On 7/7/18 11:46 PM, Jim Brown wrote:
On 7/7/2018 11:33 PM, Jim Brown wrote:
Another question about the presentation is the validity of the noise
data -- how recent is it?
One issue with the strategy in the nice presentation is that they are
taking the ITU noise and assuming it is uniformly present over the
hemisphere (when they're numerically integrating using the antenna pattern).
Sky noise isn't uniformly distributed, and RFI (whether from
thunderstorms or manmade sources) most certainly isn't.
I believe the ITU curves are made using data from a vertical monopole,
which, of course, has a null at the top, and a gain distribution vs
elevation that depends quite a lot on the soil properties within a few
wavelengths. I'll have to check, but there might be some datasets using
horizontal antennas, which at least minimizes the effect of the soil
properties.
Chris Coleman published a couple papers in the early 2000s looking at
the distribution of HF noise sources. The Australians are big into HF
Over The Horizon Radar, and as you can imagine, HF noise is a big deal
for them.
THE DIRECTIONAL ASPECT OF ATMOSPHERIC NOISE AND ITS IMPACT UPON HF
COMMUNICATIONS SYSTEMS
A direction-sensitive model of atmospheric noise and its application to
the analysis of HF receiving antennas Radio Science, v37, #3, 1031,
10.1029/2000RS002567, 2002
Here's the references to the first paper -
Kotaki,M. Global distribution of atmospheric radio noise derived from
thunderstorm activity, J.Atmos. Terr. Phy.,
vol.46, pp.867-877, 1984.
[3] Kotaki,M. and C.Katoh, J.Atmos. Terr.Phy., ‘The global distribution
of thunderstorm activity observed by
theIonosphere Satellite (ISS-b)’ Vol.45, pp.833-850, 1984.
[4] CCIR, World distribution and characteristics of atmospheric radio
noise data, Rep. 322, Int. Radio Consult.
Comm., Int. Telecom. Union, Geneva, 1964.
[5] Keller,C.M., HF noise environment models , Radio Science, Vol.26,
pp.981-995,1991.
[6] Coleman,C.J. The directionalty of atmospheric noise and its impact
upon an HF receiving system, Proceedings of the 8 International
Conference on HF Radio Systems and Techniques, IEE Conference
Publication no 474, 2000.
---
Ultimately, I think that once you've got a scheme to get the receiver
noise "well below" the sky noise (so the angular distribution of the sky
noise is immaterial) the next step is to look at "how effectively can I
suppress noise coming from undesired directions" and how quickly can you
adjust for propagation effects.
Given the ease of doing DSP to combine multiple antennas, the future in
HF receiving is going to be along those lines. The burning questions
will be how to implement it effectively:
1) do you have a broadband(ish) preamp (perhaps with band select
filters) and bring coax back to the shack where you digitize it
2) Do you put the digitizer out at the antenna, and bring the samples
back by network?
3) What should the digitizer look like? A Superhet or Direct
Conversion/Low IF front end followed by a low speed high resolution
sampler? Or a wide open sampler?
I'm not sure, also, whether you can get equivalent performance from
"ground mounted" antennas vs "antennas in the air" - That's where some
analysis of patterns helps - if your skywave signals come in at an
elevation angle where you can suppress sky and environmental noise, then
you're going to be better off.
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