26 Jun 2014

G0OQK - 6m WSPR GDX via planes

G0OQK (98km) has just spotted me on 6m WSPR with a large 4Hz Doppler, suggesting reflection/scatter from aircraft somewhere along the path. I am surprised that the software decoded with this much Doppler. My signal was not strong at  -28dB S/N.

I hope G0OQK comes on TX again soon as I'd like to see his trace on my software to see if he can be decoded by pure tropo. It seems to me that planes have to be around for the path to "work" and even then the Doppler has to be low enough, ideally much lower than 4Hz. There was a successful decode of G0OQK on 6m a few days ago with 0Hz Doppler but I think that was an aircraft crossing our path and not pure tropo. I'd like to think the path between us can be worked without the help of planes.

UPDATE 1750z:   Apart from a single spot of me by G0OQK, all recent 6m WSPR spots have been by local G4IKZ (18km).  I may go back to 10m.

UPDATE 1840z:  Returned to 10m but it is very quiet with no Es, no GDX and no locals being copied on WSPR. Earlier JT65 attempts on 10m (called CQ) were no good. I had success with JT9-1 on 20m.

UPDATE 2112z:  10m "woke up" briefly here at 2012z with an Es opening to OE3ODW.  Just a couple of 2 minute WSPR slots and he was gone.

As G0LRD has pointed out, drift on 6m (and higher) is as likely to be TX drift as aircraft reflection. Looking for multiple lines in any 2 minute slot is the only sure way to know if there was Doppler, and this only works with several planes with differing Doppler shifts. What we can never know is if a single line, drifting maybe, is TX drift on a direct (tropo) path or aircraft reflection.


David (G0LRD) said...

Hi Roger,
Our recent discussion about aircraft reflection has led me to run some more calculations. I used your two 0Hz decodes from G0OQK as the source for the parameters. What I found was that less than 1Hz Doppler over 2 minutes would be possible in one specific circumstance: the aircraft would need to be travelling more-or-less along the baseline between the TX and RX, be somewhere near the middle of that path, the distance would need to be considerable (98km just about qualifies) and the aircraft would need to be at high (cruising) altitude. This geometry keeps the path length change rate down to the snails-pace required(200mm per second max).

So, I think you're right, and thanks for making me think more about it. I love these sort of technical debates!

However, I wouldn't rely on the drift figure too much in order to classify a decode as reflection. On 6m I have often seen transmitter drift by similar amounts.

Cheers! David

Roger G3XBM said...

Hi David,
Of course you are quite right about drift on 6m (even worse on 2m).

I need to get my head around the Doppler ALONG the (mid path) baseline.

Like you I have enjoyed thinking about all this but I feel you have (more than) the edge on the technicalities, HI. Thanks for the technical debate. All the best.

Nick Garrod said...

Hi Roger,
My TX has been on all of the time. I have measured my output power and it is 3W into the feeder. I think I can see partial traces of your transmission, but only for a few seconds in each transmission.
I also have my ultimate 3 TX back on the air on 10M, having updated to the latest firmware but so far only DK6UG, LB9YE, SM6WZI have spotted me. Back in the spring I was getting worldwide reports.

I used to live not far from you in St Ives, but that was back in the mid 80's when I worked in Cambridge and my call was G6JHK.
regards Nick G0OQK

Roger G3XBM said...

Thanks Nick. What is the duty cycle of the 6m TX? Want to understand if I am just seeing your 2 min transmissions when planes are in the right place or all the time by tropo. Probably can't recheck until Sat morning as rig and antennas off (storms).