2m spectrum usage snapshot

A recent email on the club mail reflector asked how much activity there was on 2m. Well here’s the answer!

The image below is a slow-motion waterfall chart of the whole 2m band showing the full 24 hours of Tuesday 18 October 2022. It was captured using Gqrx SDR software (running on a Raspberry Pi 400) and a reasonably well made RTL dongle fed from a Comet GP-1 2m/70cm vertical.

Remember that the earliest time on the waterfall is at the bottom, so the end of the day is at the top of the waterfall.

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Some comments and caveats:

The antenna was vertically polarised, so the waterfall will not show much SSB & CW activity (as that is mostly horizontally polarised).

The antenna was mounted approx 7m above ground level, on a bungalow in IO91QM locator square. Higher antennas may see more activity!

The waterfall is made up of pixels each representing a 5 min sampling period. It isn’t clear whether any activity in the 5 min period will light the pixel or whether it has to last for the full 5 minutes for the pixel to light.

The waterfall resolution almost certainly means that there is some interpolation vertically. So some brief transmissions may not be visible.

The lighter the pixel the stronger the signal. Red is strongest & just about visible from GB3AL, APRS & one local FM transmission.

The wider the line the wider the signal bandwidth. I appears that some of the repeaters may be bringing carriers up with no modulation.

Horizontal lines and bands are wideband noise.

You can see I have a noise source producing wideband noise in the early hours of the day – roughly 0200-0700. Noise then picks up again from around 1800.

Vertical lines are either RF transmissions, interference or tuner artefacts (sprogs).

The thin vertical lines are either interference or tuner artefacts.

Although interference can drift in frequency that seems to be mostly absent from this waterfall.

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