Sunday, October 13, 2019

Some surprising results with LP filters from Ultimate 3S

Nine low-pass filters were granted temporary reprieve from duty in the Ultimate 3S today to do brief time as test subjects in my NanoVNA.

In seven of them, the results were predictable and acceptable. The other two - for 20m and 60m - showed poor performance for their respective bands.

In addition to the need to investigate why these two bands are poor, I gleaned another tidbit of useful info: that I don't really need a filter for each band. For example, the 30m filter would perform well with 7 MHz selected as the band of operation...minimal insertion loss and excellent suppression of frequencies above 10 MHz.

Similarly, the 80m filter performs perfectly well for 160m operation. In almost all cases, a lower filter could be used for a higher band with no penalty for this substitution.

Here are the results of all nine filters:













80m (cursor in wrong spot - about 0.6dB attenuation as WSPR freq)






rich said...

John, thanks for the post. Now I can upgrade the firmware and check out my WSPR filters as well.

Rich, WD3C

Sverre Holm said...

Hi John. I've had the same experience when I built my second 20m filter. I had to remove a turn or two in one or more coils, but unfortunately I didn't make any notes. I'm using my U3S on all bands from 3.5-30 MHz with five filters also: 3.5, 7, 14, 21, 28 MHz filters using the 21 MHz filter both for 18 and 21 MHz and so on. Your measurements seem to confirm that this is fine.

John AE5X said...

Rich & Sverre - thanks for the comments. I think Hans G0UPL has a link somewhere on his website with an online formula for designing LP filters - the one he used to come up with the design of those for his kits. I'm tempted to tinker with the variables in it t find which coil(s) would be the most likely ones to adjust for my 20 and 60 meter filters but until that inspiration arrives, it's easy enough to use the 15m filter on 20m and the 40m filter on 60m.

If I'd had a VNA back when I bought the U3S I think I never would have bought all those filters in the first place. Completely unnecessary!

73 - John