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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.

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.

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).

> 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.


Hamish
-- 
Hamish Moffatt VK3SB <hamish@debian.org> <hamish@cloud.net.au>
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