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RE: Can anyone clarify dBic?



> 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...
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