The small loft antenna for 160m has produced some impressive results tonight with spots from all over NW Europe of my QRP signal. Best was GM4SLV in Shetland at 5W and OZ7IT at 2W.
But the real reason for erecting the antenna was to allow QSOs on QRP AM over a 3km path to G6ALB so we could use topband for a talkback link when doing other tests. Now, at 9.30pm we went onto 1.973MHz AM to see how easy it would be. It wasn't easy at all! In fact it was very hard work copying even a few watts of AM. The killer is the noise floor that is around S9 both at Andrew's home and mine. On WSPR, with its much narrower bandwidth this is not such an issue, but on AM it ruins the band for what we'd intended. There is no way that 0.25W of AM would make it, certainly not easily with simple rigs as we'd planned. 10W would be OK, but that defeats the purpose. On 70cm AM (yes AM) we had armchair copy between us with about 1W. On 160m FM it was better copy until we dropped the power down and the signal was drowned by the QRN/QRM.
So, what will we do? There is now no point in continuing with the 160m AM transceivers as this isn't going to work at our noisy QTHs with QRP over 3km, sadly. The most probable choice will be 6m AM using my SixBox rig at my end.
The lower HF bands like 160m are radically different from 30 odd years ago when the noise floor was some 20-30dB lower.
I find that the modern house is enveloped in a cloud of RF noise and it's impossible to have an antenna indoors and expect to receive anything other than very strong signals, and on 160m this must be even more so. Could you mount the small antenna hidden in a bush out in the garden somewhere so it's still invisible but it might not be overwhelmed by the indoor noise?
ReplyDeleteWe could John, but at 3km we were expecting our QRP AM signals to be well above the noise floor. We were looking for a very simple AM link with a simple antenna. 160m now looks a poor choice for our application.
ReplyDeleteRoger - How about trying a rx-only mini-loop or ferrite rod antenna - as used by MW DXers - in the shack & orientate to improve rx S/N? (VK3XU has published designs.) Sounds like TX side is fine!! Enjoy the blog & ur expts. 73 Jules VK2EXT
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