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 Repeaters Mobile Stealth (esp. if encased in fiberglass tube)

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

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

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

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

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

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

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

Active circumference

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

Alford Slot Cookbook SL= 28.25” Circum.= 5.20” SL= ” Circum.= 3.715” SL= ” Circum.= 3.910” SL= ” Circum.= 2.120” Use waveguide

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

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