Milton Keynes Amateur Radio Society (MKARS)
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1 Milton Keynes Amateur Radio Society (MKARS) Intermediate Licence Course Feeders Antennas Matching (Worksheets 31, 32 & 33) MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 1
2 Feeders Coax cable RF is contained WITHIN the cable No radiation from the cable External features (metal pipes etc.) will not have any Coax effect on the RF flowing through the cable Essential that connectors are attached properly to ensure earth continuity Coax cable has a characteristic impedance Amateurs use 50Ω Coax (TV is 75Ω) Coax is an unbalanced feeder (not electrically symmetrical) When used to connect to a balanced antenna the shielding effect of the coax braid will only be effective if a balun is used MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 2
3 Balanced or Twin feeder Twin Feeder / Ribbon + Two conductors in parallel in a plastic sheath - no screen May also be open wire feeder Balanced Feeder - electrically symmetrical Does not radiate RF, providing nothing alters the balance between the two conductors Characteristic impedance is usually 300Ω or 450Ω MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 3
4 Why a balanced feeder does not radiate RF MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 4
5 Feeder Characteristic Impedance B A Feeder wires have some resistance The two conductors also act as a capacitor Introduces Reactance Together these gives us Impedance B A All Feeders have a Characteristic Impedance, Z 0 (eg 50 or 75 Ohm Coax, 300 Ohm twin feeder etc) Z 0 is based on the Ratio of A and B (and the nature of the spacing dielectric) MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 5
6 Feeder Losses ALL feeders have loss - the longer the feeder the greater the loss Losses tend to be higher at VHF / UHF than HF Twin Feeder has a lower loss than Coaxial cable The loss affects both the Transmit and Receive paths For some standard cables the loss is: Per 100m RG58 RG MHz 4.8 db 2.0 db 30 MHz 8.2 db 3.2 db 144 MHz 21 db 8.6 db MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 6
7 Decibels (db) (Expressing losses and gains using db) Gains and Losses may be expressed in db s 3dB x2 or a half 6dB x4 or a quarter 9dB x8 or an eighth 10dB x10 or a tenth A loss of 3dB, halves power A gain of 3dB, doubles power Example 1: Tx supplies 50 watts Feeder loss is 6dB per 100m Feeder length is 50m Feeder loss is therefore 3dB Power at the antenna is 25 watts Example 2: Coax has 1dB loss per 10m Use 30m coax Loss in coax is 3dB Example 3: Tx supplies 50 watts Antenna has 9dB gain ERP is 400 watts MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 7
8 Dipole Radiates from sides (not ends) Antenna size is determined by the operating wavelength (λ) A dipole is roughly half a wavelength long An 80m dipole will be roughly 40m long; a 20m dipole will be roughly 10m long Each leg is λ/4 long; λ/2 in total Radiation pattern looking down on dipole antenna MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 8
9 Polar diagrams MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 9
10 Yagi antenna (designed to radiate in one direction) Directors focus to give Gain Rear Reflector Dipole Directors Reflector gives back/front isolation Effectively suppresses rear lobe Directional MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 10
11 Polarisation Electromagnetic radiation comprises both an Electric (E) and a Magnetic (H) Field The two fields are at right-angles to each other and the direction of propagation is at right-angles to both fields The Plane of the Electric Field defines the Polarisation of the wave H E MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 11
12 Matching a Tx to an antenna Tx s need a load Tx s generally designed to transfer energy into a load with a characteristic impedance of 50Ω What makes the antenna system impedance equal to 50Ω? Feeder (with a 50Ω characteristic impedance) The antenna (ideally with a 50Ω feed point impedance) MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 12
13 Antenna feed point impedance All Antennas have a feed point impedance This is determined by the dimensions and height of the antenna Different antennas will have a different feed point impedance Dipoles in theory are 73Ω but in practice they are more like 65Ω Close enough to the nominal value of 50Ω, but won t provide a perfect match If an antenna is not the correct length for the transmitting frequency there will not be a perfect match The greater the mismatch between the Tx frequency and the antenna s resonant frequency the greater the mismatch MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 13
14 Matching If antenna is not the correct length for the frequency, the feed point impedance will not be 50 ohms Where there is a mismatch, some of the RF will be reflected back from the antenna This sets up standing waves on the feeder Points of high and low voltage Has the potential to damage the Tx The amount of power reflected back is referred to using a Standing Wave Ratio (SWR) Correctly VSWR (Voltage Standing Wave Ratio) 2 I V 0 1/4 WAVELENGTH 1/4 WAVELENGTH 1/4 WAVELENGTH 1/4 WAVELENGTH MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 14
15 Antenna Matching (1) Not 50Ω - poor SWR 50Ω - good SWR Better described as AMU MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 15
16 Antenna Matching (2) Where there is a significant mismatch between Tx and antenna it is possible to use a matching device ATU / AMU ATU / AMU does not: tune the antenna remove the mismatch reduce standing waves change the feed point impedance of the antenna ATU / AMU does: allow the Tx to see the correct impendence and therefore transfer maximum energy into the antenna system If the antenna feed point impedance is very different to the Tx impedance, then the antenna may not radiate very well A dummy load is an excellent match but a poor radiator! MKARS Intermediate Licence Course - Worksheet Antennas Feeders Matching - Andrew Thomas G8GNI/M5AEX V1.1 16
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