Another Omni 2.4Ghz

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Build a Jig

If you do not have a handy helper to hold the sectors together, then you will find it easier to make a small jig from offcuts of wood, to hold the sectors together as you solder them. The clamps on the right hand side of the picture need to be no more than 30mm long. The base board of the jig, needs to extend out to the right long enough to take the whole length of the completed antenna, as it will need to support it during the soldering, as the antenna is not rigid enough to support itself.

Don't make the clamps too tight, as you need to be able to easily lift the cable out after it has been soldered.

When you are readly to solder the sectors together, you need to take care , that each sector is correctly spaced. The overall length of each sector needs to be 40.5mm , measure from one end of the shielding of the sector you are adding, to the same end on the next sector, and slide the sectors together/apart until the distance is 40.5 mm. Try to get it as accurate as you can, as it affects the direction the antenna transmits in if you get it wrong. There should be a small 3mm gap between the sheaths of each sector.

Once you have soldered each sector together, lift it up, turn it over, and move it down the clamp ready for the next sector. This results in a nice straight antenna. When soldering, remember to heat both the shielding and core so that the solder runs smoothly and fixes them together.

Once complete, test the cable with either a bulb and battery or a multimeter. The center of the fly lead should form a circuit to the 1/4 wave section, and the shield of the flylead to the shield of the top section. Now test that there are no crossed connections, by ensuring there is no circuit between the center of the flylead and the shielding of the top sector, and no circuit between the 1/4 wave section and the shielding of the flylead.

Now fix the N connector of your choice onto the end of the fly lead. The type of connector you use depends on what you want to connect to. I use inline connectors, but you could use any connector you like. Slide the antenna into a length of conduiting. It should be a snug fit, you may need to gently ease it in. Now find an old soft drink bottle top, and pop it on the top end of the antenna. Voila one complete antenna ! Securing the antenna in the conduit is best left until you are ready to mount it somewhere. You can cut 5cm slots in the bottom if the conduit, and use a jubilee clamp to grip the flylead, or drill a hole through the conduit and use a cable tie to hold the fly lead, or use a bulkhead mount connector on a botle cap, and glue it to the bottom of the conduit, or glue the flylead in place. It's up to you.

Testing

I will assume you are connecting the antenna to a wireless card in a laptop, and connecting to an accesspoint somewhere. You will need to a signal to noise meter to examine the signal strength. Most wifi cards come with software that does this. Now its time to test that the antenna actually works. This can be harder than it sounds, as unless you can remove the existing aerial from the card or ap, you can't tell it is using your new homebrew antenna. Well wrapping the existing antenna completely in 6-8 layers of tinfoil, has a dramatic reduction on signal strength, now connect the antenna, and the signal should go back up. Remember that omni antenna send out the signal horizontally, so don't test it from the room below your access point. Hopefully you should see that your new antenna actually works. There are three ways to test the gain of the new antenna

Use spectrum analyser in a professional radio lab compare the gain of your new antenna, to the gain of a known antenna (Not the pcmcia card) Carry out an empirical range test with your new antenna If you have access to a spectrum analyser I'd love to know your findings. If you have an existing omni or yagi antenna, then you can compare the snr readings between the two antenna. Remember to do the test outside, as bouncing the signal off walls can really give odd results. Try to stand as far away from the antenna as possible when doing the test, as even moving near it can change the results a lot. Position the two antenna in the same place for the test. Don't try and compare the snr of your new antenna to that of the wireless card in your laptop, as just by turning the laptop round, or lifting it up, or holding it, can dramatically change the snr. Try it and see what I mean. If you don't have another antenna, then wander down your street until you are out of range. Now wander back towards your AP, and wait for the laptop to chime that it has made a connection. Note how far from home you are. Try it again, with the laptop connected to the new antenna. Remember to keep the antenna vertical. I found carrying them in a bag with the antenna poking out the top works well. Try not to hold the antenna, as this will change the snr. You should now be able to pick up your AP about twice as far away, assuming there are no new massive obstacles. If you have a go at making this antenna, and get it working, drop me an email (address at bottom of page) and let me know how you found building it, whether you've found a simpler way, and ... be honest ... how long it took.

Problems

If you find that the antenna is not working, then try tilting it towards the AP, as if the dimensions are wrong, it tends to send the signal angled in a cone above and below the horizontal instead of horizontally. If this is the case, check the dimensions. If it still doesn't work, double check that the sectors are connected ok, and there are no crossed connections. If all else fails make sure you are not connecting it to a 60ft fly-lead as this will reduce the gain a lot.

Disclaimer

I should point out now that I don't claim that the above design is fit for any purpose, and don't accept any liability for use of the design, or any antenna based on this design. If you want to build an antenna using this design, then you are responsible for ensuring that it doesn't breach any laws where you are, and is compatible with any hardware you connect it to. If in doubt, buy a commercial antenna.

Source: http://wireless.gumph.org/articles/homemadeomni.html