Antenna Analyzers Analysed

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Overview. Wide Range of devices available to measure Antenna CharacteristicsTraditional SWR and Noise Bridges

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Antenna Analyzers Analysed

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1. Antenna Analyzers Analysed Dave G4UGM

2. Overview Wide Range of devices available to measure Antenna Characteristics Traditional SWR and Noise Bridges “Simple” antenna analyzers May be called “Scalar Network Analyzers” “More Sophisticated” devices Vector Network Analyzers Bulk of tonights talk will look at “scalar” devices.

3. Traditional Antenna Matching SWR Measures mis-match between TX and Antenna Valve TX Can tune for max antenna current Valve PA can cope with mis-match Solid-State TX Much more delicate – can’t withstand high SWR Passion for low SWR

4. Tuning Aids - 1 Noise Bridge White noise generator Tune for NULL in noise in RX Pros True “Silent Tune” – no TX power Cons Not easy to use on noisy bands Relies on RX impedance being 50? Does not tell us why we have no match

5. Directional Power Meter Uses transformers to sense power Tune for minimum reflected At “good” SWR reflected power very LOW SWR 1:1.5 Reflected Power about 5% At “high” SWR reflected power is LOW SWR 1:2 Reflected power about 12 %

6. Example at 2:1 SWR Forward Power = 100 Watts SWR = 2:1 Reflected power = 11 Watts

7. An SWR Bridge Classic QRP Bridge A simple Wheatstone Bridge Load is one leg of bridge Simple Diode Detector When load is 50 ohm Bridge is in Balance Detector will show Minimum When load is not 50 Ohms Detector shows a reading

8. Problems SWR of 1:1 is at knee of diode Low SWRs not easily measured Work Around Use transformer to “step-up” voltage Use Schottky or Germanium Diode Use compensated op-amp Note Reactance in load also un-balances bridge

9. The reactive load Simple Circuit Signal = 4Mhz, 50? R2 = 50? reference Load = 25? + 400pf Look at the RMS Values Vin = 1.349 Vref = .546 Vload = 1.109 Why doesn’t Vin = Vref + Vload! PHASE SHIFT

10. Voltages in Simulated Antenna Peak Values don’t add up Instantaneous values do See the green line

11. Kirchoffs Laws Can be derived from Ohms laws But they make circuit analysis easier Kirchhoff's current law At any node (junction) sum of currents = 0 Kirchhoff's voltage law Sum of the Potential Difference round a circuit = 0 When applied to AC circuits Need to use the Vector Notation

12. Rotating a Vector generates a Sine wave The length of the vector represents the amplitude The starting position represents the phase We can use the starting vector to represent the wave These vectors can be added and subtracted They then conform to Kirchoffs laws Vector Notation

13. Vector Representation Resistance Reference Phase Inductance Current Lags by 90 ? Capacitance Current Leads by 90? Combined load Some angle between

14. A simple Impedance Bridge

15. Calculating Vx & Vr

16. Calculating RL , RX and SWR

17. A Practical Circuit

18. Choice of Diode Most designs use Schotky Barrier Diodes VK5JST Uses OA91Germanium Diodes. I buuilt a simple curve tracer to test Lets see the resulst!

19. Diode Curves Tracer – 1N4148

20. Diode Curve – OA91

21. Diode Curve – BAT85

22. Analyzers of this Ilk VK5JST Cheapest and “simplest” Micro908 Similar to the VK5JST except has DDS MFJ259/b Commercial device with analogue VFO

23. More Complex Designs Measure Phase Directly AIM More sophisticated device with extra tricks. VNA & mini VNA More complex 2-port device

24. Quick Comparison

25. MFJ-259B Most common device in UK? Combination readout Analogue SWR and Impeadance Digital R, Z & SWR Wide frequency coverage up to 170Mhz Manual Tuning Uses the resistive bridge 4 detectors for direct SWR HSMS-2820 Shockty Detectors Simple Micro PIC16C73

26. MFJ-259B Pros Wide frequency coverage Comes ready built analogue SWR for quick tune Cons Manual Tuning 8-bit d/a Limited computer interface Feels expensive…

27. Micro 908 Produced by US QRP club Digital read-out only Digital R, Z & SWR HF frequency coverage up to 60 Mhz Digital Tuning Uses the resistive bridge 4 detectors for direct SWR Shockty Diode Detectors Uses and advanced micro which is why I didn’t use it

28. Micro 908 Pros Versatile Instrument Open Source design Can be used standalone and with a computer Cons Complex Surface Mount Kit 8-bit d/a Feels expensive…

29. VK5JST Produced by VK5JST Digital read-out only Frequency, SWR, R & Z HF frequency coverage only up to 30 Mhz Analogue Tuning with varicap fine tune Uses the resistive bridge 3 detectors only Point Contact Diodes Simple PICAXE Micro Programmed in basic sub-set

30. VK5JST Pros Low Cost Open Source PicAxe is “basic like” Cons? only up to 30 Mhz Analogue Tuning PicAxe is limited

31. AIM Commercial device Designed by W5BIG No device read out Serial Connection to PC HF frequency coverage only up to 170 Mhz Digital Tuning Dual DDS chips Tuned 1Khz appart Has a novel detector Dual Mixers MSP430 DSP Chip Data analyzed by PC Allows complex accurate calculations

32. AIM Pros Wide Coverage High Accuracy Great PC Program Cons Expensive Probably value for money Only works with PC

33. Other Devices VNA2180 Vector Network Analyzer From W5BIG “Similar” to VNA but with two ports N2PK VNA “DDS” type design I/Q Quadrature mixers MINI VNA DDS Style Design TAPR VNA AMD AD8302 “VNC on a chip” based DDS design

34. Future Developments AMD AD8302 Described as a VNA on a chip

35. AIM against VK5 – 40 Mtrs

36. AIM against VK5 – 40 Mtrs

37. 80 Metres

38. 80 Mtrs

39. 20 Meters

40. 20 Meters

41. Summary VK5JST is cheapest device available Offers reasonable accuracy Good around 50ohm restive less accurate at higher values underestimates capacitive reactance Could probably be improved chip has 12 bit D2A assembler code for maths

42. Questions

43. AIM Coverage 0.1 – 170 Mhz D2A 12 Bits Frequency Source Computer controlled DDS Readout Only on computer Software Closed source But very good Current Cost Ł354 VK5JST Coverage 1 – 50Mhz D2A 8-Bits (7 really) Frequency Source Manual VFO + Fine Tune Readout 2 x 16 line display Software Open Source Less sophisticated Cost Ł50-60

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