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I'm curious what kind of directional antennas could fit inside a cell phone, which would require it to be 5mm or less in its longest dimension. I know that the length of a directional antenna is often a critical factor in the gain and directionality of an antenna, so I'm wondering also what frequency would be ideal for that constraint.

So in short, what types of directional antennas would be practical at a maximum length of 5mm, and what limitations are necessary with that constraint?

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    $\begingroup$ Other than material/manufacturing issues, any antenna design can be used at any size; it's just that, to get the same operation, the wavelength scales with the physical length. So if you want to ask about smaller-than-ideal antennas, you should specify a fraction of the wavelength, not a fixed physical length. I'd recommend editing your question. $\endgroup$ – Kevin Reid AG6YO Apr 28 '16 at 21:54
  • $\begingroup$ The GPS antenna in your phone is that sort of size, 5 x 3 x 1 mm, and it's directional... a bit $\endgroup$ – tomnexus Apr 28 '16 at 22:18
  • $\begingroup$ @KevinReidAG6YO That's exactly why I asked about the frequency that would be ideal at that physical length. I don't think you're correct that all antenna designs can be scaled to any size depending on the wavelength. I've heard that high frequency microwave antennas need to use different design methodology because the wavelength is close to the size of the other electronic components. I don't know how true that is, but I want to leave my question as is because I don't want to make any assumptions that i'm not sure about. $\endgroup$ – B T Apr 28 '16 at 22:46
  • $\begingroup$ @tomnexus Only unintentionally I would imagine, right? $\endgroup$ – B T Apr 28 '16 at 22:47
  • $\begingroup$ What is the reason that directionality is important to you? It seems you can get any gain you want, just choose a frequency. G * wavelength^2 = 4*pi*(5 mm)^2. 10 dBi @ 53 GHz, 20 dBi @ 170 GHz and so on. $\endgroup$ – tomnexus Apr 29 '16 at 4:45
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You could look into "Patch Antenna" design. Some theory here: http://www.antenna-theory.com/antennas/patches/antenna.php

however: manufacturing this yourself would be quite challenging I would imagine.

HTH.

[edit due to comments]

Hi BT, it seems you asked a different question in the comments. I guess the original question:

what types of directional antennas would be practical at a maximum length of 5mm, and what limitations are necessary with that constraint?

Would be answered by above reference to patch-antennas.

Now you are asking:

How are these used in mobile devices without being omnidirectional?

This can be broken up in two questions:

1) How are these used in mobile devices ?

2) without being omnidirectional ?

Answers are simple:

1) They are not used in mobile devices, or at least not main-stream, due to size of the patch itself (there are others derived from patch antenna design, already commented below)

2) They are directional. But this does not seem to hinder mobile technology. The cell towers seem to have enough directivity/gain/coverage/power to reliably build connections even when the mobile device has a "somewhat directional" antenna.

HTH.

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  • $\begingroup$ Interesting, according to that it has pretty good directivity. How are these used in mobile devices without being omnidirectional? $\endgroup$ – B T Apr 29 '16 at 23:54
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    $\begingroup$ I don't think that patch antennas are used (at least often) in mobile phones: the size has to be compearable with wavelength making it too large for mobile phones. I'm not an expert but I would assume that the CCEs (capacitively coupled element) are the usual way to go, replacing older PIFAs (Planar inverted F antennas) used in single or dual frequency phones. $\endgroup$ – OH2FXN May 2 '16 at 17:00
  • $\begingroup$ @OH2FXN Both CCE and PIFA are derived from patch/microstrip antennas. $\endgroup$ – Edwin van Mierlo May 3 '16 at 7:45
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    $\begingroup$ @EdwinvanMierlo You seem to be correct, I knew that PIFAs and CCEs are based on microstrip antennas but didn't know that they are also classified as one. Either way, this is answer for B T's follow up question on how to make microstrip antenna more omnidirectional. $\endgroup$ – OH2FXN May 4 '16 at 16:56

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