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Taking Some of the Mystery Out of the Alford Slot

Taking Some of the Mystery Out of the Alford Slot. An heuristic study of this antenna By Robert J. Templin W5OE Roadrunners Microwave Group. Alford Advantages. Horizontally polarized Omni-directional Easy to build Small, light, cheap Six dBd gain. Alford Applications. Beacons

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Taking Some of the Mystery Out of the Alford Slot

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  1. Taking Some of the Mystery Out of the Alford Slot An heuristic study of this antenna By Robert J. Templin W5OE Roadrunners Microwave Group

  2. Alford Advantages • Horizontally polarized • Omni-directional • Easy to build • Small, light, cheap • Six dBd gain

  3. Alford Applications • Beacons • Repeaters • Mobile • Stealth (esp. if encased in fiberglass tube)

  4. Problems Encountered • Published designs often don’t work as advertised • Vertical pattern has a null at horizon (“butterfly” pattern) • Specified frequency has high SWR

  5. Austin ATV Repeater • Moved to 2429 MHz output • Input moved to 1252 MHz • Separate audio and visual carriers • Three new omni antennas needed

  6. Slotted Waveguide • Well described in W1GHZ website/antenna book • Made several out of 2 by 4 aluminum box section tubing

  7. Slotted Waveguide • Arduous machining job • Horizontal pattern full of deep lobes • Big, heavy, hard to seal against critters

  8. Slotted Waveguide • Measured 8 dBd gain • Lobe fix required huge wings on each side, 40 inches wide • Looked for a better alternative

  9. Relooked at Alford • Built about a dozen samples for bands from 902 to 3456 MHz • Applied regression analysis to get cook-book formulas to build successful antennas

  10. Alford Slot Cookbook • Critical dimension is the active circumference of copper. Slice tubing lengthwise to get 0.400 l active circumference. • Critical adjustment is slot width but it is determined by squeezing the slot closed for minimum SWR

  11. Active circumference

  12. Alford Slot Cookbook • Critical dimension is slot length it should be 2.15l between shorting tabs. • Feed with 200W balun inside tubing

  13. 902.1 1252 1296 2304 3456 SL= 28.25” Circum.= 5.20” SL= 20.3125” Circum.= 3.715” SL= 20. 125” Circum.= 3.910” SL= 10.688” Circum.= 2.120” Use waveguide Alford Slot Cookbook

  14. Alford Slot Cookbook • Model for 3456 could not be built, no room for feed balun • Models for 432 and below are large but possible

  15. Alford Slot Cookbook • Models built to these specs had gain measured at 6 dBd, good vertical patterns & uniform horizontal patterns • ND2X has used 1296 model on mobile for over a year; 902 model for a few months • They are easily sealed for environmental protection; don’t require sealing mobile

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