Re: Cellphone Reception/Transmission
- From: Jeff Liebermann <jeffl@xxxxxxxxxx>
- Date: Thu, 26 Jun 2008 19:33:34 -0700
On Thu, 26 Jun 2008 10:59:32 -0700 (PDT), mpm <mpmillard@xxxxxxx>
wrote:
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.
Handset power is controlled by the cell site. The site measures the
received signal to noise ratio (actually, the error rate) and reduces
the handset power to some threshold where additional power would not
improve the error rate.
In the other direction, the handset reports the received signal
strength to the cell site. Power per channel is reduced in the same
manner as in the first example.
A fair assumption is that the two directions are independently
controlled. That's the way I would implement it. Too bad not
everyone does it that way. Some of the old analog systems only
measured SNR in one direction, and just pre-set the opposite direction
according to some lookup table. That's a fair assumption is everyone
was running around with roughly the same antennas, belching the same
tx power, and having the same receive sensitivity. That didn't work.
There were too many phones running around with 3 watt power
amplifiers. I had one of these and it caused all kinds of grief, but
not to the cellular provider or the FCC. The problem was that the
cell site would hear a very strong signal produced by my 3 watt
amplifier, and reduce the cell sites TX power based on the assumption
that I must be using a 0.6watt max handset, and was very close to the
cell site. The result was I couldn't hear very because the site was
not transmitting enough power. That's the way Cell One and Dobson ran
their AMPS and IS-136 (TDMA) systems. When AT&T took over, they did
it right and set the power independently in both directions.
Now, attaching an external directional antenna could conceivably
increase the power above some FCC limit. It doesn't. The handset
power is controlled by the cell site. If it thinks you're belching
too much power, it will turn it down. Anyway, the limit is 600mw.
There hasn't been a phone made for perhaps 8 years that will belch
600mw of RF. Most small handsets will perhaps do 150mw max and
typically run at about 50mw RF out. The max specs are on the FCC ID
data which I'm too lazy to lookup for examples. (Translation: I'm
probably close, but not exact).
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).
Good point. I have no idea what the minimum tx power output of the
handset. Presumably, the dynamic range of the ALC must be equal to
the added gain of the external yagi for this to work. For example:
<http://wilsonelectronics.com/ViewProductB.php?ID=13>
the Wilson cellular yagi claims 14dbi gain at 900Mhz. Assuming that
the internal handset antenna has 0dbi gain (or slightly worse), if the
power amplifier is capable of 14dB ALC range, the minimum power should
be the same as with just the handset antenna. For example, the
MAX2507 RF sub-system chip has a 20dB dynamic range on the detector,
and about the same on the TX ALC control. 14dB should be no problem.
Also, the rules generally under 47CRF2 Subpart-J relating to the
requirement to test transmitters with the antennas they will use.
Yep. Type certification is suppose to be as a system. That's been
tweaked for Wi-Fi where one is allowed to replace the antenna with one
of an identical type and with lesser gain. The lack of enforcement
should offer a clue as to the level of compliance. However, you're
correct. Replacing or attaching an external antenna that was not
certified with the handset constitutes a violation of the rules.
(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,
That actually varies. The FCC 15.247 rules are a bit vague, where to
measure the +30dBm into +6dBi antenna power (i.e. does it include coax
losses). However, that's a diversion and separate topic. We're
talking cellular here.
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.
Yep. Customer installing car kits, magnet mount antennas, flashing
LED's on the antenna, RF shields, and other contrivances constitute a
lack of control by the service provider. These are probably fairly
innocuous and can be ignored. Antennas can easily get out of hand.
For example, a small yagi would probably not be a problem. Putting
then handset at the feed of a big dish might not be so innocuous.
Finally, you can actually make matters WORSE by DIY antennas. (I
won't get into all these details..)
I think I gave some clues has to how a power amplifier might cause
trouble. I'm curious. How would a do-it-thyself antenna cause a
problem? I can see trouble if it's used fairly close to the cellular
base station, where intermod, blocking, and other forms of overload
might kick in. However, presumably a high gain antenna would not be
used adjacent to a cell site. There may also be linearity issues
across the band what will cause problems with CDMA system. It would
need to be a fairly badly built antenna, with a very narrow bandwidth
(which implies lots of gain), to cause problems. Did I miss anything?
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..
Somehow, I don't see Joe Sixpack holding a cell phone in one hand, a
high gain antenna in the other, and pointing it at his own head. Even
if Joe Sixpack did that, I don't think he would yack long enough to do
any real damage. However, rules must be obeyed, so perhaps a warning
label on the antenna "Don't point at your head" might be useful.
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.
The cell site is going to try to adjust everyone's handset power so
that everyone signal level in the receiver IF, at the cell site, is
identical.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@xxxxxxxxxxxxxxxxxxxxxx
# http://802.11junk.com jeffl@xxxxxxxxxx
# http://www.LearnByDestroying.com AE6KS
.
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