Hi Chuck, I wanted to commend your reply in particular
The difference a good balun makes can be profound - as much as 20~30dB reduction in noise
It sure can and it's a thrill to achieve such an improvement cheaply and easily.
The word "balun" can cause confusion so I thought the simplest thing to do was show-n-tell with a cheap and easy example I use:
• the toroids (Australia source) are cheap and easily obtainable from Jaycar, they have excellent HF performance even below 1MHz, eg.https://www.jaycar.com.au/l8-25x15x10mm ... 4/p/LO1234
• the diodes (Australia source) eg. 1 amp 1000 volt high speed 1N4007 or similarhttps://www.jaycar.com.au/1n4007-1a-100 ... 4/p/ZR1007
• the wire is salvaged from an old CAT5 ethernet tail
The two diodes are across the coax, this clips voltage spikes as a protection for the SPR, I've not noticed any cross-mod effects.
The coax not only connects to the two diodes but to the two wires on the left of the left hand toroid.
The left toroid is a common mode choke, helping get rid of unwanted RF on the coax braid.
The output of that toroid is connected to the two red wires, being the input of the right hand toroid.
The right toroid is a galvanically isolated transformer, upping the impedance to something better than 50 ohm for the HI-Z input.
The two yellow output wires of the right toroid go to the RSP's Hi-Z connector via the black and white wires.
Given many people are using a non-resonant random antenna over a wide range of frequencies with a random length of coax rather than a tuned antenna on a ham band, the actual impedance at the end of the coax could be anything, so it's all non-critical except for the benefit of the galvanic isolation between the two windings.
The really important part for anyone new to radio and wanting to lower their noise floor is to read as much as they can about common mode RF interference and common mode chokes for radio hams.