Impedance, Reflections, and Transformations

Transcription

1 Impedance, Reflections, and Transformations Rocky Mountain Ham Radio University Chris Hamilton AE5IT 2017 December 16

2 Conventional wisdom: My antenna is useless above 1.5:1 SWR (Or is it 2:1? Or 3:1?) Antenna tuners only make your transmitter happy. Noooooooooo Chris Hamilton AE5IT Rocky Mountain Ham Radio University 2

3 Waves behave just about the same in any medium! Ripples in water Wake from a boat Guitar strings Radio waves in coax line Chris Hamilton AE5IT Rocky Mountain Ham Radio University 3

4 Wave Superposition: Waves sum when they intersect Chris Hamilton AE5IT Rocky Mountain Ham Radio University 4

5 Wave Superposition: Waves sum when they intersect Chris Hamilton AE5IT Rocky Mountain Ham Radio University 5

6 Wave Superposition: Waves sum when they intersect Chris Hamilton AE5IT Rocky Mountain Ham Radio University 6

7 Wave Superposition: Waves sum when they intersect Chris Hamilton AE5IT Rocky Mountain Ham Radio University 7

8 Wave Superposition: Standing waves! Forward & Reverse waves identical amplitude; 100 % reflection - peaks and troughs 2x amplitude - nodes have zero amplitude Chris Hamilton AE5IT Rocky Mountain Ham Radio University 8

9 Wave Superposition: Standing waves! Forward & Reverse waves different amplitude; partial reflection - peaks and troughs somewhere between 1x & 2x - nodes have non-zero amplitude Chris Hamilton AE5IT Rocky Mountain Ham Radio University 9

10 Standing Wave Ratio Vmax Vmin 100 % reflection Voltage SWR = Vmax 2.0 = Vmin 0 = Chris Hamilton AE5IT Rocky Mountain Ham Radio University 10

11 Standing Wave Ratio Vmax Vmin partial reflection Voltage SWR = Vmax 1.0 = Vmin 0.54 = 1.85:1 Chris Hamilton AE5IT Rocky Mountain Ham Radio University 11

12 Standing Wave Ratio Vmax Vmin 0 % reflection ( 100% absorption! ) Vmax 1.0 Voltage SWR = = = 1:1 Vmin 1.0 Chris Hamilton AE5IT Rocky Mountain Ham Radio University 12

13 Reflections come from impedance mismatches Standing waves come from reflections Big impedance mismatch = big standing wave ratio True in radio, water, mechanical! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 13

14 Four ways to determine SWR Use whichever is simplest to measure but understand what you re doing Chris Hamilton AE5IT Rocky Mountain Ham Radio University 14

15 What does this mean for radio? Chris Hamilton AE5IT Rocky Mountain Ham Radio University 15

16 What does this mean for radio? Open line demo! 50 Ohm TX ( FT W MHz ) 4:1 half-wave coax balun 200 Ohm open wire line (1/8 brass rods, 1/3 on center) Changeable loads Directional power indicators Voltage indicators Power RX Chris Hamilton AE5IT Rocky Mountain Ham Radio University 16

17 Open line demo Matched condition: dummy load Directional power indicators show forward power No reflected power Voltage indicators equally bright along entire length of transmission line No meaningful received power Chris Hamilton AE5IT Rocky Mountain Ham Radio University 17

18 Open line demo Mismatched condition: open circuit Directional power indicators show forward power 100% reflected power Voltage indicators show standing waves! High & low voltage points 90 degrees apart Repeats every ½ wavelength No meaningful received power Chris Hamilton AE5IT Rocky Mountain Ham Radio University 18

19 Open line demo Mismatched condition: short circuit Directional power indicators show forward power 100% reflected power Voltage indicators show standing waves! High & low voltage points 90 degrees apart Repeats every ½ wavelength Phase shifted compared to open circuit! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 19

20 Open line demo Mismatched condition: ½ wave dipole ~ 3:1 SWR Directional power indicators show forward power Some reflected power Voltage indicators show standing waves! High & low voltage points 90 degrees apart Repeats every ½ wavelength SWR not as severe as open or short Note signal at receiver Chris Hamilton AE5IT Rocky Mountain Ham Radio University 20

21 Open line demo Mismatched condition: full wave dipole ~ 11.5:1 SWR Directional power indicators show forward power Much reflected power Severe mismatch, feeding at high voltage point Voltage indicators show standing waves! High & low voltage points 90 degrees apart Repeats every ½ wavelength SWR pretty bad Note signal at receiver Chris Hamilton AE5IT Rocky Mountain Ham Radio University 21

22 Received signal strength 3:1 SWR vs 11.5:1 SWR Chris Hamilton AE5IT Rocky Mountain Ham Radio University 22

23 Received signal strength 3:1 SWR vs 11.5:1 SWR Only a few db difference, not catastrophic! Why??? Chris Hamilton AE5IT Rocky Mountain Ham Radio University 23

24 Received signal strength Low-loss line! Reflected signal re-reflects Eventually radiates Minus some heat loss Chris Hamilton AE5IT Rocky Mountain Ham Radio University 24

25 Can we make it better? Impedance transformer antenna tuner! ½ wave dipole: 4.7 pf shunt cap 23.2 cm from load (or 73.2 cm ) Full wave dipole: 1.9 pf shunt cap 56.4 cm from load (or cm ) Note received signal strength! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 25

26 Has SWR changed? Is the transmitter happy? Is the transmitter fooled? Is the receiver happy and fooled? Reflected power is real power let it radiate! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 26

27 Scenario 144 MHz 15 feet 450 Ohm window line 52 Ohm antenna 8.5:1 SWR 63% reflection db (1.3%) loss per 15 feet 100 W Tx 1.3 W Heat 0.8 W Heat 62.2 W Refl W Radiated W Fwd 0.8 W Heat 0.5 W Heat 37.7 W Fwd 0.5 W Heat 38.2 W Refl W Heat 23.4 W Refl 13.8 ( and so on ) Total 93.8 W radiated 6.2% lost to heat! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 27

28 Scenario 144 MHz 15 feet RG Ohm antenna 8.5:1 SWR 63% reflection 1 db (20%) loss per 15 feet 100 W Tx 20 W Heat 10 W Heat 50.4 W Refl W Radiated W Fwd 8.1 W Heat 4.1 W Heat 16.3 W Fwd 3.3 W Heat 20.3 W Refl W Heat 8.2 W Refl 4.8 ( and so on ) Total 49.6 W radiated 50% lost to heat! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 28

29 Open line demo Let s make it lossy Change Tx line dielectric to water Place thawed otter pops on high voltage points. Note received signal strength! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 29

30 Lesson 1: line loss is everything Lesson 2: every antenna is multi-band if your feedline is low-loss. A single all-band dipole was common approach in the days before coax. Patterns and gain become interesting. But that s not necessarily a bad thing! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 30

31 Feeding a dipole above resonant frequency 1/2 λ nd harmonic From ARRL Antenna Book 22 ed Feeding a dipole on a harmonic is like building a collinear and not paying attention to element spacing. Chris Hamilton AE5IT Rocky Mountain Ham Radio University 31

32 Feeding a dipole above resonant frequency rd harmonic th harmonic From ARRL Antenna Book 22 ed Chris Hamilton AE5IT Rocky Mountain Ham Radio University 32

33 Feeding a dipole above resonant frequency th harmonic th harmonic From ARRL Antenna Book 22 ed This won t stop you from making WAS. Interesting gain & patterns if you can match the load! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 33

34 SWR: Where do we measure? Most SWR meters are really directional power meters. If measuring in shack, reflected power is attenuated by line loss! With 15 ft RG :1 SWR at antenna will look like 4:1 in shack! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 34

35 How to have a perfect SWR reading Transmit into a dummy load Use several hundred feet of crummy old coax Saturate your coax braid with water (Does your SWR reading get better over time? You have a problem.) Chris Hamilton AE5IT Rocky Mountain Ham Radio University 35

36 How to have a perfect SWR reading Put a tuner as close to your antenna as possible probably NOT on your desk! Run lowest-loss line you can manage from tuner to antenna Window line is okay when it s dry Homemade ladder line is a cheap & simple project Chris Hamilton AE5IT Rocky Mountain Ham Radio University 36

37 SWR s real downside High SWR means high voltage by definition! Can burn through insulators Can give other unpleasant surprises It s all manageable but be careful Chris Hamilton AE5IT Rocky Mountain Ham Radio University 37

38 Acknowledgements Bill Hays AE4QL Standing Up For Standing Waves Eric Nichols KL7AJ Lecher Lines Joel Hallas W1ZR The Doctor is In Chris Hamilton AE5IT Rocky Mountain Ham Radio University 38

39 Now play with the fun toys! Chris Hamilton AE5IT Rocky Mountain Ham Radio University 39