The 1296 beacon is s3 here … but at s3 it is perfect copy and the signal is peaking 40 dB over my noise level. Lest you think all I do around here is sit and listen to the beacon — I can explain. I am battling 1296 mast mounted preamp issues. I built both of mine from DEMI kits — just the board. I mounted them in boxes I built out of scraps of this and that. They each measure less than .5 dB noise figure and right at 17 dB gain. My noise floor is a calibrated -144 dBm with the SDR narrowed down. The issue is that after the preamps have been on for a while, they have a tendency to break into self oscillation mode. I believe heat must be a factor because they work ok when I first turn them on in the morning and after the sun gets up a bit, the problem occurs. . What I have been doing all day is testing preamps and trying to find a solution to the problem. I finally stumbled on to something that works. I won’t call it a solution. One preamp is up at the antenna as a standard, mast mounted unit would be. I took the spare preamp and inserted it at the end of the transmission line, just where the jumper from the radio connects. I now have a total of 34 dB of preamp. The amplifiers seem to like this. They don’t oscillate! Perhaps I need to add an internal 50 ohm load to the output of the preamps and that will stabilize them. I’m still thinking that one over. Ps — the feed line up the tower is good — and it is 1 5/8” hardline. And, I just replaced the feed line from the feed point on the antenna to where the preamp is inserted. I also put a new piece of Buryflex from the preamp down to the loop around the rotator. I will watch it for a few day and see how it looks. Standby for regular beacon reports :-) 73, Dave

Hi Dave... If I understand correctly, your first configuration had the two preamps cascaded in close proximity at the top of the tower. If so then your relocation experiment is perfectly ok. Your system noise figure and overall gain will not be seriously degraded by the new configuration since the first stage sets the NF and the net gain is unchanged. One possible cause of the oscillation is a potential impedance mismatch between the preamps, or simply leakage between the enclosures. But since the relocation has probably fixed it, all is good. From Tom Holmes, N8ZM > On Jul 20, 2017, at 10:43 PM, Dave Sublette via mvus-list <mvus-list@febo.com> wrote: > > The 1296 beacon is s3 here … but at s3 it is perfect copy and the signal is peaking 40 dB over my noise level. > > Lest you think all I do around here is sit and listen to the beacon — I can explain. I am battling 1296 mast mounted preamp issues. I built both of mine from DEMI kits — just the board. I mounted them in boxes I built out of scraps of this and that. They each measure less than .5 dB noise figure and right at 17 dB gain. > > My noise floor is a calibrated -144 dBm with the SDR narrowed down. > > The issue is that after the preamps have been on for a while, they have a tendency to break into self oscillation mode. I believe heat must be a factor because they work ok when I first turn them on in the morning and after the sun gets up a bit, the problem occurs. . What I have been doing all day is testing preamps and trying to find a solution to the problem. I finally stumbled on to something that works. I won’t call it a solution. One preamp is up at the antenna as a standard, mast mounted unit would be. I took the spare preamp and inserted it at the end of the transmission line, just where the jumper from the radio connects. I now have a total of 34 dB of preamp. The amplifiers seem to like this. They don’t oscillate! > > Perhaps I need to add an internal 50 ohm load to the output of the preamps and that will stabilize them. I’m still thinking that one over. > > Ps — the feed line up the tower is good — and it is 1 5/8” hardline. And, I just replaced the feed line from the feed point on the antenna to where the preamp is inserted. I also put a new piece of Buryflex from the preamp down to the loop around the rotator. > > I will watch it for a few day and see how it looks. Standby for regular beacon reports :-) > > 73, > > Dave > > > > > _______________________________________________ > mvus-list mailing list > mvus-list@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/mvus-list