| Home |
| Search |
| Today's Posts |
|
|
|
#1
September 6th 09, 11:48 AM
posted to rec.radio.amateur.antenna
|
|||
|
|||
Corriolis force
"Jeff Liebermann" wrote in message ... On Sat, 5 Sep 2009 20:30:30 -0500, (Richard Harrison) wrote: Art wrote: "Chris, what I believe he is referring to is that computer programs support a tipped vertical over one at right angles to earth." Quick! Tell your local broadcaster. We don`t need theory to show that antennas for transmitters and receivers work best together when they are exactly parallel in space unless something in the transmission path is redirecting the polarization of the signal. In aligning microwave antennas humdreds of times, I`ll swear this is true every time, regarfless of Corriolis, no matter which side of the equator I was on. Try it. You`ll be convinced too. Best regards, Richard Harrison, KB5WZI Ummmm.... Faraday Rotation? http://en.wikipedia.org/wiki/Faraday_effect It's not too horrible at microwave frequencies, but becomes noticeable as the frequency decreases. I've done some crude polarization studies at VHF and UHF frequencies with a rotating dipole, and found some rotation on line of sight paths. However, we were looking for a polarization distribution and didn't really spend any time getting accurate numbers for a line of sight path. * Fraday rotation is much greater in the ionosphere where there are more free electrons, apparently. L-band and C-band links with geostationary satellites use circular polarisation for this reason (as well as others when a mobile terminal is involved) - the degree of twist is measured in turns. At Ku-band it's down to a few degrees. Also many broadcasters use elliptical polarization (a mix of horizontal and vertical). However, that's not to eliminate any Faraday Rotation effects. It's to deal with the effects of reflections and refraction, which create nearly random polarization at the receiving end. Also, because stationary broadcast antennas are horizontally polarized, while vehicle mounted broadcast antennas are usually vertical. * In the UK, VHF FM radio was launched with horizontal polarisation mainly because of a belief that vehicle-generated nois was predominantly vertically polarised. This belief derived from an experiment, but the conclusion may have contained an error. However, it was clear that VHF reception in vehicles was impaired by this choice because a simple vehicular antenna is vertically polarised. As vehicular reception grew, and when commercial radio started at VHF, a vertical component was added to the transmitted signal to improve matters. Nowadays, most of the transmitting antennas for VHF FM radio provide 'mixed polarisation'. I was once asked why the British drive on the left side of the road. My instant answer was Coriolis Effect. I claimed that due to the earth's rotation, it's easier to make left turns on the left side of the Atlantic Ocean and easier to make right turns on the right side of the Atlantic. It took an excessively long time for even the sharpest student to catch the joke/hoax. Sigh. * Isn't the real answer connected to knights in shining armour on their horses and the side that they, traditionally, carried their jousting pole? .... but where did the tradition come from? ... is that to do with the traditional way a right-handed person mounts a horse? Drivel: At 2.4GHz, most wi-fi wireless routers use vertically polarized rubber ducky antennas. Yet, most laptops have their antennas in the top of the LCD frame which are horizontally polarized. Same with PCB antennas found on most PCMCIA cards. * A vertical whip on a device with a horizontal chassis, and horizontal power/data cables, probably radiates plenty of HP as well as VP. Years ago (before the widespread use of normal-mode helix antenna) I worked on a scheme that involved STC/ITT 'Starphone' hand-held transceivers. The base-station antenna was VP but the handsets each contained a PCB loop antenna where the axis of the loop was parallel with the long dimension of the handset, that is, generally wrong! ... but the scheme worked quite well and there was no chance of whip antennas getting broken. Anyway, nobody cares much about antenna polarization: * Well, some still do! Chris http://picasaweb.google.com/lh/photo/HC8B4F-AnCQF6I_u0k3MYg http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn03.html -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
| Reply |
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Forum | |||
| Force 12 - C3S | Antenna | |||
| Air Force 1 | Shortwave | |||
| Air Force One | Shortwave | |||
| FS: Force 12 | Swap | |||
| Force 12 C-4 | Antenna | |||
Hybrid Mode
