Home-brew Compact 6dBi Collinear Antenna
(first published September 2005, details of higher-gain version added July 2013)
This page details the construction of an easy-to-make collinear 360 degrees omni-directional,
vertically polarised, antenna for 802.11b/g wireless networking.
The antenna is very robust and compact, and has a gain of approximately 5-6dBi.
Background
Various websites detail the construction of a collinear antenna suitable for 2.4GHz wireless networking,
including
wireless.gumph.org
and
guerrilla.net (no longer available, but mirrored here).
However, these antennas are quite complex to build, being made up of numerous short lengths of
coax which must be accurately cut to length.
You also need to know the velocity factor of the coax that is being used, as most of the measurements
are based on it.
A variation
of the coax collinear is constructed from brass rod and brass tubing, but is still just as fiddly
to construct.
Some time ago, I made an 8-element coax collinear, using the
wireless.gumph.org
instructions.
Antenna comparison testing confirmed the gain was approximately
8dBi.
However, it took many hours to construct, and the antenna has very little
physical strength. I resorted to cable-tying a length of dowel to the coax, and then
encasing it all inside some 25mm electrical conduit.
I was particularly intrigued when a friend pointed out a much simpler collinear, consisting
of just a length of copper wire appropriately bent, as detailed
here,
with a claimed gain of about 6dBi.
This version provides a number of benefits over the coax construction, requiring far less effort
to construct, and providing a smaller and more robust antenna.
While the 6dBi gain is less than that of an 8 element coax collinear, the gain can be
improved by increasing the number of elements.
Doubling the number of elements will increase the gain by almost 3dBi, ie, double the gain.
Parts Required
The materials required:
- approx 300mm 2.5mm2 copper wire
- panel-mount female N-connector
- 250mm length of 20mm light-duty electrical conduit
- 2 end-caps to suit 20mm conduit
and optionally, for mounting of the completed collinear antenna:
- 2 clamps to suit 20mm conduit
or
I used a length of copper wire from some scraps of 2.5mm2 electrical
cable I had lying around. This cable has a diameter of approximately 1.6mm,
and is flexible enough to bend into the required shape without too much effort
or specialised tools.
2.5mm2 3-core electrical cable
A panel-mount female N-type connector is also required, to allow the antenna to be
connected to a wireless device.
Note that other connectors (ie, TNC, SMA, etc) can also be used, depending on the connectors
on your pigtails.
I used a panel-mount female N-type connector, as shown in the photos below.
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panel-mount female N-type connector
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Design Details
This collinear simply consists of a length of copper wire with some loops located
at specific locations.
The dimensions of the sections of the antenna are important, and are shown in the
diagram below.
dimensions of the collinear
The length of the bottom section is 1/2 wavelength (the left section in the diagram above), the centre section is 3/4 wavelength,
and whip section on the top is slightly less than 3/4 wavelength, apparently to reduce the
capacitance effect.
The 802.11b standard uses 2.412GHz to 2.484GHz frequency range, so at the centre of that frequency range,
1/2 wavelength is 61mm, and 3/4 wavelength is 91.5mm.
These dimensions appear to be consistent with similar commercial antennas.
Construction
Start construction of the antenna from the bottom end, and solder one length of the copper wire
into the N connector.
Measure 1/2 wavelength from the top of the N connector, and create the first loop.
loop detail, and completed bare antenna
Note that the loops are offset from the wire which makes the vertical section of the
antenna.
Then measure an additional 3/4 wavelength, and create the second loop.
Trim the whip section on the top so it's the correct length.
If you are intending to use a length of 20mm conduit to house your antenna, be sure
to keep the diameter of the coils to approximately 15mm or less, to ensure they will
fit inside the conduit (20mm light-duty electrical conduit has an internal diameter of 16mm).
completed collinear
However, a length of copper wire isn't particularly robust.
One of the easier ways to address this shortcoming is to enclose the antenna
inside a radome.
Note that you need to use something that's transparent to 2.4GHz, else it will
adversely affect the performance and operation of your antenna.
I used a 250mm length of 20mm light-duty electrical conduit, with some end caps to suit.
The 20mm light-duty conduit has an internal diameter of 16mm, and the loops bent in the copper
were a snug fit inside the conduit.
If you need something a little more roomy, then 25mm light-duty conduit can be used.
Two small bends are required in the wire near the base of the antenna, to ensure that the
loops are centrally located above the N connector, thus allowing the entire antenna to be
inserted into the conduit.
Testing indicates these bends to not have any noticable impact on the performance of the antenna.
20mm conduit and end caps
To mount the antenna, a suitably sized hole was drilled in one of the end caps,
and after discarding the N connector's nut and washer,
the N connector was screwed into the end cap from the outside.
N-type connector screwed into an end cap
The antenna can now be inserted into the conduit, and the other end cap can be installed.
enclosed collinear
If the antenna is to be used outdoors, the end caps should be glued on with
appropriate conduit glue, to ensure a weather-proof seal.
Note that the antenna should be tested before gluing the end caps in place.
Mounting
The collinear antenna is designed to be mounted vertically.
The conduit radome provides a robust and sturdy enclosure for the antenna, and if required,
it can be mounted outside in the weather.
If mounting it outside, be sure to wrap the N-connector appropriately in self-amalgamating tape
to prevent any moisture ingress.
Plastic clamps made specifically for conduit can be used to attach the antenna to a vertical
surface. As these clamps are plastic, they will not interfere with the operation of
the antenna, while all-metal mounts placed in the radiation pattern could affect
the operation of the antenna.
20mm conduit clamps
After attaching the clamps to the surface where the antenna is to be mounted,
the antenna can easily be clipped into the clamps, and can also be easily removed.
sample mounting using clamps
An alternative method of mounting the antenna is with a short length of right-angle
galvanised steel.
A suitable hole needs to be drilled in one side, and the antenna can be attached to the
galvanised steel by removing the N-connector and attached copper antenna, and passing
it through the hole in the galvanised steel.
The antenna will be firmly attached between the N-connector and the end cap.
using a metal bracket
If you just want to use the antenna for casual stumbling or wardriving, there is no need
to mount it, and it can just be used as a handheld antenna.
demonstrating just how small it is
Usage
For information on connecting an external antenna to a wireless radio, have a look at the page on
using wireless antennas.
Testing
When building my first collinear using this design, I made the loops in a similar way to those
seen on many commercial antennas which have multi-loop coils, with the wire above and below
the coil being centrally located with regards to the loop.
first attempt at the loops
However, after doing some further research, I realised this was incorrect, and made another
collinear using the loops as per the approach described above.
Some quick comparison testing indicates the collinear with the offset loops performs
much better than the collinear with the centered loops.
Need More Gain?
More gain can be achieved by adding additional segments to the middle of the collinear,
as shown in the diagram below.
dimensions of a longer variation of the collinear
Some quick comparison testing indicates this longer collinear has approximately 2dB more gain than the 6dBi
version, with a gain of about 8dBi.
Note that lengthening the collinear antenna in this way will reduce the vertical beamwidth of the radiation pattern.
References
Collinear Omni-directional Antennas:
Credits
All photos are copyright .
last updated 12 Feb 2018
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