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
 metal bracket

I used a length of copper wire from some scraps of 2.5mm 2 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.

 panel-mount female N-type connector

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:

 How To Make a cheap Wireless LAN 6dbi collinear Antenna
 Easy Homemade 2.4 GHz Omni Antenna