Receiving EME Signals below the Visual Horizon
Rex Moncur VK7MO and Howard Ling G4CCH
This brief report analyses signals received by VK7MO from G4CCH on 11 September 2006, when the signal continued to be received almost 6 degrees below the elevation when the moon was visible.
The situation at VK7MO’s home location is that the setting moon is visually lost behind the metal roof of the house next door at around 9 degrees elevation. The top part of the dish would have visible sighting of the moon for a further degree.
The station parameters are a 5.4 meter dish at G4CCH with 400 watts to the feed, and a 2.3 meter dish at VK7MO.
The weak signal mode JT65 and WSJT Version 5.9.6 was used for these tests. Under best conditions signals can be decoded at 5 watts and thus there is almost 20 dB in reserve. It is assumed that this large reserve of almost 20 dB provides the opportunity to decode signals that in other circumstances could not be detected.
The following graph shows the received signal as the Moon dropped from 17 degrees to 3 degrees elevation at VK7MO.
At 17 degrees the signal increased in signal strength as the moon rose above the horizon at G4CCH. Interestingly signals were decodable at elevations of 4 degrees at G4CCH even though horizon prevented visual sighting of the moon until 7 degrees.
At 16 degrees the moon was visible at both ends, and G4CCH's signal rose to a peak of -12 dB. At this time a QRP QSO was attempted and power at G4CCH dropped from 400 watts to 5 watts.
When the moon had dropped to 11 degrees at VK7MO power was returned to 400 watts and signal levels increased to around -18 dB.
At 10 degrees the bottom of VK7MO’s dish would not have visual sighting of the moon and ground noise would be substantial. At 9 degrees elevation the centre of the dish lost visual sighting and at 8 degrees the top of the dish would loose visual sighting. While signals dropped off gradually as the moon’s elevation dropped they continued to be decoded well after visual sighting was lost for the full antenna and a decode was received at 3 degrees.
In a previous version of WSJT, Joe Taylor, made allowance for the radio refractive index in the atmosphere and this did suggest that at low elevations another degree beyond the visual horizon should be possible thus a weak signal down to 7 degrees can be explained by the combined effects of the top of the dish and radio refractive index. However it is more difficult to explain the reasons for signal being decoded, even though weakly down to 3 degrees. One explanation is that knife edge diffraction over the peak of the metal roof was producing a useable signal.
KNIFE EDGE DIFFRACTION
It is possible to estimate the loss due to knife edge diffraction using a calculator that has been provided on the Web by Green Bay Professional Packet Radio at:
This calculator gives the estimated loss due to knife edge diffraction as shown by the red dots on the same graph as above for comparison with observed signal strength.
The graph shows a very close correlation between the calculated loss due to knife edge diffraction and the observed loss and thus one can reasonably conclude that the signals are copied below the visual horizon due to knife edge diffraction.
In the above case the metal roof that causes the obstruction and allows knife edge diffraction was located 32 metres from the antenna at VK7MO. By applying the calculator to other situations it is found that the further the distance of the obstruction from the antenna the greater the rate of change of loss with elevation below the obstruction. Thus this phenomena of receiving EME signals well below the visual horizon is only significant where the obstruction is close (within a few tens of metres) to the antenna. The calculator shows that at higher frequencies the obstruction needs to be even closer but at lower frequencies it can be further away and still produce this effect.
In cases where an obstruction is relatively close (within a few tens of metres) to the antenna one should not rule out receiving EME signals up to several degrees below the visual horizon.
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