Spectrograms

As well as monitoring the bands by ear, I have also been trying a couple of different Spectrogram programmes, another benefit of today's soundcard software! My first interest here has been in watching meteor scatter signals, especially on some of the European Band 1 TV transmitters. Their high ERP makes for very good MS reflections, and of course, other modes of propagation show up too.

The images on this page have been derived from Easygram, by Petr Maly (OK1FIG) and Gram6 by Richard Horne. See links to the left for more info. Gram8 is now available, but I haven't yet tried that. Both programmes produce the goods, the only real differences to my eyes were the ergonomics. In Easygram, scan parameters can be saved, and once you have set up your favourite 'mode' it can be recalled from a menu, which is the "Easy" part, I guess. On the other hand, I preferred Gram for tweaking and twiddling with. What I found particularly nice with Gram is that it keeps working whilst I'm also using the soundcard to record with CoolEdit.

The vertical axis is time (beginning at bottom), and the horizontal axis is frequency. The red traces are markers at 500 and 700Hz, and the timescale from bottom to top is 2 minutes.

As so often happens, two carriers are visible, offset by about 100Hz, and the MS reflections are different on each.

Short pings appear as horizontal streaks across the main signal representing Doppler shift, whilst longer bursts spread out in time, and generally show higher signal levels. Red areas are approx. 30dB above noise. On the longer bursts, Doppler effects can cause the carrier to split into two Doppler-shifted components, as happens here. Note also the thin diagonal trace caused by aircraft scatter.

 

This is a similar scan to the one above, but taken at 55.250MHz when there was much less meteor activity. The various signals trailing around the screen are Doppler-shifting aircraft scatter. Mt QTH is close to East Midlands airport, and lies under its southern flight path, so aircraft scatter is a very common occurence here.

Some of the traces look "chopped", and presumably this is the rapid fading caused by interference between signals over varying path-lengths. This TV transmitter, which I believe is in Germany, is always audible by one mode or another, either scatter or tropo. Just goes to show what you can do with enough ERP.

 

This horizontal trace clearly shows the pronounced frequency-spreading observed on auroral signals. Note the cw can be read easily from the amplitude trace at the top, yet the waterfall just looks like a mass of noise with no discernible centre frequency.

Taken with Gram6, whereas the first two images came from Easygram.