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The AntCaptenna-II (12+ dBi)
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The AntCaptenna (ACA) was born from a desire to make an antenna
that:
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could be used as a 'client' antenna (i.e. used to connect to
an AP) |
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had a reasonable gain (12-14 dBi) |
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was waterproof |
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was rugged |
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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 some of the harder to get parts, or completely
assembled AntCaptennas here
 .
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Click
image to enlarge |
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.
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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. |
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So the conclusion is: Best offset = 30.5 mm = 1/4 wavelength
| 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. |
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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.
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| 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 |
This NetStumbler file shows two sections:
| First = Standard BiQuad (no PCB, no enclosure) |
| Second = AntCaptenna |
The test setup was:
| AP |
30mW radio connected to a 180-degree waveguide
(See BICTON-AP for details) |
| Distance |
870m |
| Test End |
30mW RoamAboutDS PCMCIA card, 200mm pigtail,
plus
antenna under test. |
|
Click image to see
full scale
Download
file |
Price
See Prices
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