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receive polarity
On Feb 15, 6:02*pm, Jeff Liebermann wrote:
On Mon, 15 Feb 2010 08:01:18 -0800 (PST), Art Unwin wrote: I have not seen the like printed any where soto me *it is good stuff. This might offer a clue as to how such antennas are built: http://ieeexplore.ieee.org/iel3/4812/13333/00608613.pdf?arnumber=608613 http://www.springerlink.com/content/g215405815642611/ Plenty more under IEEE Ants and Props search. *Check if your local library or college library has a subscription: http://www.ieeeaps.org http://ieeeaps.org/aps_trans/ When I model a polarization independent antenna the individual gains confuse me as each of the individual gains are some what 3 db down from the "total" gain. In other words "total" is not the addition of all the polarizations gains. If you use a circularly polarized antenna, and feed it a linearly polarized signal (either vertical or horizontal) you'll see a -3dB polarization mismatch loss. http://www.antenna-theory.com/basics/antennapol.php I find it very difficult to get my mind wrapped around that fact. On the surface it would suggest that competition types would benefit from a polarization independent antenna. Nope. *According to my friends that do contesting, the major requirement of an antenna is NOT to maximize the gain in all directions. *It's to reduce the gain to the side and back, where all the other interfering stations are usually located. *Directionality is important or all you're going to hear are other local hams. *A truely isotropic antenna is fairly useless for contesting. *(Note: *I don't do contesting). -- Jeff Liebermann * * 150 Felker St #D * *http://www.LearnByDestroying.com Santa Cruz CA 95060http://802.11junk.com Skype: JeffLiebermann * * AE6KS * *831-336-2558 To be honest Jeff the antennas I design are based on starting with a full wavelength radiators which I presume you are already aware of. Initially I was basing efficiency on all forces being accounted for with a higher gain resulting. In fact this aproach to design provides diversity of polarizations instead of linear thus linear seamed to supply top gains. The full wave length aproach gives the option of dual polarity or even all forms. The penalty is usually in the 1db range where as the multiple polarity may drop down a bit on gain but makes use of signals that a linear design cannot hear as well as zero side lobes and good front to rear figures. So without knowing what polarizations one has to deal with a reasonable choice is hard to come by. On top of these questions one has to look t what "gain" really represents since cross polarization can be reduced to just noise with the rest of the db gain value representing quality signal. Thus it is difficult to quantify gain when the real advantage comes about on weak signals that others cannot hear. In other words gain itself is not important unless it is a measure of discernabilitity or quality above noise or none matching polarities. Two antenna designs come to mind 1 is the two element array that can supply 2 polarities, horizontal and one direction circular and 2 the helical that can accept all that is thrown at it with a prime gain around 13 db and 10 db for the others. Thus if polarizations are random with weather fluctuations in city or wooded area it would seam reasonable to discard linear forms in favour of helicals. To sum up, all the above has placed me on a zero level as to what antenna efficiency really means which to the reader must now be obvious, as one does not know what variables should be weighted and by how much., |
#2
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receive polarity
On Feb 15, 6:55*pm, Art Unwin wrote:
I was basing efficiency on all forces being accounted for with a higher gain resulting. Whether or not you account for all forces will not have any effect on gain. :/ This is another case of the hopeful free lunch.. But the cupboard was bare. The radiation from the vertical and horizontal polarizations added together will never end up being more than you started with. And an antenna that is a mix of both will be a compromise if the other antenna is purely one way or the other assuming no reflections, etc skewing the path. No free lunch.. |
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