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*Subject*: RE: [amsat-bb] Can anyone clarify dBic?*From*: Tony Langdon <tlangdon@xxxxxxxxxxxxxxxxxx>*Date*: Thu, 14 Jun 2001 08:43:23 +1000

> On Wed, Jun 13, 2001 at 10:57:34AM +1000, Tony Langdon wrote: > > antenna is linearly polarised and is in the exact same > plane as one of the > > transmitting Yagis, we can deduce the folloring: > > > > 1. The receiving antenna will be excited to the maximum > extent by the field > > of the Tx antenna in the same plane. > > I don't think this is correct. If circular polarisation is > done correctly, > you don't have a pair of antennas transmitting one each in > the horizontal > and vertical planes. You have a single effective antenna producing a > circular wavefront. Yes, but I was using a special case to illustrate a point. It doesn't matter how the CP wave is generated, it can still be treated as equivalent to the sum of two linearly polarised waves that are 90 degrees out of phase with each other and aligned at rightr angles. It just happens that it's also possible to synthesise the circularly polarised signal with 2 linearly polarised waves with the proper (as discussed above) relationship. Even if the signal is generated by a helis, it still (IN A MATHEMATICAL SENSE) can be considered to be composed of two linearly polarised components at right angles to each other and 90 degrees out of phase. Again, the fact that a CP signal can be synthesized by the summation of two orthogonal waves 90 degrees out of phase is proof of the analysis, not necessarily saying it _has_ to be done that way. If it looks like am duck, walks like a duck, quacks like a duck... > Since the transmitted signal is not in a single plane you > cannot say that > the Tx antenna is in the same plane. The transmitted signal > is continuously > rotating from 0 to 2pi (0 to 360 degrees). A linear polarised receive > antenna will receive some signal from this source at all but > two angles > of the rotating transmission. Again, break it up into the two components, one in the same plane as the linearly polarised antenna, one at right angles. Of course this is arbitrary (the two planes could be at any angle, as long as they are orthogonal), but it simplifies any analysis. > For example, if you have a horizontally polarised receive > antenna, then > your antenna will receive something except when the transmitted signal > is vertical (+/- 90 degrees, or pi/2 radians). When the transmitted > signal is horizontal (0 or 180 degrees, pi), you receive all > of the signal. > In between, you receive something according to cos or sin(angle). Yes, and if you do the sums, I bet you get exactly the same results as I obtained by splitting the CP signal into 2 linear components (again, the maths works out). > > 4. Since it doesn't matter how a CP wave is generated, the > same holds true > > for any CP to linear circuit. > > This doesn't follow with the rest of your argument. A helical antenna > is not composed of antennas in two separate planes. It does hold true. CP is CP, regardless of how it's generated, and CP can be treated as a summation of two LP waves (as discussed above). Can you tell me that you can detect the difference between CP generated by a helix and CP generated by crossed Yagis in the far field? For your argument above to hold true, you have to be able to detect a difference... If anyone has the maths, do the sums... ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

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