On Thu, 15 Jul 2010 08:43:04 -0700, Jim Lux
wrote:

I doubt they're using regeneration or some such similar scheme (too hard
to control for manufacturing variability)..

I'm not suggesting they did it intentionally. I'm suggesting that the
front end is unstable and that nobody noticed.

However,
the idea of self interference is interesting... (e.g. your finger
couples the WiFi to the cellular front end, which blocks)

I was looking at the FCC ID web site photos again. I can't even tell
which end of the antenna is hot or ground, much less where it's fed.
If the hot end of both antennas are facing each other, it's certainly
possible. However, if Apple did that, they wouldn't need two separate
pieces of metal for the two antennas. They could simply declare some
point along the frame as being the official ground point, and have the
cellular antenna go one direction, and the wi-fi/BT antenna go the
other. Because they didn't do that, my guess is that the hot end of
the cellular antenna is next to the ground end of the wi-fi/BT
antenna.

It could even be as dumb as a dc bias shift thing (save a penny by
leaving out the capacitor, oops, that removed the DC block)

Methinks not. The duct tape solution just reduces the effect, not
eliminates it. Assuming duct tape to be a good insulator, that would
also suggest that it's not a DC effect.

As far as match goes, in very compact systems, the idea of matching 50
ohms goes out the door unless one of your components happens to be 50
ohm impedance.. It's not like they're stringing together building blocks
with SMA connectors inside there. Lots of these RF circuits are
implemented with differential in and out these days, and the Z is
whatever it happens to be.

If you look at the inside photos of the BCG-E2380A on the FCC ID web
pile,
http://www.fcc.gov/oet/ea/fccid/
(don't forget to include the "-" as part of the product code), there's
a module labeled "Acoustic and Cellular Antenna Feed" on Pg 9 and 10.
https://fjallfoss.fcc.gov/eas/GetApplicationAttachment.html?id=1300576
There's a coax cable going across the module, which I assume is 50
ohms.
http://802.11junk.com/jeffl/iPhone4/iPhone4-accoustic-and-cellular-ant-feed.jpg
It doesn't appear to do any more than connect a tiny coax connector to
the RF module on one end, and a gold screw lug to the frame antenna on
the other. No matching in between. Due to the coax, someone must
think the antenna is 50 ohms at 800/900 and 1800/1900.

I've never seen a differential RF amp in a cell phone. If so, I would
expect either a balanced antenna feed, or possibly two antenna
connectors.

Disclaimer: I'm doing quite a bit more guessing than usual. Please
do not treat the above as authoritive.

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