The AntCaptenna is an easy to make, relatively high-gain directional antenna. A pair of AntCaptennas, pointed at each other with clear Line Of Sight (LOS) can make a 5km link.

Introduction

The AntCaptenna (ACA) was born from a desire to make an antenna that:

could be used as a 'client' antenna (i.e. used to connect to an AP)
had a reasonable gain (12-14 dBi)
was waterproof
was rugged
could be pole-mounted

The result was the AntCaptenna - which gets its name from the Ant-Cap used as the back reflector.

[Ant Caps are used on wooden stumps in Western Australia, to prevent white ants (termites) from coming up from the ground into the house. The ant cap used here has a center section of 125x125mm that is flat, and has 50mm 'wings' that are bent 'inwards'. The total size is approx 225x225. I have absolutely no idea if the wings add or subtract to the overall performance of the antenna, but they look important! If you can't buy an ant-cap, just make a piece of galvanized metal approx 0.4mm thick, 150 x 150, or bigger.]

While the full instructions for building your own AntCaptenna are given below, you can buy a kit of parts, or an assembled antenna from our Online Store .

Click image to enlarge

The Design

There is no new science in this antenna. It is nothing more than a BiQuad - ready for external mounting.

Note:

The original AntCap design specified a 15mm offset between the antenna element, and the back reflector. This dimension came from Trevor Marshall. This has always worried me, as that is 1/8th of a wavelength, and 1st principle physics says if you want a reflected wave to be in phase (i.e. constructive interference) then the reflector should be an odd number of 1/4 wavelengths. However - I trusted the "1/8" design.

I decided to put theory to the test and built a range of AntCaptennas with offsets ranging from 15 to 35 mm. The best RX signal level was at exactly 30.5mm (1/4 wavelength) and the peak was 4dB higher (better) than the 15mm (original) version.

The chart at left shows the peak value (-61 dBm) at 30.5 mm offset.

A 4th-order polynomial of best fit is also shown.

So the conclusion is: Best offset = 30.5 mm = 1/4 wavelength

SWR Test

The SWR of the BiQuad PCB (alone, in free space) was measured as shown.

The minimum SWR falls nicely within the 2400 MHz WiFi band.

The Cookbook

Ingredients

My apologies in advance to those that have no access to these suppliers. They are places generally available in Australian capital cities.
All dimensions in mm. (unless otherwise stated)

Item	Description	Supplier	Part	Qty
1	AntCap 125x125x50	Bunnings	1079234	1
2	Bracket ("Angle-Pergola 88x63x36mm")	Bunnings	1071032	1
3	M4x20 screws (pack of 20)	Bunnings	643277	4
5	1/8" or 3mm rivets	Bunnings		4
6	Tube of flexible, waterproof caulk	Bunnings		1
7	V-clamp (size to suit pipe)	Jaycar	LT-3235	1
8	Enclosure, IP65 rated, 115x64x40x3	Jaycar	HB-6122	1
9	PCB Standoff, Adhesive, 6.35mm	Jaycar	HP-0760	1 pk of 25
10	Plastic cable tie, (small)	Jaycar		1
11	N-connector, jack, RG58, crimp	Prices		1
12	9006 low-loss coax (RG58 size)	Prices	9006	0.4m
13	PCB, BiQuad	Prices		1

Tools

Drill
Drill bits (3, 4, 5 mm)
Screwdriver
Soldering iron
Rivet gun

Method

Enclosure Holes
- Decide what polarization antenna you want to make. The pictures above show a Horizontally polarized antenna (the enclosure is up and down) to be mounted on a vertical pole. A Vertically polarized antenna would have the enclosure going sideways.
- Locate the bottom of the enclosure. The bottom is the one with the groove for the gasket strip.
- Insert the supplied gasket into the groove in the enclosure bottom. The gasket is too long; cut as needed.
- Place enclosure bottom in the center of the front of the AntCap. Align enclosure to be 'square' with the AntCap sides, and to match the polarization you want.
- Drill 4 x 4mm dia holes (one in each corner of the enclosure) through the AntCap.
Attach Bracket To AntCap Side
- Place short arm of Bracket against the back of one of the 4 'sides' of the AntCap, so that the end of the short arm aligns with the edge of the AntCap, and it is roughly centered along the AntCap side. Note: The bracket purchased from Bunnings has 4 small and one large hole pre-drilled on each face.
- Mark the location of the 4 small holes. Drill 3mm holes through the AntCap sides at these marks.
- Rivet Bracket to AntCap.
Enclosure Top
- Locate the top of the enclosure. The top has 4 brass mounting nuts embedded in the plastic (these are NOT the ones at the corners; they are 'inside' the enclosure.
- Using a hot soldering iron, remove and throw away each of these embedded nuts. Insert soldering iron into the nut, and gently rotate it out as the plastic starts to melt.
- Drill a 5mm hole (for the coax cable) in the side of the of the enclosure top that will face down. The hole should be approx. centered in the enclosure face.
BiQuad PCB
- Cut PCB as shown in the figure. [tba]
- Drill holes in the PCB as shown in the figure. [tba]
- Assemble the N-connector onto one end of the coax cable.
- Remove 5mm of external insulation of other end of coax. Separate and fold back the shield. Remove 4mm of internal insulation. Twist the shield so that it resembles a piece of wire than can go through one of the small holes in the PCB. You may have to use only 50% of the shield wire or it ends up too thick. Solder the shield braid together to form the 'wire'.
- Thread the un-terminated end of the coax through the 5mm hole in the enclosure top.
- Pass the ends of the coax through the two small holes in the PCB, from the 'back' side to the 'solder' side. Solder the two ends of the coax to the PCB tracks.
- Use a cable tie through the two holes near the coax, and tighten onto the coax.
- Insert 3 plastic standoffs into the PCB, from the 'solder' side to the 'back' side.
- Apply a dap of caulk to the coax, just near the PCB. Make sure it goes all around.
- Remove the protective paper from the 3 plastic standoffs
- Slowly pull the coax so that the attached PCB moves into the correct position in the enclosure top, with the sticky pads on the standoffs securing it in place.
Assembly
- Using the M4 screws, assemble everything together. The screws pass as follows:
  - Through the BACK of the AntCap
  - through the bottom of the enclosure
  - into the top of the enclosure
- Tighten the screws