- peak and back to zero, so we need a way of describing the average 180° 360° 0° +V peak -V peak 90° 270° Time NS AC Volts Output Brush Slipring

5 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance Peak & Peak-Peak Sine waves and other waveforms have varying amplitude with time The variation has positive and negative values during the cycle The peak value is the level of a positive or negative peak The peak-peak is the difference between the negative and positive peaks V pk Time One Period, T V pk-pk + - Amplitude

6 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance RMS Value RMS = Root Mean Square, which is a form of averaging The RMS value of any varying waveform is the equivalent of the constant DC Voltage that would have the same power or heating effect For a sine wave, the RMS value is equal to 1/  2 of the peak value. V rms = V pk /  2 or V rms = x V pk V pk V rms Time One Period, T V pk-pk + - Example: AC Mains is 230V rms So… V pk = 230/0.707 = 325V V pk-pk = 2xV pk = 650V

7 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance In air the velocity, v of radio waves is a constant ( ~3x10 8 m/s) So if the frequency increases, the wavelength decreases, and vice versa, determined by: v = f x Example 7MHz=40m, 10MHz=30m, 14MHz=20m approx Frequency & Wavelength v m/s f Hertz metres v Wavelength,  Frequency, f

8 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance Transformers consist of coils of wire sharing the same magnetic field and may have an iron or ferrite core to concentrate the field Energy is transferred from one coil to the other by AC changing the magnetic field - which can not happen with constant DC Voltages (such as AC Mains) can be stepped down to a lower level if fewer turns of wire are on the secondary coil than on the primary - or can be stepped up if the secondary has more turns AC & Transformers Basic Transformer Symbol Primary Secondary More General Transformer

9 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance AC & Components Components that store energy in electric or magnetic fields have a finite reaction time before they reach a steady state or the stored energy may oppose the change being applied –Most prominent in Capacitors and Inductors/Transformers This behaviour is different from simple Resistance If the input is changing (ie AC) then a time difference occurs between the current flowing and the voltage being applied –Charge rushes in or out of capacitor plates –Magnetic fields in coils create back EMFs which oppose current You can still apply Ohms law (‘R’=V/I) to such situations to assess this form of AC Resistance, known as Reactance, X

10 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance Reactance in Inductors As AC current flows (or tries to) Magnetic fields create ‘back EMFs’ which oppose the input current This AC Resistance, is termed Reactance For an inductor, the Inductive Reactance has symbol X L Applying Ohms law:- X L = V rms / I rms L AC Source Freq, Hz V rms I rms

11 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance Reactance in Capacitors When an AC Voltage is applied, charge rushes in/out of a capacitor plate, attracting/moving charge on the opposite plate This induced charge effectively enables an AC current to flow (unlike DC) So at AC, we have Volts and Current and its AC resistance is termed Capacitive Reactance, X C Applying Ohms law:- X C = V rms / I rms C AC Source Freq, Hz V rms I rms

12 Chelmsford Amateur Radio Society Intermediate Licence Course Murray Niman G6JYB Slide Set 17: v1.0, 25-May-2009 (3) Technical Basics -2: AC & Impedance Impedance In combinations of capacitors, resistors, or inductors, current will result in energy transfer (into heat) in the resistors and energy storage and release in the capacitors or inductors. R and X are in Ohms, but distinct in nature When correctly combined the overall term used for the resistance is Impedance, Z (as the components impede current flow) In such a circuit Ohms Law applies to the ratio of the overall potential difference to current so we have: Z = V rms / I rms RC V rms R L