On 6/9/21 11:25 AM, Zach Metzinger wrote:
You could also build both diplexers and band-pass filters yourself, usually for far cheaper, depending on your transmit power. A NanoVNA comes in handy to tune these homebrew creations. However, it all comes down to money vs. time. hihi
Since Herb got me thinking and searching, here's another website about filter designs:
https://wiki.batc.org.uk/Filters
I really like the G3SEK article on inter-digital filters, much like some of the filters that W1GHZ has on his website.
A while back, I related an overload problem I was having on a MAX2640-based 70cm LNA. A water tower with an impressive array of LTE (and other) antennas around its mid-section blasts out energy in the 600-800 MHz range, only a mile from our house.
I solved this with a lumped (discrete) filter on the front-end that had only a few tenths of a dB loss in the band of interest, and then followed the first stage with a 435 MHz SAW filter.
This design has RF relays that engage for RX when powered by DC up the coax, as convention for these devices, but I had to add a PIN diode at the output of the 2nd amplifier to keep the transmitter from destroying the MAR-3 device. A proper sequencer, perhaps using the ALC to keep the transmitter from going to full power for a few milliseconds, might have also solved the problem.
Definitely not the prettiest thing around, but it works:
http://www.n0zgo.net/lna/70cm/max2640/70cm_lna_max2640_annotated.jpg
Keep building!
--- Zach N0ZGO