With poor propagation recently on 40 m there has been a move lately to activating SOTA and WWFF parks on 80 m during the day. I have wondered how well this might work as I have assumed the sun during the day increases D layer absorption and blocks lower frequency signals, yet I have worked stations on 80 m during the day.
Decided to test 80 m propagation by measuring the signal level of a constant transmitter, the VK2WI CW practice beacon in Dural Sydney. This transmits 24 hours on a frequency of 3.699 Mhz and provides a handy beacon about 250 Km away.
A search of the net found a suitable S meter logging program, S-Meter Lite by Greg W8WWV. This is freeware. On installing it found my Kenwood radio isn't supported... A bit more googling found a workaround, using N4PYs Pegasus software to get the S meter signal, feed this into a virtual crossover modem, made with another program called VSP Manager, and fed the output of the virtual modem into S Meter Lite. A confusing mess of virtual com ports that I eventually got working... :(
After sorting out the software side set up S-Meter Lite to log to a csv file, and collected S meter readings on the Dural 80 m CW beacon from just before sunrise at 6:55 am, past sunset at 5:24 pm, stopping the program at 9pm local time. Opened the CSV file up in Excel and created a chart.
Here is the result. I've marked sunrise and sunset.
I used some attenuation to help scale the signal. Note the low noise floor, S1, just before sunrise. Initially the Dural CW beacon was low, but shot up around sunrise around S6 to S7. So did the background noise, up to S3-S4... The signal came in quite well, with some QSB, eventually fading away into the noise around 12:30 pm, and returning around 3 pm. The signal continued to build, peaking a bit after sunset at 5:24 pm, dropping down around 7pm to a steady S4 to S5.
So you can use 80 m during the daytime for SOTA/WWFF activations, just avoid the afternoon!...
The CW beacon is not the most steady of signals, as it transmits CW at varying rates, making it hard to get a steady signal on the S Meter. Might be better to use a broadcast shortwave AM signal close to the 80 m band as a beacon instead, something to investigate.