Diurnal LF-Propagation

For this study I chose five transmitters that are 24 h per day on air, supply a stable signal and transmit at different frequencies in the low frequency range, see table 1:

Transmitter

Power

Frequency

Location

Distance

Direction

DHO38

1.2 MW

23.4 kHz

53° 4’ 55’’ N
7° 36’ 59’’ E

185 km

301°

MSF

50 kW

60 kHz

54° 55’ N
3° 15’ W

923 km

294°

HBG

25 kW

75 kHz

46° 24’ 30’’ N
6° 15’ 10’’ E

705 km

204°

DCF77

50 kW

77.5 kHz

50° 0’ 56’’ N
9° 0’ 39’’ E

259 km

196°

DCF39

50 kW

139 kHz

52° 17’ 10’’ N
11° 54’ 29’’ E

132 km

88°

Location of the receiver is Algermissen, northern Germany (52° 15’ 9’’ N 9° 58’ 43’’ E).

Monitoring was performed for two days: 03.01.2011, 20:30 to 05.01.2011, 20:00 with antenna DX500 (RF-Systems). This results in two 24 h datasets. Time is given as local time. Sunrise was at 8:29 h and sunset at 16:20 h. Solar activity shows no significant irregularities during the monitoring period, see table 2 (data from www.solen.info/solar):

Date

Solar sun spot number (NOAA)

Solar flux (NOAA)

Planetary A-index

03.01.2011

51

92.1

4

04.01.2011

54

90.6

4

05.01.2011

50

87.7

2

The software AR5000RA was used to set the five vfos A - E of the AR5000 receiver according to table 3 (see below). The feature “Monitor” > “Observe up to five distinct frequencies” was chosen to perform the monitoring.

VFO

Frequency

Mode

IF-Bandwidth

AGC

HF-Attenuation

A

60 kHz

CW

3 kHz

slow

0 dB

B

74.5 kHz

CW

3 kHz

slow

0 dB

C

77,5 kHz

CW

3 kHz

slow

0 dB

D

139 kHz

CW

3 kHz

slow

0 dB

E

23.4 kHz

CW

3 kHz

slow

0 dB

Integration time: 2s, Sampling rate: 20s

For each moment the mean is calculated from the two 24h-datasets of recorded signals from the transmitters DHO38 (magenta), MSF (blue) and HBG (red) and plotted against time, see Fig. 1:

Plot: Diurnal course of radio signals

In Fig. 1 the yellow bar indicates the time span between sunrise und sunset. As can be seen, the change between day and night has a major effect on the received signal strength. According to Barin Kumar De, S.K. Sarka and Pinaki Pal (Characteristic Features of 40 kHz LF Signal Amplitude) a sunrise minimum (A), recovery effect (B), afternoon maximum (C) and sunset minimum (D) can be detected. The sunrise minimum (A) is observed before sunrise at the receiver location occurs. This is because the sunlight reaches the atmosphere in 70 - 90 km height earlier (and can build up the ionosperic D-Layer) than the surface of the earth beneath. Also the sunrise minimum of the signal of HBG takes place earlier than of the signal of MSF. This is due to the location of the both transmitter. HBG is situated about 9° eastward and 8° southward to MSF.

The diurnal variability in signal strength of the transmitters DCF77 (squares) and DCF39 (triangles) is different from the above results. The recorded signals of both days are plotted together against time, see Fig. 2:

Plot diurnal course of radio signals

In Fig. 2 the yellow bar indicates the time span between sunrise and sunset. As can be seen, the DCF39-signal (upper scatter line) shows no significant dependence on time of day. This changes only a little with the DCF77-signal (lower scatter line). During nighttime the signal indicates a stronger variability than during daytime. There seems no interaction of the arriving transmitted signals with the ionosphere. So they must arrive as unaffected ground waves.