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.
|
