Re: Cellphone Reception/Transmission

On Jun 26, 1:02�pm, Jeff Liebermann <je...@xxxxxxxxxx> wrote:
On Wed, 25 Jun 2008 20:00:29 -0700, D from BC

<myrealaddr...@xxxxxxxxx> wrote:
I had lots of dropped calls and blanks when I was using my cellphone
near the BC/US border.
I noticed that my phone would sometimes go into roam mode.
I guess a US cell tower and a BC cell tower were competing for my
phone.

I was told that if I faced away from the US border, my cellphone would
connect to the BC cell tower and not the US tower. I'm trying to avoid
roaming charges especially when I'm still on my side..

Are cell phones omnidirectional or have directivity?

Yes. �In order to meet the SAR (specific absorption rate)
specification for RF exposure, cell phone makers are directing as much
of the energy away from the head and body. �See chart at:
<http://reviews.cnet.com/4520-6602_7-5020355-1.html>
<http://www.cellphonespec.com/index.php?option=com_content&task=view&i...>

Some cell phones have external antenna connectors, where a direction
panel or yagi antenna can be attached.
<http://www.wilsonelectronics.com////Antennas.php?Type=B>
There are also various plans for do it thyself antennas. �For example:
<http://bigredball.blogsome.com/2006/07/29/how-i-got-mobile-phone-rece...>
Hmmm... I don't really like the way the driven elements are mounted
(the 50 ohm coax should extend all the way to the feed point), but
it's good enough to work for what you're doing.

You can also fabricate a reflector to redirect most of the signal. �I
think this would be the most practical solution. �Just a piece of
cardboard covered with aluminum foil. �See:
<http://www.freeantennas.com>
These are for 900 and 2400MHz, but can be scaled for 800/1900MHz
cellular.

Incidentally, I had a somewhat similar problem making calls when I was
on a local beach, below the cliff line. �The local cell site was
blocked by the cliffs, so my phone roamed cross the bay to the
competition. �It wasn't worth dragging a yagi with me to avoid
roaming, but I did take one when I was across the bay to get to my
home system. �Eventually, the problem was solved by carrier
negotiation and cross roaming agreements, so it's no longer an issue.

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

Please be advised that while Jeff's information relating to the use of
"do-it-yourself" antennas has technical merit, designing your own
antennas and attaching them to your handset likely violates several
FCC Rules.

Particularly: Rule 24.232(c) which, for PCS spectrum, mandates use of
the minimum power necessary to make the call. (which if close enough
to the tower can still be too high if the handset is already at
minimum power and excessive antenna gain is employed).

Also, the rules generally under 47CRF2 Subpart-J relating to the
requirement to test transmitters with the antennas they will use.
(Not as separate piece-parts). Granted, this is more of an issue for
Part-15 unlicensed (low-power) devices that are certified according to
field strength, but there are separate rules under Part-22 for handset
licensing (as a consequence of being a "subscriber in good standing")
under the respective carrier's FCC licensing. Coupled with this rule
is a carrier requirement to maintain "control" over their networks
(22.927, etc..) and that cannot be accomplished if handset antenna
patterns / gains are uncontrolled. So, you have to consider both of
those rules together, when rolling your own antenna designs.

Finally, you can actually make matters WORSE by DIY antennas. (I
won't get into all these details..) So, be careful. Also, there are
statutory limits to maximum output power (for both PCS and Cellular,
and even SMR stuff like Nextel/Sprint), and additional limits for
potentially harmful radiofrequency radiation off cell phone antennas -
whether or not they are next to your face, etc..

For the vast majority of MOBILE applications, the cell site is going
to turn down your handset output power anyway, so the use of high-
gain, directional antennas has limited application.


.

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