On 3/6/17 9:40 AM, Grant Saviers wrote:
Re 4sq receive vs yagi - which to listen to on 80 and 40.
For my site, it depends a bit on the yagi, heights, and arrival angles.
My 4sq receive, 70' per side, DX Eng, rarely beats the F/S of a 2L 80m
at 156' or the F/S & F/B of the 3L 40m at 140'. The 4sq usually beats
an 86' rotatable 80m dipole at 100'. The RDF numbers suggest this as well.
OTOH, a 40m 2L Moxon at 102' is more often (25%?) bettered by the 4sq.
On 160 it is no contest 4sq vs T vertical, 4 sq to the rescue. With very
high angle noise my 4sq generally does less well although overall it is
a great antenna, RDF seems as advertised.
As Jim(s) note, the best strategy is often to null the noise rather than
aiming azimuth for maximum gain.
there's a lot of interesting and very powerful approaches to nulling
noise in an array. It's not just about forming a "negative beam" in the
direction. You can use what's called Space Time Adaptive Processsing
(STAP) in the radar world and adaptively figure out what the interfering
signal is, and subtract it from the whole input array. So what you'd be
left with is your desired signal(s) and things that are sort of "blob
like" in the spatial or frequency domains (e.g. lightning noise at a
distance, or broadband interference).
This kind of processing could very easily knock out slowly drifting
noise sources with underlying structure (like PWM battery chargers), far
better than some simple time domain impulse noise notcher or a tone
notcher. And it has almost no comparable implementation in the analog
domain - you can't get there without digital processing without a room
full of incredibly twitchy analog circuitry (look at papers by Widrow,
Griffiths, etc. from the 60s for analog examples)
Those kinds of interferers can be de-emphasized in the spatial
processing chain (where you *are* forming a null in that direction).
Then, there's just straight out adaptive beam forming on the desired
signals.
None of this is trivial, but neither is it mind bendingly complex.
Someone who's done MUSIC or ESPRIT, for instance, could probably whack
out an initial implementation in a week.
The key things to figure out for ham use would be
a) how quickly do the interfering signals change (so you can pick
appropriate adaptation rates for the cancellers)
b) how to make the implementation have constant time delay - the
Flexradio experience pointed up that a fixed delay isn't a huge problem
(after all, the signal you're listening to came from 100 milliseconds
away), but if the delay through the processing chain varies, it will be
impossible to copy CW at reasonable speeds.
c) how many receiver nodes works, and what should the arrangement be.
In general, you probably want your receiver nodes spread in a 2 D array
(although having some 3D-ness might be useful.. a node at the bottom of
your transmit radiator, and another at the top, 10-20 meters into the
air, might be useful. Or even a node at the top of a tower, or one at
each end of a Yagi, in combination with a bunch of ground mounted nodes.
But I see some sort of architecture like this for each node:
1) a active whip - there's a ton of these out there ranging from $20 - $150
2) a bandpass filter, lumped LC, passes the ham bands, but not others - $20
3) a HF receiver dongle - $25 (I don't know if the RTLSDR has the right
dynamic range, but it's easy to find out)
4) a small single board computer to drive the dongle and stream the data
over a long wire (or WLAN) back to the shack. (like a beaglebone green
or BBG wireless) - $30-40
5) power supply (PoE if you're running network cables would be
convenient. Solar power at each node might be better if you're running
wireless)
So call it $100/node
with $1000 in hardware plus a dedicated (inexpensive $500) PC to run the
signal processing - For $1500 all told, you'd probably have a receive
system that would outperform just about any sort of Yagi or pair of
Yagis, on receive.
You wouldn't necessarily get all the bells and whistles and integration
with the rest of the shack - not that it couldn't be done, but I'm
thinking more about the signal processing, with the goal of generating
an audio stream for listening, or feeding something like CW skimmer or
your digital modes program - But, again, looking at the Flexradio
experience 10 years ago, they spent a LOT of time and effort dealing
with creeping featurism and user experience and compatibility issues,
not on the core signal processing.
That is one thing.. a yagi on a rotator is a pretty darn intuitive user
experience. You turn it until the Signal to (Interference+Noise) ratio
is the best. One variable to optimize - azimuth, maybe 2 if you have a
stack and a switching network.
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