This PCB Module is shipped Built & Tested, tuned at 520 KHZ.
The low-pass section is a 9-pole filter and its output goes to an end SMA for a soundcard or other SDR or to an analog receiver.
The high-pass section is 9-pole and its output is terminated with two 100 Ohm resistors in parallel to make 50 Ohm.
As is, an external antenna can be connected to the diplexer input, the low-pass output provides a cleaned VLF/LF spectrum while the signals at higher than 520 KHZ get terminated on the load resistor. A high-performance receiver can be made as is used in our VLF/LF upconverter.
It can be used the other way, just the load resistors must be removed and a load should terminate the low-pass port.
The input is 50 Ohm and protected by a Gas Plasma Discharge Tube at 75 V.
Another higher Z input can be made by adding a 16:1 or other transformer at the empty T1 footprint.