[IP] iPhone Antenna

Interesting bit about cellphone antennas: radiation and reception.  
Sent from my iPhone
Begin forwarded message:

From: Dave Farber <dfarber@me.com>
Date: June 25, 2010 10:55:26 PM PDT
To: “ip” <ip@v2.listbox.com>
Subject: [IP] iPhone Antenna
Reply-To: dave@farber.net

Begin forwarded message:

From: Joly MacFie <joly@punkcast.com>
Date: June 26, 2010 12:06:10 AM EDT
To: dave <dave@farber.net>
Subject: iPhone Antenna


I received a phone call today from PC Magazine.  They were running a story on the new Apple iPhone 4, specifically the reports (PC MagGizmodo,Engadget) that people are experiencing decreased reception on their cell phone when they hold the phone by the metal frame.  That frame has been touted by Apple, in the keynote address by Jobs, as being part of the antenna system.  Here is a brief summary of what I told the reporter who called me, and a little extra. (I will update this with his name when he emails me.)

I saw the photo of the frame of the iPhone in the slideshow at the end of Steve Job’s keynote address at the Developer’s Conference.  There are three gaps in the stainless steel band which are allegedly part of the antenna system.  I have not had alot of time to analyze their structure, nor do I have one in my hands yet.  So, either it is public relations hokum, or those slots are really part of the antenna structure.  They do appear to be active, based on observations.

In the first generation iPhone (which I am currently using), the antennas were on the back of the phone, near the bottom.  There was a piece of plastic on the bottom covering the antennas, so you knew where they were.  I developed a way to hold the phone which avoided covering this area with my hand, similar to the Gizmodo article linked above.  It is worth stepping back a moment and asking the question, “Why are the antennas placed where my hand is MOST likely to cover it?”  It’s a fair question.

The FCC puts strict limits on the amount of energy from a handheld device that may be absorbed by the body.  We call this Specific Absorbtion Rate, or SAR.  In the olden days, when I walked ten miles to school in three feet of snow, uphill in both directions, cell phones had pull-up antennas.  This allowed the designer to use a half-wave antenna variant, and put the point of maximum radiation somewhat away from the users cranium.  Of course, most people did not think it was necessary and kept the antenna stowed.  Motorola’s flip phone acutally had a second helical antenna that was switched into place when this was the case.  But, more importantly, SAR rules were not yet in effect.

Flip phones became yesterday’s style, and phones were becoming more monolithic.  Some phones, like the early Treo, kept the antenna in the traditional location at the top of the phone, near one edge, but reduced it to a short stub.  Whips became stubs, stubs became bumps, and finally antennas were embedded into the rectangular volume of the phone.  The trouble was SAR; if you left the antenna at the top, the user was now pressing it into their head, insuring lots of tissue heating.  Enter the bottom-located cellphone antenna.

Just about every cell phone in current production has the antenna located at the bottom.  This insures that the radiating portion of the antenna is furthest from the head.  Apple was not the first to locate the antenna on the bottom, and certainly won’t be the last.  The problem is that humans have their hands below their ears, so the most natural position for the hand is covering the antenna.  This can’t be a good design decision, can it?  How can we be stuck with this conundrum?  It’s the FCC’s fault.

You see, when the FCC tests are run, the head is required to be in the vicinity of the phone.  But, the hand is not!!  And the FCC’s tests are not the only tests that must be passed by a candidate product.  AT&T has their own requirements for devices put on their network, and antenna efficiency is one of them.  I know because I have designed quad-band GSM antennas for the AT&T network.  The AT&T test similarly does not require the hand to be on the phone.  

So, naturally, the design evolved to meet requirements – and efficient transmission and reception while being held by a human hand are simply not design requirements!

OK, back to the iPhone 4.  The antenna structure for the cell phone is still down at the bottom (I won’t address the WiFi nor GPS antennas in this blog entry).  The iPhone 4 has two symmetrical slots in the stainless frame.  If you short these slots, or cover them with your hand, the antenna performance will suffer (see this video I found on YouTube).  There is no way around this, it’s a design compromise that is forced by the requirements of the FCC, AT&T, Apple’s marketing department and Apple’s industrial designers, to name a few.

One of the questions the intrepid reporter from PC Magazine asked me was, “Will putting the phone in a pocket and using a Bluetooth device help?”  Good question.  The answer is yes, to a point.  The first generation iPhone clearly had a conductive surface below the antenna (I hesitate to call it a ground plane, because it it too small).  So, putting it in your pocket with the screen toward your body and the antennas facing out while using your Bluetooth earpiece will work better than holding the phone with your hand.  In fact, in my car my iPhone sits forward on the dashboard, under the winshield, screen down while I use my Jawbone.  Works great.  (However, if you put your iPhone in your left back pocket, and your earpiece in your right ear, you may have issues.  This is a failing of the Bluetooth system in dealing with severe body losses at 2.4GHz, not the cellphone’s problem.)

The iPhone 4, however, moved the antenna action from the back of the phone to the sides.  This probably improves the isotropy of the radiation pattern, but only when the phone is suspended magically in air.  Not too helpful.  Putting this iPhone 4 in your pocket will likely couple more energy into your body (you bag of salt water, you) than did the first generation model.  Yep, I predict it will be worse.

So, what’s an iPhone lover to do?  Well, I voted with my dollars.  I ordered my iPhone 4 to replace my Original.  I already know how to do the Vulcan Antenna Grip on the iPhone, and I am wearing out my current model.

And sometimes an antenna that’s not great, but good enough, is good enough.

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