A Working Alford Slot Antenna, Bob Templin (W5OE, SK)

Many articles have been written on the so-called Alford Slot antenna. This is a vertical stick-like antenna that producing low-angle horizontal radiation. At its' essense, the Alford Slot is simply a horizontal dipole - hence the horizontal pattern. Yet it is more than that. The Alford is effectively a vertical stack - a continuous series - of of those dipoles. They radiate in phase such that vertical radiation is suppressed.

Bob Templin (SK) addresses the issues caused by misinterpretation in the literature. He teaches a relatively simple system to create a well-working Alford Slot.  Ed. Note: The Alford is omni-directional when left as-is. Others have shown how to make it directional, e.g., with a 180 deg. dispersion by using backplate "ears".  Enjoy!

(For Bob Templin's PowerPoint presentation on this topic, click here and save it.)

The process for building a good Alford slot is surprisingly simple.   Instead of trying to define the slot length, width, and tubing size and then duplicating these dimensions as accurately as possible, do the following:

  1. Select a copper tubing tubing diameter that is commensurate with the desired frequency.  The finished product will have a total circumference of 0.416 free-space wavelengths. This determines the operational resonant frequency of the antenna. See details at the bottom of this page.

  2. Cut the tube lengthwise on an overhead radial woodsaw using a blade designed for non-ferrous cutting.

    1. Slot length is determined later by soldering shorting bars across the slot.
    2. It is usually about 2.15 free-space wavelengths but is not especially critical.
    3. When cutting make sure tube is securely anchored to a board otherwise it may fly out of the saw table.
    4. Two small holes drilled at the ends 180 degrees away from the cut and using screws to a board is usually sufficient anchoring.
    5. Don't take too deep a cut at a time.
  3. Slot width determines the antenna SWR.  Ordinary copper tubing, once sliced lengthwise, will spring open to a much wider slot width than needed, but you fix this later see below.

  4. Heat the copper tubing to red heat progressively along its length with a propane torch until it is annealed dead soft.

  5. Cool the tubing in water.

  6. Deoxidize the tubing in muriatic acid (outdoors only - bad fumes).

  7. Rinse the tubing with water and dry.

  8. Drill two feed point holes, one on either side of the slot, across from each other, in the middle of the intended slot length.

  9. Attach a 200 ohm balun (see below) feed to the feed point holes.  Run the coax either up the center of the tubing or on the outside back edge, 180 degrees from the slot, where the voltages are zero. UT-141 semi rigid coax is quite useful for this.

    Be careful when making the bend at the "bottom" of the loop for the balun. Semirigid coax tends to kink, not a good thing!!

  10. Put shorting bars across the slot two wavelengths apart (top and bottom).

  11. Excite the antenna at the desired frequency through an SWR bridge or return loss coupler.

  12. Squeeze the slot length closed uniformly along its length until the SWR is minimum.  You should be able to get down to less than 2:1 SWR.

  13. Add small gimmick capacitor tabs at either side of the feedpoint to further minimize swr.

  14. See picture of a 23cm version below, right, which started as a 1.25" copper tube.  Note that the tube below the bottom shorting bar is not part of the antenna and may be filled with anything.  In this case, a 1" hardwood dowel was inserted to the bottom of the shorting bar and the tube was drilled (4 places, 2 on either side of the slot) and screwed to the dowel. The dowel was then inserted into a 3' section of threaded 1" black water pipe (which had a pipe flange installed on the end) and cut off flush with the bottom of the pipe flange. The pipe flange was bolted to the deck to secure the antenna.

  15. Put the antenna in a weather proof housing, if desired, and readjust for best SWR.  Use largest diameter RF-transparent plastic or fiberglas tubing available to minimize detuning effects. Note that the example pictures below has no housing - as long as the vehicle is in motion water has no opportunity to build up inside the antenna, it functions well without one.

NOTE: The table below shows the "cookbook" values. Active Circumference is the tube circumference minus the width of the slot (see top view below, right). As an observation, the finished tube cross-section will often be more egg-shaped than round.
 

Alford Slot Antenna Details
Freq.
(MHz)
Slot Length
2.15 l
(inches)
Active
Circum.
0.40l
(inches)
Slot Width
0.016l
(inches)
Final
ant.
dia.
(inches)
902.1 28.25 5.20 0.190 ~1.72
1296 20.125 3.910 0.170 ~1.3
2304 10.688 2.120 0.090 ~0.7

The diameter of a 3456MHz Alford slot antenna is too small to fit the balun inside the tube. If you need an onmidirectional antenna for 3456MHz and above, use slotted waveguide antennas with the big "wings" to make the pattern cicular instead of 4-leaf clover-like.

W5OE (Bob Templin)

 
 
Length of the slot from the bottom of the top shorting bar to the top of bottom shorting bar is 2.15 l
 
23cm Alford Slot
used by ND2X/M,
made by W5OE

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