How do (wifi) antennas get their shape?

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Flumble
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How do (wifi) antennas get their shape?

Postby Flumble » Tue Aug 25, 2015 11:11 pm UTC

The reason I'm asking is mostly because how the hell can this even work?
Image
inside my tablet

I guess the length of the metal plate is what matters "most" and that the curving makes it more omnidirectional, and I guess one antenna is for bluetooth and the other for wifi. To clarify: the top metal is connected to a black wire, the bottom metal is connected to a white wire.

So, yeah, my questions for now are:
A) Why does this piece of handiwork (note the random soldering) actually work instead of missing the right bands completely?
B) Is this a decent antenna compared to other antennae? (in my experience it has a harder time finding networks than a decent smartphone, but this may also be a controller issue)
C) If not, is it easy to make a better antenna oneself? (the current wires have ipex connectors at the end already, so those are reusable)

Cradarc
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Re: How do (wifi) antennas get their shape?

Postby Cradarc » Wed Aug 26, 2015 4:08 am UTC

A) An antenna is just two floating wires that act as capacitors. One wire must be grounded (relative to the attached system) and the other end catches the EM wave. When an E-field is applied across a capacitor, charges flow until the capacitor accumulates a certain amount of charge. When the E-field reverses, it discharges and charges in the opposite direction. The charging and discharging produces an AC signal which can be detected and amplified.
The shape of the antenna is everything. This is because you have to catch the wave in the right phase and absorb as much of the energy as possible. It's kind of like how the shape of a bathtub can influence the wave patterns which result when the water is disturbed. Antenna theory is a complex topic and I'm not familiar with it. However, you can be sure that the manufacturer has done rigorous tests to insure that antenna design is good enough for the application.
That being said, "random soldering" is not very important, as long as the general shape of the antenna "bulk" is correct.

B) There's always going to be a lot of noise in the received signal (unless you're standing very close to the access point), so getting a perfect antenna isn't going to improve your reception noticeably. The exceptions are: 1) your antenna is busted 2) the manufacturer is bad and does not have a better design 3) you did not pay enough to deserve a better design from the manufacturer.
Regardless, most of the heavylifting occurs in the processing that's done to recover the target signal from the noisy channel. That processing is most likely occurs in circuitry (although there is a growing shift towards more software-based processing). Your controller also could be buggy, but that's a whole different issue.

C) No. If you can make a better one yourself, you should start a company selling them. Replacing an antenna is different than creating a cantenna to boost your router signal.
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Qaanol
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Re: How do (wifi) antennas get their shape?

Postby Qaanol » Wed Aug 26, 2015 4:10 am UTC

wee free kings

SAI Peregrinus
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Re: How do (wifi) antennas get their shape?

Postby SAI Peregrinus » Fri Aug 28, 2015 5:05 pm UTC

The soldered bit is the feed line, its position doesn't matter much in some antenna designs. Eg the Inverted F Antenna.

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Re: How do (wifi) antennas get their shape?

Postby Minerva » Wed Sep 09, 2015 6:29 pm UTC

How are they designed that way?
It's a little bit of a craft, and sometimes a little bit of black magic, and sometimes a little bit of finite-element computational simulation of Maxwell's equations.

An antenna, basically, is an impedance-matching transformer that goes between the 50-ohm transmission line (or whatever) and the impedance of free space.
But it's a special, and very diverse, class of transformers.

In trying to understand how these things work, it's worth remembering that a piece of wire isn't a piece of wire at microwave frequencies - it's a transmission line, with resistance, inductance and capacitance.

So you might have a piece of metal that is connected to both ground and to the signal, for example, and a DC resistance test would show a signal short to ground. But that's not really a "short" at RF, because the stub that connects the signal source to ground is likely to be a quarter-wavelength long - so it will always be at a maximum or minimum signal amplitude relative to ground.
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