Dear all,
nearly two years ago we had a discussion about Phased Arrays
and Diversity on this reflector.
Diversity is a method to choose the temporarily best signal
from receivers that are many wavelength apart from each other.
Especially when the antenna must be omnidirectional, this is
advantageous. The 10 m relay in Hamburg is a well known example.
Phased Arrays combine several antennas coherently. For an
acceptable antenna pattern the distances of the individual
antennas should be about a quarter of a wavelength (as in a Yagi).
The combination can be done in several ways:
1. Direct combination of the antenna feeds as in Yagi-groups
2. Combination after preamplification
3. Combination after conversion
The first way is not possible on LF because of the distances.
The third way needs coherent oscillators at all wide spread
receiver frontends or a sufficiently strong pilot signal
in the LF band that could be used later to make all converted
signals coherent. Since, in any way, the signals must be combined
from distances of some hundred meters to a few kilometers
the optimum choice are IR-links that can transmit the 136 kHz
bands directly to a central station (way 2.).
The combination at the central station can be done in two ways:
a. Delaying the individual signals appropriately to realize
one directional pattern, then adding them and receiving with
a conventional receiver.
b. Downconverting all signals in parallel using the same
oscillator (which in this case is no problem), digitize
the signals and implement delay patterns for all directions
you want in parallel by a DSP. This is equivalent to a
strongly directional antenna with high gain that simultaneously
looks in many directions.
73 de Klaus, DJ5HG
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