Wifi Dongle Suitable for Custom Antenna

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Wifi Dongle Suitable for Custom Antenna

Postby wlan0 » July 30th, 2017, 9:37 pm

I'm building a Piratebox. It's a wireless network in a box, hosting a website with chat, filesharing, and forum-like functionality. It runs continuously on a Raspberry Pi, like a wireless dead drop. This requires, of course, wireless networking capabilities.
Unsatisfied with the range and power of consumer wifi antennas, my group set out to make our own.

After some crude metalworking, we ended up with a double biquad antenna:
here
We wired it up to an Edimax EW7811un:
here

And we plugged it into a laptop. It worked! We got an insane range boost, a couple hundred meters.

Then we left it on overnight to test continuous operation. In the morning, range had gone down to about twenty meters. Shortly after it stopped working entirely. Upon inspection we discovered that the Edimax was totally fried.

Now we're looking for a new wifi dongle to use, and we need one that will be able to handle having a big antenna rudely hacked onto it and run 24/7 indefinitely.
This isn't exactly a use case we can get advice on from the local electronics shop, so do any of you have experience in this area?
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Re: Wifi Dongle Suitable for Custom Antenna

Postby asheets » July 31st, 2017, 1:48 pm

Probably not the answer you'r looking for, but the failure is probably related to an impedance mismatch between the antenna and the NIC.. I'd look at using a compatible reverse-SMA connector on your antenna and a NIC equipped with a matching stub.
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Re: Wifi Dongle Suitable for Custom Antenna

Postby k-ww » August 1st, 2017, 8:04 am

This may be an easier answer for you:

http://www.tmart.com/Comfast-CF-WR302S- ... -2017-up11
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Re: Wifi Dongle Suitable for Custom Antenna

Postby wlan0 » August 2nd, 2017, 8:54 am

asheets wrote:Probably not the answer you'r looking for, but the failure is probably related to an impedance mismatch between the antenna and the NIC.. I'd look at using a compatible reverse-SMA connector on your antenna and a NIC equipped with a matching stub.


I've been reading about impedance. Please correct me if my understanding is incorrect:

Because our antenna and the Edimax had mismatched impedance levels, (resistance?) the Edimax burnt out. If we wire the antenna to an SMA connector, and then connect that to a wireless card that uses SMA, instead of wiring it straight to a wireless card, the matching impedance levels of the SMA connectors will ensure there's no loss or reflection of signal?

Also, how can I determine what SMA connector is "compatible" with my antenna?

Thanks for your direct and helpful reply.
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Re: Wifi Dongle Suitable for Custom Antenna

Postby k-ww » August 3rd, 2017, 8:21 am

If you look at these videos they will give you a better 'feel' for what is going on in your hookup.

[We had the actual setup from Bell Labs in high school back in the late 60's]

mismatched impedance:

https://www.youtube.com/watch?v=AjzBGC4tGTo

impedance matching:

https://www.youtube.com/watch?v=FaCyg0dB_aY

good overall visual explanation:

https://www.youtube.com/watch?v=DovunOxlY1k
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Re: Wifi Dongle Suitable for Custom Antenna

Postby markbng » August 3rd, 2017, 8:36 am

A few years I discovered the VNA. This is a thing you need when you are working with developing antenna's. The VNA plots the impedance over the frequency domain. The antenna could be optimal is it's in open air, but building it in a box is a totally different story. For instance....carbon in the box can influence the antenna. The VNA can detect this and you can calculate a filter to optimize your antenna setup. Some cheap VNA's are the VNWA and the miniVNA tiny. Good luck with experimenting.

The VNWA goes only up to 1.3GHz. so that won't work :(. Because many people face the antenna problem, I want to start a VNA project on hackaday in the future. I don't have time for this project now (for me to make the complete project). So RF hardware engineers, please leave me a message.
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