8m backscatter?

In recent days, my QRP 8m FT8 has been received at odd distances. I have also received G8JNJ (247km). Now this could be tropo, aircraft reflection or MS. However, I am wondering if it is some kind of backscatter?  It is far too close for Es.

VHF/UHF FT8

 As recent activity sessions have shown, FT8 on 2m and 70cm can be very effective. Although many QSOs are due to tropo, I am convinced that some sort of scatter is often involved. 

This maybe aircraft scatter or troposcatter. With FT8, scatter is possible with far smaller stations. It would not surprise me if scatter was more likely as the frequency is increased. It might be very effective at 23cm or higher with FT8, although I have no evidence.

If I had 23cm FT8, I might try some tests.

UHF/VHF scatter

Recent QRP FT8 operations on 2m and 70cm have got me thinking. On the face of it, aircraft and tropo scatter might actually be better on 70cm than 2m. 

Certainly for aircraft scatter, the smaller wavelength at 70cm might mean better scattering. I am no expert and may well be wrong. By my argument 23cm might be better still!

Certainly, path losses would be higher on 70cm, but antenna gain at each end can be easier as you go up in frequency.

What are your views? 

2m scatter using QRP FT8

My recent article in RSGB RadCom postulates that FT8 is opening up scatter for QRP set ups.

This becomes possible at any time with really low ERPs. Unlike aircraft scatter, this seems possible whenever I go on suggesting it is troposcatter.  DX is possible without lifts!

My ERP is probably less than 5W. I use the FT817ND and a big-wheel omni. Ranges over 450km are possible every day, even with my set up! It has totally transformed 2m possibilities for me: no towers, no beams, QRP ..... and DX!  GI and EI are possible every day from JO02 (East Anglia) even with my QRP.

See https://sites.google.com/site/g3xbmqrp3/vuhf/scatter .

VHF scatter?

Stations spotting my
2.5W 2m FT8 to the
big-wheel omni today

The perceived wisdom is that long distance VHF scatter communications is due to aircraft. Time and again I have questioned this as I seem to get spotted at great distances at almost any time with 2.5W and the big-wheel omni antenna on 2m using FT8.

FT8 works with signals that are weaker than SSB and CW and I wonder if we are able to make use of some other scatter mode? Either that or there are so many planes in the air that the time of day doesn't matter. Somehow the latter doesn't seem likely as at the beginning of lockdown I could still get as many distant spots and QSOs on QRP 2m FT8.

It would be interesting to hear the views of others.

2m aircraft scatter?

As many will know, I frequently try 2m FT8.  What puzzles me is that even will 10W and the big-wheel omni antenna (3dB over a dipole maximum) I seem to get great ranges at any time.  

This could be aircraft scatter, but I am puzzled why this is there all the time. If it was just at certain times I would be more inclined to think this is aircraft scatter. Just maybe, there are enough aircraft in the sky to allow brief 15 second periods of FT8 to be reflected at any time.

Another possibility is that FT8, with its additional sensitivity, allows other scatter modes to be possible. Whatever, 2m FT8 seems to allow me to reach incredible distances every single day, even with my very simple antenna and low power. If I had a big beam and 400W, some amazing distances should be possible.

Optical forward scatter and frequency?

Perhaps some of my readers can help me - what optical frequency is likely to be best for forward scatter off air molecules and dust particles? My tests to date have been over non line-of sight paths up to about 9km at 481THz (red light), but IR should work in daylight with filtered PIN diodes on RX, but I don't know whether infra-red frequencies scatter more easily or worse than red light?  With red light, the RX can easily be de-sensitised by bright sunlight, and this should be (?) less of an issue with filtered IR detectors, I think.

As the atmosphere scatters blue light very well (giving it a blue colour) one might expect IR to scatter less well than shorter wavelengths like visible red or ultra-violet? See http://en.wikipedia.org/wiki/Atmosphere_of_Earth .

Successful 481THz NLOS test (8.63km)

8.63km non line-of-sight reception of 481THz optical beacon

Bernie G4HJW set his high power optical beacon running a 1.082kHz tone this evening pointing in the Burwell direction on a non line-of-sight path from his home QTH at Little Wilberham. He then came over to Burwell to compare his RX and my own from a test site on the edge of the village clear of the village lights. Distance was 8.63km.

Both on Bernie's RX kit and my own we got solid copy by ear of the beacon at a distance of around 8.63km NLOS using 100mm optics. Once found, the signal was solid.  I then set up the optics on the tripod and fed the signal into Spectran where the signal was 30dB over noise in a 0.17Hz bandwidth. Listen here for a recording (best played back using Spectran with 0.17Hz bandwidth settings)

G4HJW's beacon TX

There was hardly a cloud in the sky, so this was largely by scatter from the mist/dust in the air. Bernie says signals have been stronger on other nights. Best reception was with the RX aimed just above the horizon, maybe 5-10 degrees at most above.

This was very gratifying as it is the furtherest NLOS signal I have copied and it verified my RX kit is now working at good sensitivity.

When I got back home I did try to copy the beacon by ear out of the bedroom window but the tree cover and lights made copy not possible. However I will try again later in the week with Spectran running and narrow bandwidth.