mikey@trsvax.UUCP (05/15/84)
#R:ecsvax:-242100:trsvax:53400026:000:1667 trsvax!mikey May 15 09:12:00 1984 MOST cordless phones use the AC line to communicate to the handset. Until a very recent FCC change, they used a carrier just above the AM broadcast band. BTW, the base to handset is usually AM and the handset to base is FM just below 50 mhz. The new FCC policy allows both directions to be FM and in the 49 mhz range. Being AM to the handset is not necessarily that bad. The person on the other end still gets a really clean signal, although you don't, and unless they hear the lawn mower or something, they can't tell your wireless. Besides, they were a relatively cheap status symbol. I don't know if the phone manufacturers were actually restricted from using FM at 49 mhz before both ways. It's just that the number of channels were severly restricted (i.e. 5 or 6) and manufacturers designed their phones to get the least interference with one another. Besides, when they first came out, no one thought they would take off like they did. As for interference with BSR controllers, the BSR puts its signals on the AC line only at zero crossing, and I seem to remember that it had something to do with a 500 khz carrier, but I may be wrong there. Radio Shack's controller that hooked up to the TRS-80 cassette port still has a service manual available through their National Parts department. I looked at one and it used the same circuitry to couple to the AC line that some of the intercomes used that had carriers at 500 khz or nearby. If you're interested, you could order a copy of the service manual, but it won't have information on the actual protcall for the controllers, just technical informantion on the interface. mikey at trsvax
msc@qubix.UUCP (05/19/84)
> MOST cordless phones use the AC line to communicate to the handset. How do they manage that piece of magic? The handset doesn't plug into a wall socket. Do you mean they use the AC line as an antenna? Even that is a little bizarre. Many clock-radios use an additional wire in the AC cord as an antenna but they do not use the AC wiring. Most of the cordless phones that I have seen in the stores have telescopic rod antennas. -- >From the TARDIS of Mark Callow msc@qubix.UUCP, decwrl!qubix!msc@Berkeley.ARPA ...{decvax,ucbvax,ihnp4}!decwrl!qubix!msc, ...{ittvax,amd70}!qubix!msc "I'm a citizen of the Universe, and a gentleman to boot!"
mikey@trsvax.UUCP (05/24/84)
#R:ecsvax:-242100:trsvax:53400027:000:1513 trsvax!mikey May 24 09:41:00 1984 The AC lines will radiate the carrier current signal to some extent, but not that much. You can help the range of your cordless phone by making sure that the AC cord is unwound and extended as opposed to coiled until it plugs into the wall. If you are in a house with foil lined insulation, stretching the cord near a window can really increase your range. If you put a cordless phone in an office that has all the AC lines in conduit, the range will be very short. In this case, use an extension cord to the base set and stretch it around the room. The actual signal will travel throughout your entire area or household on the AC lines until it hits a high impedence, such as your power transformer. The same thing happens with wireless intercoms. Thats why you can use the intercoms between some houses and not others. It depends on wether you share a power transformer tap or not. Some universities use carrier current AM broadcast stations on their campus. To provide wider coverage, the power company can install RF bypasses to let the RF get around the transformer. You can drive all over the campus and pick up the signal on your car radio from the RF leakage, but it will fade off to nothing within 500 feet of campus, or the last power line. Most of these college carrier current stations operate around 50 watts. By the way, I believe the FCC still lets experimenters operate up to 5 watts of carrier current without a license on one of the bands below the 535 khz. mikey at trsvax
mikey@trsvax.UUCP (05/25/84)
#R:ecsvax:-242100:trsvax:53400030:000:414 trsvax!mikey May 25 07:45:00 1984 Inside the cordless handset there is a ferite rod antenna, very similar to what is in a standard AM protable radio. Trust me, the low frequency signal is radiated through the power lines. At the frequencies involved, it is a very effecient/cost effective method of getting the signal from the base to the handset. The whip antenna ONLY is for the VHF signal from the handset to the base. mikey at trsvax