Taking Some of the Mystery Out of the Alford Slot

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 dont 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
Alford Slot Cookbook

• 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

• 902.1
• 1252
• 1296
• 2304
• 3456

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 dont require sealing mobile