I just decided to use MS paint to illustrate so here is the pics.
LNB is an aerial view of the LNB circuit board.
LNB2 is a little graphical representation of the circuit layout in the switch area.
LNB3 is highlighting the changes I made.
I added the diode and replaced one of the transistor switches. The other transistor switch that powers the
C band LNB was still good so I left it in place. The one that switches the KU LNB was bad so I replaced it with a TO92 style transistor that has a little more tolerance to abuse.
The transistor switches just switch handle the power to each LNB. They are isolated from the RF patch via radio frequency chokes. So short lead length is not a high priority. I left the leads a bit long so it would be easy to solder them in place.
The signal switching is handled by two RF switching diodes that are forward biased when it’s particular LNB is powered up.
You want to put the reverse protection diode into the circuit after the RF choke (not directly to the input terminal) so that it is isolated from the RF path, so the diode’s capacitance doesn’t shut the signal to ground.
On the LNB 2 picture I gave a graphical representation in of the reverse protection diode in the 7808 in the c band LNB so that it would be clear where the reverse current is shunted to in when the input swings negative. The KU LNB also has a regulator which has a reverse protection diode too. So there is two protection diodes but both are after the switch.
The reverse protection diode I put in basically mirrors the one in the 7808 voltage regulator. But it’s before the switch so the reverse current doesn’t have to go through the switch to get to it.