After several contests, monitoring of the softrock skimmers has turned up a bit of a problem with using softrocks as the skimmer platform. Very strong signals are producing a mirror image that is often reported as a spot to the RBN. Certain to be annoying for the S&P packet crowd during a contest. Annoying enough that a few flame mails have arrived.
The volume of the bad spots is relatively low on the lower bands, and more common on the higher bands. 40m is somewhere in the middle, with most of the bad spots being sent for domestic USA stations.
The problem is a combination of the hardware and software, both contributing to the problem. A software fix could potentially be made to CW skimmer or to the RBN aggregator to correct for the problem. Will inquire to the authors…..
In the meantime the best solution available is to throttle the RBN aggregator to allow only spots below the center frequency to be reported. For example, the 15m skimmer is based on a softrock with a center frequency at approximately 21044.5Mc. So for the duration of the ARRL DX CW contest, an entry in the “Notched Frequencies” will be active to not report 21044.5-21100 to the RBN.
That solution does nothing to correct for half of the possible bad spots(i.e., a strong signal above the center frequency whose mirror image is being spotted below the softrock center frequency). But it should alleviate many/most of the actual bad spots, since most run stations prefer to operate as low in the band as they are able.
Open to other suggestions short of replacing the softrocks with better (yet unaffordable) hardware.
Update 20130217, 2140Z: There are new versions of both skimmer and aggregator. Perhaps upgrade will help.