Tuesday, March 22, 2011

WSPR Beacon update

Now that I have liberated GPIO pins for the RTC clock, I have incorporated it into my breadboard design.  I have the code to the point where it will begin transmitting on the next even minute.  I now need to implement the TX percent control, however I am uncertain what it means.  Obviously, TX percent set to 100% will transmit every 2 minute window and 0% will not transmit at all (Idle).  However, what does 50% mean?  Does it mean I will transmit every other 2 minute window?  Don't think so, but not sure...  I did see one online reference that 20% means transmitting every 10 minutes or so.  This came from the WSPR 2.0 user manual.  Here is that text:

WSPR uses two-minute time slots for transmitting and receiving. The slider labeled
Tx fraction sets the average proportion of time allocated for transmitting. The default setting of 20% is a good compromise under typical conditions: it means that you will transmit approximately once every ten minutes and receive the rest of the time. The exact T/R sequence will be randomized so as to maximize your chances of receiving other WSPR stations. For receive-only operation, set the Tx fraction slider to zero
My brain refuses to understand this for some reason...

I think what is intended here is the following:

transmit time = (1/transmit percent) * 2 min

If I round this up to the nearest integer, the following transmit times result:

100% - every 2 minutes
90% - every 4 minutes
80% - every 4 minutes
70% - every 4 minutes
60% - every 4 minutes
50% - every 4 minutes
40% - every 6 minutes
30% - every 8 minutes
20% - every 10 minutes
10% - every 20 minutes
0% - never transmit

I am not sure this is precisely what Joe Taylor intended, but I think it will be adaquate for the beacon project.  I will further randomize the precise amount of time between transmissions up to the calculated value to better match what Joe Taylor suggests for minimizing collisions between transmitting stations.