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Old 8th Apr 2020, 1:00 pm   #15
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 2,056
Default Re: 6 metre 4 pole filter

If it helps, a typical 50MHz BPF with 4 resonators and about 5% bandwidth should have an insertion loss of maybe 2dB - 4dB depending on the quality of the components. There are several BPF types that can be used for a 5% BW and I would normally choose one that has the steepest rejection on the same side as the LO and image.

The best place for this is just after the preamp as per Tim's impressive design. If you have RF switching here, the same BPF can be used for both Rx and Tx.

It's more than 30 years since I operated on 6m but I doubt there is much advantage in having a system noise figure of much lower than about 6dB. This would include feeder loss in the overall system. It might be different for diehard DXers in a rural location with a beam but for most operators the noise levels on the band will be dominated by manmade and galactic noise.

With this in mind it might be possible to use a basic preamp made with a U310 JFET in grounded gate and this will be easier to manage in terms of stability at the expense of a slightly higher noise figure. Otherwise, some care will be needed with a dual gate mosfet as Tim warns. They can go unstable very easily and the BF981 can generate negative resistance at the drain up past 2GHz if care is not taken with the design.

I think TR10 should have a PNP symbol on Tim's schematic but it's a bit fuzzy in places so it might already be OK. It looks like an NPN symbol to me.
Regards, Jeremy G0HZU
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