Diurnal Shortwave-Propagation

This examination gives a brief insight into diurnal ionospheric dynamics at shortwave. The reflecting capacity of the ionosphere can be observed by radio waves from distant transmitters. The transmitter should be on air 24 h a day mostly. It must not vary its transmitting power in an unknown manner. Also antenna direction and beam pattern should be constant. International broadcasting stations do not fulfill this requirements.

For my experiment I choose the NCDXF radio beacon system. Each station of this worldwide net is every three minutes at a defined frequency for 10 seconds on air. The stations transmit cw at 100 W, reducing output stepwise to 10 W, 1 W and 0.1 W respectively. These beacons produce only small signals with my receiving setup that have no measurable influence on the receiver's agc. So I use the audio output for examination of the signal. The weak signal mode of the program AR5000RA (“Monitor” > “Observe weak signals”) in combination with the audio software Audacity v. 2 was employed to record the signals.

Receiving site is Algermissen, northern Germany (52° 15’ 9’’ N 9° 58’ 43’’ E). I decided to make measurements of 180 seconds duration every 30 min at 14100 kHz. Recording was performed from 06.05.2012, 20.30 hours to 07.05.2012, 20.03 hours local time. Antenna is the DX500 manufactured by RF-Systems. For solar activity see table 1 (taken from www.solen.info/solar)


Solar sun spot number (NOAA)

Solar flux

Planetary K-Indices






1 M-Class





2 M-Class

The settings within AR5000RA were: CW-Mode, IF-Bandwidth 3 kHz, Volume 30 units and within Audacity: 11025 kHz sampling rate, Sound activated recording On, Sound activation level -50 dB (AR5000RA mutes the audio out of the measuring periods).

The monitoring results in an audiofile of 48 x 180 seconds = 144 minutes duration. It is an ease to split up this file in 48 single audiofiles using Audacity's command "regular interval labels" followed by the command "Export Multiple". The 48 audiofiles were fed to the program Spectrogram from Richard Horne running in batch mode. The result are 48 graphical representations (waterfall diagrams) of the audiofiles in the frequency domain.

Picture: Spectrogram of different beacon signal strengths

For the purpose of this examination a coarse differentiation of signal strength is adequate. Four steps are defined by visually rating the waterfall diagram on base of the one-second dashes of the beacon signal. The dashes are sent with 100 W, 10 W, 1 W and at least with 0.1 W. The first three dashes are used to rate the signal strength as shown in the picture to the right.

This rating system is applied to the signals of the russian beacon RR9O (position within file: 70 - 80 s) and the finnish beacon OH2B (position within file: 130 - 140 s) on all waterfall diagrams. The results are plotted as bars on a 24 h time scale together with their sliding average (see reception diagrams below).

A prediction of ionospheric conditions for the path between this two stations and the receiving location was achieved from the website VOACAP Online from Jari Perkiömäki (OH6BG), James Watson (HZ1JW) and Juho Juopperi (OH8GLV). These plots are shown below the corresponding reception-diagrams:

Plot: received signal strength during 24h from beacon RR90 Plot: Prediction of signal strength for beacon RR90 Plot: received signal strength during 24h from beacon OH2B Plot: Prediction of signal strength for beacon OH2B

The sliding average (black line) of the received diurnal signals of RR9O and OH2B corresponds in its course well with the prediction. Looking at the individual signals (bars) their strength varies vastly from one half hour to the next indicating remarkable ionospheric perturbation along the path of propagation. So shortwave listening remains an exciting challenge.