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Antennae

June 28, 2010

While visiting our daughter on Friday, we stopped at the King of Prussia Mall for lunch and a little shopping. On our journey, we passed an Apple Store, where there were about fifty people in line to buy the just-released iPhone4. My father would often use the phrase, "hand over fist," to describe certain money-making schemes. Through the power of the internet, I now know that this expression is a nautical term for hauling on a rope. The expression morphed over the years to mean something that's done quickly, like making money. Hand over fist is a good summary of Apple's electronic gadget business that includes other recent items, such as the iPad. The iPhone4 is technically interesting because of a design flaw in its antenna.

Ask any radio frequency engineer what he considers to be his most hated component, and the answer will be, "the antenna." Antennae, a.k.a., antennas, are an essential part of any radio system. They couple the electromagnetic signals that your circuit creates to "free space," an archaic term for the vacuum. Early antennae were dipoles, something easy to construct, since they're just conductors a quarter of a wavelength long, and easy to analyze. In the hundred or so years since Hertz's discovery of radio, many novel antennae have been invented. There are several books on my bookshelf that are filled with designs for different antennae [1-3]. The trouble with antennae is that they look good on paper, but they are difficult to implement. One major problem is that they are sensitive to their surroundings, and that's the problem that besets the iPhone4.

Helical antenna for NOAA TIROS satellite reception

Helical antenna for NOAA TIROS satellite reception


Since the bit error rate in a radio channel depends on the signal to noise ratio, it's important to have a high signal strength to get high data rates. The iPhone4 needs a high data rate, so Apple's design engineers decided to put its antenna on the outside of the phone to couple more signal into free space. Since Apple sells style as well as technology, the antenna is concealed as part of a stainless steel band that surrounds the periphery of the phone. For this to work, there must be a gap in the band, and this gap is at the lower left corner. The problem is that when people grasp the phone, their hands will sometimes bridge this gap, causing attenuation of the radio frequency signal [4-8]. There's less of a problem if the phone is contained in a decorative or protective case or boot, since these isolate a person's hand from the antenna. The iPhone4 is apparently the first cellphone to place its antenna in this location. [6]

One iPhone4 user posted the following as his own experience [5]. "Signal drops from 4-5 bars to 'searching for signal...' when I hold it in my palm or cover up the line on the lower left side of the phone..." The "line," of course, is the gap. It appears that the signal will drop to only 10% of its original value if the device is held in a certain way.[8] One quick fix [6] is to put a piece of tape over the gap, something that goes against the principles of anyone who bought the device because of its styling. Unfortunately, aesthetic principles are often inconsistent with physical reality.

A cellphone emits about 3.5 watts of transmitted power. Imagine what happens when antennae go awry at broadcasting stations that emit kilowatts of power. Two antenna incidents happened on my watch at a 1kW AM radio station. In one case, the radio frequency energy arced to ground through an insulated wire at the base of an antenna. That was a simple fix, but we needed to turn off the transmitter for a few minutes to move the wire and add some insulating tape. You can't touch that sort of antenna while it's operating! Another problem occurred when a connection became corroded in a circuit used to set the phase difference of signals to each of four antennae. The phasing sets the directional pattern of the antenna system to prevent the station's signal from interfering with other radio stations, so the corroded connection caused the directional pattern to change. The change was slight and it could be compensated with some panel controls, so the problem wasn't repaired until early Monday morning when the transmitter was off for regular maintenance.

References:

  1. J.D. Kraus, "Antennas," McGraw-Hill (New York, 1988).
  2. R. Dean Straw (Editor), "The ARRL Antenna Book," American Radio Relay League (2007).
  3. American Radio Relay League, "The ARRL Handbook for Radio Communications," American Radio Relay League (2010).
  4. "Apple Acknowledges Antenna Flaw In New iPhones," RedOrbit.com, June 25, 2010.
  5. Gregg Keizer, "Hardware expert explains iPhone 4 antenna problem," ComputerWorld.com, June 25, 2010.
  6. Erica Ogg, "iPhone 4 antenna issue: User error or design flaw?" CNET.com, June 25, 2010.
  7. Dylan F. Tweney, "Apple's Response to iPhone 4 Antenna Problem: You're Holding It Wrong," Wired Magazine Online (June 25, 2010).
  8. Jens Nielsen, "Er iPhone 4 født med antenne-problemer?" June 10, 2010.

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