Pedersen Ray Images

Pedersen Rays:

A series of images accessible from the links below were recorded from a 2 milliwatt 40 meter continuous wave transmitter located some 380 Kms south of me. The recording duration for most of the images was approximately 80 minutes (X axis). Frequency variations can be observed on the Y axis and show daytime Doppler shifting and multi-path signal effects which I believe were attributable to the Pedersen Ray phenomenon.

As the refractive ionospheric layers shift their positions relative to ground and to each other, those elevation changes result in variations of signal path length. Hence a readily observable Doppler shifting occurs over the signal path. The resultant doppler shifting is easily measurable as can seen in the images presented below. The effect is most noticeable over short, high angle signal paths. That is - for NVIS propagation. The images below were recorded from equipment having low dipole antennas at each end of the signal path, and using an FT-920 as the receiver. Audio frequency output from the receiver was viewed using Spectran software by I1PHD & IK2CZL. Spectran was configured with it's Y axis (frequency range) set to about 6 Hz.

This is a fertile experimentation area for the enterprising radio amateur. All that is required is a stable CW signal source feeding a low dipole antenna at one end of the path, and a similar antenna connected to a stable receiver at the distant end of the signal path. The receiver's audio output is best isolated from ground prior to entry into the PC's sound card. I use an old 600:600 ohm modem transformer and shielded cabling for that purpose. Spectrum Analyser software such Spectran or Spectrum Lab may be used to view the resultant (AF) signals on your PC's screen. Set up your receiver in USB mode to provide say a 1000 Hz beat note with the received signal. Then adjust your Spectrum Analyser software to be centred on 1000 Hz, and arrange a narrow spectral window of around 10 Hz in which to view the received signal frequency variations. Commercial transmissions also provide convenient signal sources from which to record Pedersen Rays.

(NB - CW mode, LSB, and / or narrow filters may also be used, but the setup may become somewhat more involved than when employing the method described above.)