Quarter wave omni-directional

This omni directional antenna, nicknamed the Spider Omni, was built in response to finding a simple diagram in an old book. After scaling to the correct frequency, it was built to help augment an 802.11b vehicle-based community wireless network effectively and cheaply. The original prototype is still working well after three years. This document explains how it was made.

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The omni directional antenna, with a cigarette for scale.

Note - This antenna is for use with 802.11b wireless computer networks or 2.4GHz video sending equipment. It is not for FM / AM / SW / LW radio useage.

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1. Overview

The omni-directional antenna was built using an N-type chassis mount connector with short lengths of stiff fencing wire soldered into each corner hole. The driven element (the wire soldered into the centre, or conductor) was a quarter wavelength (32 millimetres) as was each ground plane (four soldered around the body of the connector). Each groundplane was cut to length and then bent over at 30 degrees below the horizontal to attempt to match the impedance to 50 Ohms (although see the note on impedance below).

A semi-technical drawing of the antenna. Enlarge.

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2. Construction

2.1. The antenna body - an N-type connector

The N-type connector was obtained from R.S. at £2.93 GBP (part number 112-2139). On the packaging it had printed: Telegartner (presumably manufacturer) Type: N-Flanschbuchse J01021H1082 Tel +49 (0) 7157/125-0 Fax -120

2.2. The wire

Prototypes were made with thinner wire (twin and earth from domestic main installations, 16 Amps), but the resulting antennas were too flimsy to store or use reliably. We ended up using the thickest, stiffest fencing wire that would fit in the holes of the N connector body. It was only bendable with two decent sets of pliers which means that in use it remains in place, even with quite a heavy bird on it...

2.3. The solder connections

Soldering to the "solder bucket" in the centre of the connector was easy because it has been pre-tinned. The same was not true for soldering to the holes in the body of the connector. The area around these needed a good roughing up with sandpaper, then tinning by applying solder until it flowed, then the wire soldering into the holes until all the solder flowed together, while the whole connector is hot. This means the connector gets really sodding hot (ensure it's in a safe place and well held - a vice, mole-grips), fortunately the dialectric (insulating plastic) did not melt, as has happened on cheaper connectors. All solder connections were tested by "wiggling" when cool - a very telling test.

I have had many emails from people who have made this design and have had good results, some of them looking much better than the original prototype above. Since making the first one I have bought an 80 Watt soldering iron (though not temperature controlled) and this has made the soldering much easier, so I would recommend using nothing less than 80 Watts.

This picture was sent to me (enlarge image). Antenna made by John, thanks for the photo. This one is made using brass rods bought from a local hobby shop, heatshrink tubing and some neat soldering! (he used 135 Watt iron)

There is another good example of one made by Josh at http://home.att.net/~jjmorrissiey/JMorrissiey.html

2.4. Insulation

The central conductor was shrouded with the outer plastic stripped from wire of the same gauge. Initially this was not even pushed all the way down to the dialectric (the gap shows in the photo at the top of the document).

Since then heatshrink tubing has become hot favourite for insulating, it is more effective and easier to fit, and should insulate right down to the dialectric. Be careful to cut it square and clean so that it does not split when heated, as splits can travel over time.

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3. Impedance

This antenna should have an impedance of 50 Ohms.

I have been emailed by people using this design with a bend of 45 degrees, and that this worked better to give 50 Ohms. If the impedance is wrong then it would be possible to damage the amplifier or 802.11 card to which it is connected.

The original prototype (with 30 degree bends) is still working well after a year and a half. Although looking back at the photos, the prototypes may well have been more like 45 degrees, both seem to work well. I have mended one in the field by eye, after sustaining damage whilst in use on an internet rickshaw, and still it worked well. I will investigate the angle further.

Ideally an SWR (Standing Wave Ratio) meter should be used to "tune" the antenna once made.

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4. Weatherproofing

So far this antenna has not seemed to need weatherproofing.

For temporary mounting outdoors sealing the joint between the N connectors with self-amalgamating PTFE (Polytetrafluoroethylene) tape, silicon sealant , heatshrink etc. helps to avoid ingress of moisture into the cable.

For long term installation outdoors it would be prudent to house the antenna and N connection in a plastic food container (check that it is safe for use in a microwave oven first, and be aware that some plastics effect signal strength).

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5. Mounting

When we have used this antenna it has been sufficient to screw it to the cable and affix the cable securely; this is using URM67 coaxial cable which is 10mm outer diameter and resists bending . The antenna is so strong and light the cable seems to provide adequate mounting in itself.

Antenna mounted on N-type cable.
The antenna mounted on an N-type cable.

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6. Cost

What can I say? A bargain at £2.93 GBP, assuming you find the fencing wire in the rubbish (or a local fence) and don't really have to pay for your solder.

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7. Performance

From experience of useage in a variety of situations and surroundings, I would estimate the gain of this antenna to be around a 3 dB improvement over a Buffalo pcmcia card internal antenna , with a much smoother polar coverage (allowing for the losses in a pigtail, and 6 or 7 metres of URM67 cable, and connectors).

In use on a flat field, using two of these antennae attached to pcmcia 802.11b cards via cable and pigtails, we were able to maintain an 11Mb/s network at between 400 to 500 metres with clear line of sight.

You may be able to tell from the lack of absolute measurments in my results that I do not have access to calibrated test equipment, or the time to carry out well documented tests. If anybody does, please let me know the results!

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7.1. Examples of use in "the field"

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8. Warning

Apart from the fact it works really well, no-one has yet popped on their lab-coat and done any high-brow tests on this "homebrew twig", and of course manufacturers recommend you don't do anything which they don't recommend, or attach non-proprietary stuff to their stuff. Of course.

The cigarette shown in the picture is for scale only. Do not try and smoke it.

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