[TowerTalk] Re; Need Help Remembering Something I Read

[jimlux]

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.