iPhone antenna

Question

Level 4

1,722 points

iPhone antenna

I have a few questions about the iPhone antennas, which have been prompted by the experiences posted by many on this forum regarding the iPhone's variable signal reception, in the US and the UK.

To summarise my understanding, the iPhone has 3 antennas: GSM 850/900 MHz; GSM 1800/1900 MHz; and a WiFi/Bluetooth antenna. They are all positioned at the base of the iPhone, i.e. the bit covered by the black plastic cover. Also, rather than being placed close to the RFIC (as in many other mobile phones), they are connected by wires.

I am assuming that the antennas are similar to those found in other mobile phones, as I doubt Apple will have wanted to "reinvent the wheel", so to speak. I'm also assuming that they are large enough to handle these frequencies, i.e. a quarter wave aerial where the antenna length = a quarter times the wavelength. Therefore, for 850/900 MHz, wavelength = approx. 35 cm, therefore the antenna length should be approx. 8.75 cm long. Any significant decrease of this length would inevitable cause signal losses.

If the above is correct, then I'm wondering whether the fact that the iPhone antenna seems to be less sensitive than that found in other phones is simply due to the fact that the presence of the wires connecting the antennas to the RFIC is causing signal bandwidth changes and attenuation, due to impedance mismatches at both ends?

Any thoughts would be appreciated.

Dual 2.3 GHz PowerPC G5, Mac OS X (10.5.1), 2 GB DDR SDRAM

Posted on Jan 10, 2008 2:20 AM

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Answer 1

jia10 Author

Level 4

1,722 points

Jan 10, 2008 6:05 AM in response to Tamara

Most older phones have the antenna at the top. However, new phones have to comply with existing regulations on minimum safe exposure to EM radiation, hence, locating the antenna at the bottom of the phone, away from the skull and temporal bone. Many other brands include some advice on holding the phone so that the antenna is not obstructed by the hand, which can cause some changes to the the effective wavelength of the antenna, and therefore its transmission efficiency. I haven't found any information in the iPhone user's guide to this effect.

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Answer 2

Jan 10, 2008 7:06 AM in response to jia10

Can you point me to these regulations? I really can't find one for cell phone? All I can find are debates.

Besides, wouldn't your hand eventually fall off from to much radiation. Or both hands if you switch from time to time...part of an ear..your lip....sorry, I have to ask.

Is your hand thicker...or you head?

Sorry, I have to ask.

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Answer 3

jia10 Author

Level 4

1,722 points

Jan 10, 2008 7:24 AM in response to David M Brewer

Microwaves create a heating effect. This is strictly limited by international regulation and is measured by the value called the SAR (specific absorption rate). All phones sold in the EC must have a SAR less than 2.0 (in the USA it is less than 1.6). The SAR is a measure of microwave absorption into the head, so it makes sense to choose a phone with the lowest possible SAR.

You can live without a finger or nose or lip, but without a brain is somewhat trickier. The specific concern are acoutic neuromas, which arise in the auditory (VIII) cranial nerve. Having said that, there is no strong evidence that mobile phone use causes these tumours.

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Answer 4

Jan 10, 2008 8:50 AM in response to jia10

The length of the antenna connection would be taken into account by any competent RF engineer, and I would think that any cell phone manufacturer would have competent engineers experienced in microwave antenna design. I was doing this 30 years ago designing TACAN receivers. An antenna does not have to be physically 1/4 wavelength; it has to have an apparent length of 1/4 (or 1/2) wavelength to avoid reflections and present a low SWR. An antenna can present an apparent wavelength different from its physical wavelength; this can be done by winding it around an insulator, or by adding inductance (or capacitance) in the circuit. The length of the connection to the antenna must also be included in the calculations at microwave frequencies (actually at any frequency, but lower frequencies don't have the same problems).

While the impedance match is important, capture area also matters. The iPhone has a smaller capture area than many other cell phones. The location of the antenna is also conducive to users wrapping their hands around it, thus inadvertently adding shielding to the antenna.

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Answer 5

DanH

Level 5

4,372 points

Jan 10, 2008 8:58 AM in response to David M Brewer

David,

It's a regulation, not a law. The law says the FCC can make regs and they did (grin). You can find the background info and links to the actual language here:

http://www.fda.gov/cellphones/qa.html

-dan

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Answer 6

jia10 Author

Level 4

1,722 points

Jan 10, 2008 10:18 AM in response to Lawrence Finch

Many thanks for this informative reply. The point about the apparent length of 1/4lambda is well taken. I'm intrigued by your statement that the iPhone antenna has a smaller capture area than other phones, and consequently has less gain? Is this because the physical length is shorter than approx. 9cm (if 900 MHz is used)? From memory, and it's been a long long time, the capture area is determined by multiplying the antenna's radiator length by 1/4 lambda, isn't it?

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Answer 7

Goshia

Level 4

1,400 points

Jan 10, 2008 10:40 AM in response to jia10

Here is the SAR report: https://fjallfoss.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachmentid=766946&native_orpdf=pdf

SAR Photo's: https://fjallfoss.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachmentid=766948&native_orpdf=pdf

Here are the official internal antenna photo's: https://fjallfoss.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachmentid=766954&native_orpdf=pdf

Message was edited by: Goshia

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Answer 8

Jan 10, 2008 10:42 AM in response to jia10

The loss of gain occurs any time the physical length is less than than the apparent length. My memory is probably fainter than yours, but that sounds familiar.

There are other factors that can affect gain such as polarization, directionality, etc. Directionality shouldn't be an issue in a phone (there is no attempt to improve gain by making the antenna directional), but polarization could be a factor.

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Answer 9

jia10 Author

Level 4

1,722 points

Jan 10, 2008 11:02 AM in response to Lawrence Finch

Lawrence Finch wrote:
There are other factors that can affect gain such as polarization, directionality, etc. Directionality shouldn't be an issue in a phone (there is no attempt to improve gain by making the antenna directional), but polarization could be a factor.

Yes, I would expect the antennas on any mobile phone to be omnidirectional, as you cannot predict what the orientation of the handset (or the user) will be at any time. Having said that, hand or body shielding would be expected to modify the antenna's polar plots, so it would not be truly omnidirectional.

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Answer 10

Jan 10, 2008 1:13 PM in response to DanH

Thank you Dan...I did see that site during my search. All in all it does says that a wireless phone doesn't poses any health threat with the available scientific evidence. And there no proof that they are safe too!

The way I see people on a cell phone these days you think their tongues or thumbs would fall off first...ha ha.

So this is still is still up in the air.

Again Dan thanks...and I'll stop beating the dead horse (that died from the rider's cell phone radiation, rider is ok)

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Answer 11

Tamara

Level 6

13,744 points

Jan 10, 2008 4:56 AM in response to jia10

http://antennaguy.com/Images-iPhone/iPhoneAntenna3.jpg has a picture of what the antenna setup looks like.

I believe most other phones have the antenna on the top of the phone.

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Answer 12

Jan 10, 2008 7:59 AM in response to jia10

I mean an official UN...USA...UK...EC, document. That is law. Like i said, all I find on the internet is debates about this. No hard laws.

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Answer 13

Goshia

Level 4

1,400 points

Jan 10, 2008 7:16 PM in response to David M Brewer

Your eyes go first, its the softest tissue.

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Answer 14

jia10 Author

Level 4

1,722 points

Jan 11, 2008 12:09 AM in response to Goshia

Actually, it's the eyes and testes, as they are the organs with the poorest blood supply, therefore can't dissapate the heat generated by microwave frequencies.

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