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Date: Tue, 20 Nov 2001 11:13:53 +0000
To: rsgb_lf_group@blacksheep.org
From: "Walter Blanchard" <blanch@pncl.co.uk>
Subject: LF: Phased rx
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I asked what this synchronous reception proposal was about because 
diversity reception and synchronous reception are two completely  different 
techniques and it seemed to me they were being confused.

Jim M0BMU said a lot of what I was going to say so won't repeat it again. 
But he missed out the very important point that no matter what you do at 
the Tx end the received sigs will be at random phase because they'll be 
skywave with varying and variable path lengths. So, prima facie, you cannot 
do synchronous reception at long range.  If you try a simple addition at 
base frequency (136/73) and they're 180 degs out of phase they'll cancel, 
not add, just like the QRN.  At a fixed site the phase coherency of skywave 
LF signals around 100-130 kHz only persists for a few minutes and even then 
only if it happens that one skywave mode is predominating. Averaging over 
an hour would produce nothing.

As for getting "beams", if you add together a lot of antennas on a long 
baseline (tens of kilometres) whether for Tx or Rx you will get what 
superficially look like multiple narrow "beams" with very deep (40-50 dB) 
nulls between them.  The longer the baseline or the more antennas you use 
the more "beams" and nulls you will get. In fact, an interferometer. The 
problem is that the "gain" in any one beam is miniscule, perhaps 0.5 
dB.  Example - three antennas on a 24 km base would produce something like 
16 "beams" and nulls.  You could steer these nulls by phasing the feeds to 
the antennas but who's going to go to all that trouble for virtually no 
result?  This technique has been used in the past for navigation aids - the 
four-course radiorange; the Consol system, the German "Wullenweber" but the 
object in all of them was to produce nulls and beams to give bearings, not 
to get antenna gain. Together with all the problems of maintaining absolute 
phase stability at the transmitters as well as at the receivers this is 
really a non-starter. No off-the-shelf ham gear is capable of this sort of 
performance even with allegedly "high-stability" options.

Diversity might have more going for it but you'd have to space the antennas 
by many kms, maybe even hundreds of kilometres.  Closely spaced antennas as 
suggested by 3KEV do nothing at these frequencies; they've got to be 
several wavelengths apart (one wavelength at 136 kHz = 2.2 kms) . However, 
if you want to experiment this is the way to go, as (I think it was) KK7A 
suggested.  Set up several Rxs 50-100 kms apart and combine the detected 
results in a summing program somebody would have to write, I don't know of 
any that would do this job right now.  Transfer the data by an MB7LF-like 
relay to get it in real time; internet far too slow for this job (and 
totally incoherent).

Incidentally the Loran timing somebody suggested would not work because the 
accuracy of Loran timing depends critically on maintaining ground-wave 
3rd-cycle selection which in turn means maintaining pulse shape via 
wide-band selection at the fundamental 100 kHz.  The Loran energy around 
136 is totally insufficient.  Peter Martinez G3PLX once synthesized a Loran 
pulse using only 136 kHz energy and it was 8 uS out.  Anyway what most 
people are seeing is skywave. You need a specialised Loran timing receiver 
but if you really need this sort of timng then buy a good GPS set instead - 
it'll be far better.  Fortunately, you don't for diversity.

Walter G3JKV.