From: "Klaus von der Heide" <v.d.heide@on-line.de>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 19 Nov 2001 18:23:43 +0100
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed
Content-Transfer-Encoding: 8bit
Subject: Re: LF: Long-Baseline Phased Arrays on LF
In-reply-to: <5.1.0.14.0.20011119112735.00a76080@gemini.herts.ac.uk>
Message-ID: <E165s4B-0007KS-00@mail1.isys.net>
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>


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