The Format of Charts I Produce.
The charts show the signal strength as received. The receiver is set to CW or SSB mode to produce a heterodyne with the signal's carrier. Spectrum Laboratory (SpecLab) is set to export a text file to record the amplitude of this audio signal. The receiver is operated with no AGC, provided it is not overloaded the audio volume represents the actual signal strength received. SpecLab produces a waterfall, showing signal strength as a change in brightness or colour, see below. A chart shows this more clearly.
These charts are produced with Excel. Open Office or SpecLab's "Watch and Plot" may be used. "Watch and Plot" charts may be saved by a "Scheduled Action" eg. plot.capture("C:\SpecLab2\5045\5045Chart-"+str("DD-MM-YY",now)+".jpg") Look here for more information.
The top, purple line represents the signal strength of Macapa, the station being observed, gradually rising until it is at it's strongest just before the yellow line showing sunrise. Do not take this as a typical example, if you look at my other charts you will see great variations in propagation patterns.
I have complicated this chart by recording a second signal seen about 30Hz away from Macapa. The blue trace shows it has a similar sunset time and that it switches off at 22.40.
The black line at the bottom represents my local background noise level, giving an indication of the signal to noise ratio. Sometimes this may rise with interference and swamp the signal showing that the recording at that time is unreliable.
The green line represents the received frequency of the signal. It is adjusted so it fits into the chart with the dB scale giving an indication of variation in HZ. For example the above chart shows that at about 23.00 the frequency begins to rise about 7Hz, dropping 3Hz then gradually rising again. This is shown on the "waterfall", see below, the frequency scale here shows the actual received frequency before I subtracted about 3160Hz so it fitted the chart. Various effects affect the frequency recorded, a jump often indicates that there is a stronger signal on a slightly different frequency that is disturbing the recording. My illustration appears to be due to the transmitter varying, a slight break in transmission occurs immediately before. Receiver or propagation effects also cause variations. Sometimes frequency variations can help identify the station. Some show a zig-zag line, probably due to the crystal oven, this pattern can be confirmed by another SWL who can actually hear the station. ZL1BPU helped me confirm ZLXA by simultaneously noting frequency jumps of this station.
This screen capture shows Macapa between 3020-3030Hz and the other signal at about 3060Hz. These are easily separated into separate lines on SpecLabs's text export. The receiver is a Softrock, using SpecLab sampling at 8KHz to minimise CPU usage with the local oscillator approximately 3KHz below the signal . A conventional receiver using CW mode will produce around 800Hz +/- half the filter bandwidth, SSB can be used, the actual audio frequency is not critical, SpecLab export is just set to it.
Greyline Index Page Links to more information of how observations are made and some of my current recordings.