Would anybody be interested in exploring signal path reciprocity for shortwave communication? Or rather changes in reciprocity over time. It sometimes happens that for a period of time the path to a distant station seems to be more or less unidirectional. And I’m not talking about different noise levels but actual signal level.
The ionised plasma together with the Earth’s magnetic field will twist and deflect radio waves travelling through it making the path more or less non reciprocal.
I’ve been using WSPR to measure the reciprocity between SM and North America and it indicates that there at times could be a difference of up to 10 dBs. But there is a problem using WSPR. The output is not signal strength but SNR. On higher bands the QRN level is fairly constant over time, if not beaming into the sun, but there could be local noise or sudden QRM from say RTTY stations. At my end I’ve solved that by listening to a local station some tenths of km away and using that signal as a reference (the difference over a 24 hour period was very little). The DX station though did not use that feature.
The means of measuring the reciprocity is open to discussion.

I’m using an ICOM IC-735 and a Drake L7 linear amplifier.

For portable use and to find local QRM there is also a Yaesu FT-817.

Sometimes I experience a lack of reciprocity for the two directions of a connection. As an example it has happened in contacts with Japan at 17 m. During a period of about an hour, I have received several S-units better reports than I could give. Later, conditions have been leveled and more reasonable reports exchanged. Is it true or imaginary? And then meant “S-meter deflection” and not SNR, Signal to Noise Ratio.

To investigate the phenomenon I started a series of measurements using WSPR. An explanation of WSPR are in another post, but broadly it’s about that a station transmits its identity at different intervals and in between listening for other stations. The received stations are automatically logged with an indication of the SNR.

Actually, we want to have the absolute signal strength and not the SNR. However, I have found that the background noise in my location does not vary so much throughout the day on the higher bands from 20 m upwards. If one makes the bold assumption the same is true for most other stations, WSPR can be used to examine the reciprocity of the transmission path.

Typical back ground noise over 24 hours:

The measurements were performed during the end of the year 2013 and beginning of 2014.

I couldn’t demonstrate any systematic difference in propagation. However there were short periods of non reciprocity.

Below is an example of a series of measurements towards Japan on the 17 m band:

Towards the US on 20 m:

Others have looked at this phenomenon earlier.

Tests have been carried out by Glen Davis Falcon vid Georgia Institute of Technology November, I960 och1956 av Laver, F.J.M. ; Stanesby, H. vid ”Post Office IngeneeringDepartment” UK:

” Tests have been carried out under carefully controlled conditions to see whether the attenuation of high-frequency signals sent over a given long-distance radio path differs according to the direction of transmission. The results obtained both across the North Atlantic and between Australia and the United Kingdom show that at times the loss in both directions is substantially the same, and that at other times the loss difference can rise to values of the order of 5 or 10 dB”.

I’m located in Huddinge 10km South of Stockholm City centre. It’s a very rocky and hilly terrain. Our house is on the South East end of a small hill stretching SE-NW with a plateau 150x20m wide 30m above the surroundings. Best direction is just south of east along an old sea bay, now filled with clay but stretching all the way out to the Baltic sea.

Scandinavia:

The Greater Stockholm Area:

A close up:

House and tower: