n6gn's blog

I reported in the Forum that we have been seeing a very large and interesting difference between two very nearly identical paths on 2m WSPR. After about a week of 2m we have shifted to 70cm to compare. Perhaps tellingly, we also consistently see a rather large difference though this time only 10 dB but it's in the opposite direction. WW6D who with the same ERP and almost identical path had been 24 dB weaker is 10 dB stronger on 70cm.

Recently I tried an experiment, I went mobile on 70cm WSPR. I used an Icom IC706mkIIg that I have in the car along with a rubidium 10 MHz frequency reference and tripler to replace the Icom's 30 MHz master oscillator. I ran WSJT-X on an old laptop. The antenna I used was a dual-band vertically polarized whip so it was cross polarized to the signals from the N6GN home station beacon running WSPR, JT65A, JT9 and CW on ~432.301500.

See the "General" forum page or this Pointer for a summary and sample waterfall/wav file of the June/July 2m WSPR activity across the Pacific. Work is progressing on a 70cm WSPR/JT9/JT65 addition to the existing KH6HME beacon on the Big Island as well.
Glenn n6gn

When K6PZB and I first started up on 2m and 70 cm WSPR, we immediately saw interesting "Doppler" components in the waterfall displays. Sometimes, but not usually, these were such that signals wouldn't decode due to the QRM from the aircraft-scattered signal (ACS). We also saw some strange double-sided signals which we couldn't identify.

Now that a larger group has been on 2m WSPR in California for a couple of months, we have had a chance to both examine ACS better and also to study examples of these double-sided signals. I am becoming of the opinion that these are due to wingtip vortices being generated by large commercial aircraft. These pressure waves are well known and can be a problem for later aircraft. http://en.wikipedia.org/wiki/Wingtip_vortices

Because they are a cause of significant drag to powerful aircraft, a lot of energy can go into producing them and they can persist for many seconds or even several minutes after an aircraft passes by.
It is my suspicion that the large pressure differentials produced by these "curlicue pumps" may be sufficient to cause a sufficiently large and intense change in the dielectric constant of air such that significant 2m energy can be scattered from them. Since they are both (counter) rotating and moving it seems plausible that they may be presenting the two-sided Doppler components visible in the attached SignaLab shot of the strong 2m WSPR signal from K6PZB, located only 8 km to the west. The precise mechanism of the scatter, the sources and other details isn't clear. At this point, I'm only speculating. It does almost look like the aircraft is generating rapidly circulating volumes of air that can even criss-cross at times and go from net-pathlength-increasing to net-pathlength-decreasing vectors. This certainly could happen when oppositely moving WTVs cross the line between the 2m WSPR stations. There are also other possibilities.

K6PZB and I have been running WSPR on 6M, 2M and 432.3 MHz lately. To do this we both operate with external OCXO master oscillators on our IC-706 WSPR radios.
Our stations are about 5 miles apart but K6PZB is located on the wrong side of a hill on the western end of this E-W path. This makes it particularly interesting
for examining Doppler from aircraft flying in our region.