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Topic Title: Antenna inside a mobile phone
Topic Summary: What does it look like?
Created On: 28 February 2010 05:11 PM
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 28 February 2010 05:11 PM
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eswnl

Posts: 144
Joined: 29 November 2008

Hi,

What does the antenna look like inside a mobile phone? Is it a single strip of metal and does it exhibit some directivity/smartness in any way.
Can the antenna be made so that it directs radiation away from the person?
 02 March 2010 12:05 AM
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massey

Posts: 38
Joined: 22 April 2002

Originally posted by: eswnl

Hi,

What does the antenna look like inside a mobile phone? Is it a single strip of metal and does it exhibit some directivity/smartness in any way.

Phew! What a question! - There is so much that could be put in an answer. It is difficult to know where to start or stop. - I'll make a start at an answer, and perhaps you can tell us what sort of detail you would like.

Well, there are many many designs for the internal antennas of mobile phones. For the small handsets we use nowadays, (almost) all of them have an "antenna element" whose real job is to couple to the RF currents on the handset ground (which is either in the PCB or sometimes is a metallic chassis). It is the currents on the handset ground that do the radiating and coupling to incoming EM waves.

The most popular families of mobile phone "antenna element" are variations on a "PIFA", which stands for Planar Inverted F Antenna. A simple PIFA is basically a monopole element that has been bent over to avoid it sticking out of the handset. Bending it over causes its radiation resistance, as measured between the base of the folded monopole and the ground, to go very low. So to match this antenna, the terminals between the bottom of the monopole and the ground are shorted out and another set of terminals between ground and monopole are added a little way along the monopole. The resulting shape - monopole bent over with a new connection looks like a letter "F" on its side - hence "inverted F antenna". The "planar" bit refers to the long arm of the "F" being a metal sheet that is coplanar to the ground, and slightly above the surface of the ground.
There are all sorts of variations on this theme to cover multiple bands.

Regarding construction, the antenna element can be cutout and folded from a metal sheet. Some are freestanding - with plastic support spacers and/or pins. Some are attached to the plastic case. Others are metal printed onto a plastic support.


Regarding smart antennas in handsets, - there are several possibilities. One is that the antenna could be optimised by switches to operate at the desired frequency. Such a switched antenna can be smaller than a conventional multiband antenna. Another is that the antenna matching could be adjusted to compensate for the influence of the head and hand when they are nearby. One could also have two or more antennas, and if one doesn't work well because it is being enclosed in the hand (very likely with today's very small handsets) that antenna is switched off and another is used.

Also perhaps the radiation pattern of one particular antenna is better suited to signal path at that time and so the antenna with the more useful radiation pattern is used.

So there are lots of possibilities. However, as far as I know, at present, most mobile phone handsets have a single multiband antenna element without much of this smart antenna stuff. (I think some have different feeds for different mobile phone frequencies.)

Can the antenna be made so that it directs radiation away from the person?

Depending on phone size and frequency, yes and no. For example, some years ago IMST and others designed metallic frames which lay at the head side of the handset and "reflected" some power away from the head. Whether this technology can be used depends on the size of the handset and whether the phone is operating at 900MHz or at 1800MHz and higher frequencies.

Even further back, in the 1990s, Hagenuk sold the "GlobalHandy" 900MHz GSM phone, which was so large, it had an antenna element that did the radiating independant of the PCB ground plane, and was directional.

Edited: 02 March 2010 at 12:19 AM by massey
 02 March 2010 10:43 AM
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cblackha

Posts: 76
Joined: 21 January 2003

Further to Massey's detailed reply, I would add

Can the antenna be made so that it directs radiation away from the person?

The "antenna" in a mobile is only part of the actual antenna that does that transmitting and receiving. A SAR (Specific Absorption Test) will identify the parts of the phone that radiate the most energy at particular frequencies, but for a typical multi-band device, this will typically be different points at different frequencies

The phone has to be made fairly omni-directional so that it can work properly in real life, meet the regulatory standards radiated performance criteria and not have a RF "hot spot" that would cause a high SAR reading or SAR failure

Charlie
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