Weíve seen a few posts here recently about our cellular phones setting off X band on our 8500s and STIs. Even Iíve experienced this in my car with the Samsung M500 (sprint) and the Bel V955 when driving on rural highways and the RD running in Highway mode. Driving in the city with autoscan, the falsing is not a problem. Hereís my take on whats happening based on some testing I did.
Phone: Samsung M500 (Sprint CDMA) operating at 1.9GHz when on Sprints towers or at 800MHz when on digital roam in the absence of a Sprint tower. Most people (myself included) leave the phones network selection to automatic; Sprint if it can find it, Roam if it can not.
What I did for this test was I pulled the RD off the windshield and place it on the seat, hoping to keep it away from any real X band sources just incase of interference. The RD was placed in Highway mode so no filtering, and the phone in automatic network selection. Since I live in the city where there are plenty of cellular towers, a quick run through the settings of the phone will reveal that it is operating on its home frequency of 1.9GHz. When I change the network selection to Digital Roam only and therefore locking the phone into the 800MHz band, the falsing becomes periodic on X band. Alternating between the menu showing the frequency and channel settings and the previous screen, I noticed the channels kept hopping in no specific order very rapidly. When the RD alerted to an X band signal, Iíd immediately check the channel number and itíd be around 7xx. When the RD is not alerting, a review of the channel number indicates it is around 5xx. What this tells me is that the digital roaming channels (each 1.25 MHz bandwidth) numbered in the upper 700s are in the territory of 807.69MHz to 811.55MHz. The 13th harmonic of this range would yield a range of 10.500GHz to 10.550GHz (X band). MEM-TEK indicated in the thread in the Bel section that cellphones are very good at producing higher harmonics so thatís why I think this is whats going on. When I locked the phone into Sprint only mode, therefore operating in the 1.9GHz band, the falsing is gone.
To make this concept easier to grasp I found this picture showing channels within the 2.4GHz band used for Wireless computer networks. To get an image of how this info relates to the 800MHz cellular band, think of 800MHz instead of 2.4GHz, think of the bandwidths being 1.25MHz instead of 22MHz, and instead of just those 14 channels pictured below, imagine hundreds of those little bands taking up the space. What I believe is happening when the channel hopping lands in a channel in the range of 807.69-811.55, our RDs are alerting on X. The most important thing I want people to see from the picture is that just because WiFi is advertised as 2.4GHz, it doesnt mean its operating exclusively at 2.4. And I bet the same applies to cellphones; the 800MHz figure just identifies a whole range of frequencies.
There was some discussion recently about 700MHz UHF 3G phones causing the problem as the 15th harmonic of 700MHz is the lower limit for X band, but I havenít seen a 700MHz phone in a very long time in my area. Either itís just not available where I live or its being phased out for newer technologies such as GSM/CDMA.
I wanted to make a video of all this but my camera batteries are still charging. Maybe later?