🚀 go-pugleaf

On Wed, 14 Jul 2010 13:05:27 -0700, AES <siegman@stanford.edu> wrote:

>Jeff Lieberman:

Pleeze spel me nayme corectlee.  There's a spare "n" at the end, in
case I get rear ended.

>Is your assessment here that placing RF-lossy human flesh (with a high 
>water content) close enough to the phone antenna loads down (or maybe 
>just detunes?) the RF circuitry of the transmitter final stage to a 
>point that the active transmitter circuitry in the phone no longer 
>oscillates effectively?  -- but just putting a dry and non-lossy 
>dielectric there doesn't, and also moving the flesh even a short 
>distance away reduces its loading on the circuitry enough that the 
>active circuitry still works OK?

Nope.  My wild guess(tm), based upon what little I can extract from
the FCC ID page and from the iFixit autopsy, is that something drastic
is happening in receive.  The -30dB (that's 1000 times drop) in signal
appears in receive, when you're NOT making a call and the xmitter is
inactive.  Well, the xmitter is sending keep alive bursts every few
minutes, but nothing more.  Methinks it's a receive problem, not xmit.

I'll go out on a limb and suggest that the receiver front end
(probably a GASFET or HEMT) might be a bit regenerative (border line
oscillatory).  This gives it lots of gain, but only if nothing else
changes.  In effect, the antenna becomes part of an oscillator
circuit, where the oscillatory conditions are partly provided by the
antenna Q (i.e. antenna efficiency).
<http://en.wikipedia.org/wiki/Regenerative_circuit>

Touch the antenna, and you reduce the antenna Q.  The front end stops
acting regenerative (barely oscillating), and the sensitivity drops
like a rock.  It's the only explanation I can conjure the will cause a
-30dB drop in received signal.  It's been demonstrated (by me and
others) that strangling the antenna of other cell phones drops the rx
signal up to -10dBm (10 times drop).  That's what I would normally
expect to see from touching a cell phone antenna.  However, the -30dB
drop of the iPhone 4 requires some extra circuit design screwups.
Unfortunately, detecting the regeneration is going to require internal
probes, test fixturing, and plenty of expensive test equipment.

There's another related possibility.  Most of the finger tests have
been with the finger bridging the gap.  That means they're touching
BOTH the cellular and wi-fi/BT antennas.  It's obviously the hot end
of the cellular antenna, but I can't tell if it's the hot end or
ground end of the wi-fi/BT antenna.  If the hot end, another
possibility it that coupling to the 2nd antenna causes this antenna to
radiate into the case, thus causing the cellular front end to
oscillate.  If the oscillations are bad enough, the signal level will
be high enough to induce blocking (overload) in the cellular front
end.  This can be easily detected with a spectrum analyzer probe in
the vicinity.  If touching/bridging the antennas causes a new signal
to appear on the SA, that's the problem.

It would be very interesting to know if touching ONLY the cellular
part of the antenna (the part that goes up the side of the phone), and
not the wi-fi/BT antenna (bottom of phone), causes the same drop in
signal.  It would also be interesting to test the wi-fi signal
strength to see if it's effected by touching its antenna.

I have a few other guesses, but these are the best I can currently
conjure at this time.

>[I'm an EE, but with no RF circuitry experience, much less any cellphone 
>experience.  So, I can understand this happening with an active 
>transmitting circuit -- though I have somewhat more trouble seeing how 
>it would interfere in such a sensitive way with a passive receiving 
>antenna.]

Think regeneration and borderline oscillation.

The bad news is that if Apple stabilizes the rx front end, and
eliminates the oscillation or regeneration, my guess(tm) is that the
resultant phone is going to have a serious rx sensitivity problem.  If
you look at the cellular antenna in the iPhone 3G and other internal
antenna cell phones, they're not very simple looking devices.  Lots of
strange lumps, traces, pads, and oddities, all designed to optimize
performance in the 800/900 and 1800/1900 MHz bands.  Getting them to
look like 50 ohms for minimum VSWR is also important.  Lots of
articles in the IEEE Antenna and Prop proceedings on the topic of
cramming a dual band antenna in the smallest possible package.

On the other foot, the iPhone 4 antenna is crude.  As far as I can
tell from here, it's a crude monopole (single wire) antenna.  There's
no matching circuit for VSWR reduction, and no attempt to optimize
performance in the desired bands.  I might have missed something in
the dissection, but all I can see is an end fed monopole, without a
counterpoise or underlying ground.  Such an antenna is going to have
resonances at odd frequencies, and miserable antenna efficiency.

Disclaimer:  I haven't found anyone willing to let me tear into their
iPhone 4 yet, so all the above is guesswork.  

-- 
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS