Port P able ort Magnet Magne ic Loop Ant An e t nna KG5EAO Rick Bono August Augus 11, 2015

Transcription

1 Portable Magnetic Loop Antenna KG5EAO Rick Bono August 11, 2015

2 Overview Develop a portable magnetic loop antenna for use on HF bands running QRP. Easy to deploy Ideally run on 40m through 10m bands For theory and discussion try Steve Yates (AA5TB) website at: Design calculations were made with Steve s Excel spreadsheet also in the link above.

3 Magnetic Loop Antenna Design Points Thefollowing pages show thedesign points for the magnetic loop using a 10 foot length of RG 213 coax. RG 213 shield diameter is.314 inches

4 40m Design Point Design Frequency = 7.000MHz Loop Diameter = 3183feet 3.183feet 0970m 0.970m Conductor Diameter = 0.314inches 7.976mm Added Loss Resistance = 0.000milliohms RF Power = Watts Calculated Results: Bandwidth = 9.573kHz (-3 db points) Efficiency = 5.770% dB Loop Area = 7.958ft² 0.739m² Radiation Resistance = 5.139mΩ Total Loss Resistance = mΩ Loop Circumference = ft 000ft 3.048m Wavelength Percentage = 7.117% λ Loop Inductance = 2.961μH Distributed Capacitance = 8.200pF Q (Quality Factor) = Tuning Capacitor = pF Capacitor Voltage = V148V Minimum Plate Spacing = mils (1/1000 in) 0.467mm Circumference a bit low for 7MHz Requires 182pF of capacitance at high limit 5.8% efficiency with 9.5kHz Bandwidth

5 20m Design Point Design Frequency = MHz Loop Diameter = 3183feet 3.183feet 0970m 0.970m Conductor Diameter = 0.314inches 7.976mm Added Loss Resistance = 0.000milliohms RF Power = Watts Calculated Results: Bandwidth = kHz (-3 db points) Efficiency = % dB Loop Area = 7.958ft² 0.739m² Radiation Resistance = mΩ Total Loss Resistance = mΩ Loop Circumference = ft 000ft 3.048m Wavelength Percentage = % λ Loop Inductance = 2.961μH Distributed Capacitance = 8.200pF Q (Quality Factor) = Tuning Capacitor = pF Capacitor Voltage = V807V Minimum Plate Spacing = mils (1/1000 in) 0.622mm

6 17m Design Point Design Frequency = MHz Loop Diameter = 3183feet 3.183feet 0970m 0.970m Conductor Diameter = 0.314inches 7.976mm Added Loss Resistance = 0.000milliohms RF Power = Watts Calculated Results: Bandwidth = kHz (-3 db points) Efficiency = % dB Loop Area = 7.958ft² 0.739m² Radiation Resistance = mΩ Total Loss Resistance = mΩ Loop Circumference = ft 000ft 3.048m Wavelength Percentage = % λ Loop Inductance = 2.961μH Distributed Capacitance = 8.200pF Q (Quality Factor) = Tuning Capacitor = pF Capacitor Voltage = V Minimum Plate Spacing = mils (1/1000 in) 0.597mm

7 15m Design Point Design Frequency = MHz Loop Diameter = 3183feet 3.183feet 0970m 0.970m Conductor Diameter = 0.314inches 7.976mm Added Loss Resistance = 0.000milliohms RF Power = Watts Calculated Results: Bandwidth = kHz (-3 db points) Efficiency = % dB Loop Area = 7.958ft² 0.739m² Radiation Resistance = mΩ Total Loss Resistance = mΩ Loop Circumference = ft 000ft 3.048m Wavelength Percentage = % λ Loop Inductance = 2.961μH Distributed Capacitance = 8.200pF Q (Quality Factor) = Tuning Capacitor = pF Capacitor Voltage = V801V Minimum Plate Spacing = mils (1/1000 in) 0.558mm

8 12m Design Point Design Frequency = MHz Loop Diameter = 3183feet 3.183feet 0970m 0.970m Conductor Diameter = 0.314inches 7.976mm Added Loss Resistance = 0.000milliohms RF Power = Watts Calculated Results: Bandwidth = kHz (-3 db points) Efficiency = % dB Loop Area = 7.958ft² 0.739m² Radiation Resistance = mΩ Total Loss Resistance = mΩ Loop Circumference = ft 000ft 3.048m Wavelength Percentage = % λ Loop Inductance = 2.961μH Distributed Capacitance = 8.200pF Q (Quality Factor) = Tuning Capacitor = pF Capacitor Voltage = V Minimum Plate Spacing = mils (1/1000 in) 0.514mm

9 10m Design Point Design Frequency = MHz Loop Diameter = 3183feet 3.183feet 0970m 0.970m Conductor Diameter = 0.314inches 7.976mm Added Loss Resistance = 0.000milliohms RF Power = Watts Calculated Results: Bandwidth = kHz (-3 db points) Efficiency = % dB Loop Area = 7.958ft² 0.739m² Radiation Resistance = mΩ Total Loss Resistance = mΩ Loop Circumference = ft 000ft 3.048m Wavelength Percentage = % λ Loop Inductance = 2.961μH Distributed Capacitance = 8.200pF Q (Quality Factor) = Tuning Capacitor = pF Capacitor Voltage = V Minimum Plate Spacing = mils (1/1000 in) 0.452mm Circumference a bit high for 28MHz Requires 10.5pF of capacitance at high limit 89% efficiency with 169.9kHz Bandwidth

10 Loop performance vs. Frequency (assumes no additional Losses) Loop Performance Efficiency (db) Frequency (1-30 MHz) Bandwidth (khz) db khz

11 Construction Frame built from ¾ PVC pipe. Four 2 sections of pipe on a central PVC cross. Frame supported on a standard camera tripod. 4 x4 x2 4x4x2 Plastic Junction box used to hold an air variable capacitor, the wiring and two SO 259 connectors. Loop made from a 10 foot RG 213 patch cable 6:1 gear reducer mounted to capacitor shaft for easier tuning Junction box mounted to Lower PVC pipe section with metal strap. Wooden dowel stick attached to capacitor for tuning Unshielded coupling loop made from a RG 8X patch cable. Loop approximately 1/5 diameter of mainloop. Capacitor is a two gang air variable capacitor with a plate spacing of Gang #1: 7 65pF Gang #2: 5 46pF

12 Loop Photos

13 Initial Results Antenna will tune up on 20m through 15m. 40m will tune up with a 120pF Silver Mica capacitor in parallel with variable Capacitor Tested extremes are at MHz and MHz Capacitance calculates to 16.6pF to 80pF Did not take into account the distributed capacitance Will achieve tuning on 17m and 10m with a smaller loop (i.e. 5 pr 6 ft). At 14Mhz, capacitor was at 45.35pF (not counting any distributed capacitance).

14 Cost Estimates AirVariable Capacitor $8.99 Ebay 3ft RG8X Patch Cable $4.65 Ebay 10ft RG 213 Patch Cable $17.95 Amazon (2x) UHF Panel Jacks $4.38 Amazon Two hole 3/4 strap $ Home Depot #10 32 x 3/4 Screws $1.18 Home Depot 4x4x2 Plastic Junction Box $6.88 Home Depot (4x) 2ft 3/4 PVC pipe pp $6.28 Home Depot 3/4 PVC Cross $1.97 Home Depot 6:1 Shaft Reducer $12.00 Xtal Set Society Total Material Cost: $65.45

15 20m Frequency Sweep

16 17m Frequency Sweep

17 15m Frequency Sweep

18 40m Frequency Sweep 120pF capacitor in parallel l

19 Power Handling & Tuning Tested on 25W using Yaesu FT 450D without issue. Higher power started arcing. Given humidity is fairly high at the QTH will limit power to 20W max. Tuning is accomplished by adjusting capacitor for maximum receive noise then bringing SWR in with a quick fine tune. Antenna has good receive properties.

20 Stations worked During Initial Testing 5W SSB, 20m band on Yaesu FT 817ND Outdoors: worked Belgium, Cuba and California Indoors: worked Bosnia Herzegovina, Czech Republic and Indiana 30W SSB, 20m band on Yaesu FT 450D Indoors: worked Venezuela 5W SSB, 40m band on Yaesu FT 817ND No contacts made.

21 Conclusion Basic design goal of an easily deployed, portable HF antenna was accomplished Trick is finding a suitable capacitor Easy to build and low cost Performs very well even with design compromises. Future F work: Test 12m & 10m with smaller loop Test 40m with larger loop diameter Build less portable with larger conductor diameter for better efficiency and with vacuum capacitor Auto tuning