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Off-Center Fed Dipoles

Off-Center Fed Dipoles. Practical Applications. Why?. Looking for solutions for low band antennas Was abused by a counterpoise as a child Looking for multiband solutions Traditional low band wire arrays use dipoles or inverted vees Applications to driven & parasitic arrays.

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Off-Center Fed Dipoles

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  1. Off-Center Fed Dipoles Practical Applications N1IW

  2. Why? • Looking for solutions for low band antennas • Was abused by a counterpoise as a child • Looking for multiband solutions • Traditional low band wire arrays use dipoles or inverted vees • Applications to driven & parasitic arrays N1IW

  3. Off Center Fed Dipole Basics • Half wave resonant antenna at lowest frequency of operation • Even Harmonic resonances(V/I ratio is approximately constant) • 160M: 160, 80, 40, 20, 17, 12, 10 • 80M: 80, 40, 20, 10 • Fed 1/3 of the way from the end vs. in the middle • Feed point impedance is approximately 200Ω • 4:1 current balun does the trick N1IW

  4. OCFD Data • 80M • Used for 10 years • 136 feet in length (located at abt 30 feet) • Fed 44.5 feet from one end • Resonant on 80M, 40M, 20M, 10M (no tuner) • 160M (measured data) • Recent addition • 264 feet long (located at abt 80 feet) • Fed 88 feet from one end; 4:1 homebrew balun • 200Ω on 160M and 166Ω on 80M at resonance • 2:1 BW: >200 kHz on 160M, 260 kHz on 80M N1IW

  5. Why They Work • It’s just a dipole! • But • ½ wave resonant element, then harmonic wire • Voltage/Current relationship at 1/3 feed point provides essentially constant ratio on even harmonics • Broadside null on harmonics N1IW

  6. Current Distribution on a Dipole N1IW

  7. Current & Voltage at Fundamental I V N1IW

  8. I & V at 2nd Harmonic I V N1IW

  9. I & V at 4th Harmonic I V (2X Expanded Scale) N1IW

  10. I & V at 8th Harmonic I V (2X Expanded Scale) N1IW

  11. But what about the 6th harmonic? • Feed point is at a current minimum • Very high impedance N1IW

  12. I & V at 6th Harmonic (Bad Dog!) V I (2X Expanded Scale) N1IW

  13. And Odd Harmonics? • Same problem: Current minimum N1IW

  14. I & V at 3rd Harmonic (Bad Dog!) V I N1IW

  15. Basic Gain Plots N1IW

  16. 160M center fed dipole at 80 feet AZ EL N1IW

  17. 160M OCFD at 80 feet AZ EL N1IW

  18. 160M OCFD on 80M (x2) EL N1IW

  19. 160M OCFD on 40M (x4) EL N1IW

  20. 160M OCFD on 20M (x8) EL N1IW

  21. 160M OCFD on 10M (x16) EL N1IW

  22. Geometry/Height • Old 80M design: 45.3 ft, 90.7 ft; 30 ft high • New 160M design: 88ft, 176 ft; 80 ft high • Both use 4:1 Guanella balun design • 80M variation: 45/65 ft flat+26 ft dropper • Feed point impedance at resonance drops as effective height above ground decreases • Recommend 1/3 λ up for 4:1 balun to work well N1IW

  23. 4:1 Guanella Current Balun N1IW

  24. 80M OCFD Construction Detail ~8 Wraps, covered with heatshrink tubing 45’ Leg 90’ Leg “Dogbone” Insulator #12 stranded/insulated copper Stainless hardware 4:1 Current Balun (PVC enclosure) UHF Connector N1IW

  25. 160M OCFD Construction Detail ~8 Wraps, covered with heatshrink tubing 88’ Leg 176’ Leg “Dogbone” Insulator #12 stranded/insulated copper Stainless hardware 4:1 Current Balun (PVC enclosure) UHF Connector N1IW

  26. 160M OCFD Normalized VSWR Plots N1IW

  27. OCFD Orientation Issues N1IW

  28. Single 160M OCFD at 125º, 160M/80M • How to deal with the broadside null… • Rotate antenna to achieve best gain compromise El of 30º Europe (50º) 80M 160M N1IW

  29. Disclaimer… • Your mileage may vary • Batteries not included • Some assembly required • Professional driver, closed course • Void where prohibited • Do not dispose of in fire • Taxes, titles, license fees extra • 10M band openings longer than 2 hours require immediate medical attention N1IW

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