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R: Can anyone clarify dBic?




----- Original Message -----
From: Tony Langdon <tlangdon@atctraining.com.au>
To: 'Hamish Moffatt' <hamish@cloud.net.au>; <amsat-bb@AMSAT.Org>
Sent: Thursday, June 14, 2001 12:43 AM
Subject: RE: [amsat-bb] 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...
>

I cannot detect any difference even considering the case of  an electrical
AC two pole synchronous motor in wich the 50 Hz or 60 Hz power is feed
with 90 out of phase in to two identical coils mechanically  ortogonal one
to the other generating a rotating magnetic field discovered by Galileo
Ferraris  the property  of wich is let the rotor rotate at a speed of
RPM = (frequency x 60) / numbar of pole

If  i remove the power from one motor pole than the other one generates only
a magnetic field wich is linearly polarized and the motor stops to rotate.

The same occurs using two crossed linearly polarized source of light
separated by an odd numbars of wavelengths wich generate a rotating disc of
light

 So all of this are only different  case of rotating vectors  resulting
from linearly polarized components having the same amplitude but 90
out of phase rotating with the same physical principle,no matter on  how
the rotating field is a CP electromagnetic wave or a squirrel cage rotor
or a disc of light.

73 de i8CVS Domenico

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