Cellphone Reception/Transmission

They're probably fairly close to omnidirectional, when in free space (i.e. not shielded by other objects). Their actual pattern is probably rather irregular and varies quite a bit from one phone to the next.

That's probably what's referred to as a "body fade". Since you're holding the cellphone near your head, your head will shield the cellphone to some extent from signals arriving from one direction.

--
Dave Platt                                    AE6EO
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In the "Mobile Networks" menu of my phone I can set "Search Mode" to Manual rather than Automatic, I can also select preferred networks to connect to. Won't help if it is the same network I suppose.

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warrior

will

nd me such ads!

Most air-interface standards (CDMA, GSM, 3GPP, etc...) update the handset output power 1500 times a second. So to some degree, handset antenna pattern issues are taken care of, although you can still drop call is the fading is bad enough or if the handset is already broadcastinng at maximum power (i.e., far from the cell site). A call can also be dropped for a lot of other reasons we won't get into here...

The other commenter is correct in stating that in actual use, the pattern is anything but omni-directional. It would be more appropirate to think of it as quasi-omnidirectional, with directional components that vary according to orientation (to the user).

Without knowing your carrier and the technology used, it is difficult to pin down the exact problem. The problem could be either with the handset (likely, its programming), or a host of possibilities on the network(s).

You would be absolutely amazed at the amount of data collected by the cell phone companies. If you can "make friends" with the right engineer, and you know the time & date of the call, some forensic engineering is usually a do-able thing...

Let me know if your carrier is AT&T Mobility, and if so, where exactly (coords) you were when this happened. Thanks!. -mpm

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:

Some cell phones have external antenna connectors, where a direction panel or yagi antenna can be attached.

There are also various plans for do it thyself antennas. For example:

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:

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

mple:

=BDI

6-2558

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.

Good post.

For mobile applications, just getting a regular old dipole (or half-dipole with the car being the ground plane) outside of the car usually does wonders and is all that is needed.

I agree that, strictly speaking, playing with high-gain antennas may not help performance, could possibly hurt it, and strictly speaking could violate some FCC rule, although the likelihood of the FCC coming after you if you're just violating the rule but not actually denying service/revenue to the TelCo are about zero.

Excellent

Not AT&T. Telus mobility.... I'm only curious if it's possible to 'point' my cellphone in the right direction on the US/Canada border.

D from BC British Columbia Canada

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).

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:

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.

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.

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.

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.

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?

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.

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@comix.santa-cruz.ca.us
# http://802.11junk.com               jeffl@cruzio.com
# http://www.LearnByDestroying.com               AE6KS

Much agreed, high gain antennas are very directional and thus difficult to aim in mobile applications. Planar cardoid patterns at

4.5 dBi are probably best, with 7 to 9 dBi quadrant being a limit tradeoff case.

Well, kinda hard to have gain without directionality, since that's where the gain comes from. ;-)

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Keith

You haven't seen me on ham radio transmitter hunts. With practice, it's possible to simultaneously swing a big yagi antenna, play with the radio dials, watch the display, and drive at the same time. I don't recommend this for the average cell phone user, but it is possible. The only problem is that the vehicle code now limits the overhang on the vehicle sides to 12 inches. Prior to that regulation, I was swinging a 10ft long yagi antenna on the roof, which would cause pedestrians to dive for cover.

Also, if you know the location of the cell sites, and have a GPS in the vehicle, it should be possible to automatically aim a motorized directional antenna on the vehicle rooftop, at the cell site.

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

Perzactly.