Well I was getting very weird results yesterday.......
........ but today I now understand them better!
I am used to using high frequency that is in this case > 1 Ghz kit! I am not however used to building it! Well I have learned a new lesson for me! Here am I trying to build a low pass filer for around 150 Mhz, I didn't think I needed to learn construction for >1 GHz, Wrong!
The filters I was building were exactly as planned and I was using double sided board with the lower side being a pure ground plane. However there behavior above 3-400 Mhz was wrong, and there behaviour at 1.2Ghz was just silly (2db attenuation when I expeced over 40db down!).
What I had not thought was that my Top layer ground fill was contiguous but greatly narrowed in places by the other components. I had made a series of resonators working in the Gigahertz ranges. I had built tuned slots and all sorts of things that were just not as they should be.
These boards are self designed and self fabricated. I had forgotten to put any vias between the top and bottom layers except at the ends where the SMA sockets connected the sides to each other.
Pick up drill, drill a number of via holes in sensible places. Insert copper wire in vias solder both ends trim off excess wire and re test.
Now my board is behaving with all below -40db above the 300Mhz level right up to 2.7Ghz. Most below 65db down and I could probably get the hump at 1.4G down if I added some more vias.
Lesson learned I may think I'm playing with 2M radio, that doesn't stop my boards playing up above 1Ghz. Grounding is critical watch how well things are grounded, and use vias!
--
Peter
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