[Milsurplus] SCR-183/-283 4-5 MC Coil on 75 MTRs

[David Stinson]

4-5 MC coil sets for SCR-183/283 transmitters
(BC-230/-430) are very common, while those that 
cover 80/75 Mtrs directly can be hard to find.
The OSC section of the 4-5 MC coil set will operate
down to around 3750 KC, but the PA/Antenna coil
is pretty much done at 4 MC.
http://home.netcom.com/~arc5/SCR-183/coilplate.JPG

You can get the 4-5 MC coil operational down into
75 meters with a few simple, non-destructive changes.
Move the Antenna coil tap up to the second turn from 
the top.  Pad the coil from the tap down to the bottom
using either a 50-100 pFd transmitting doorknob
(Thank you, Mike!)  or big, RF-rated silver mica.  
Don't use a ceramic; too lossy.
http://home.netcom.com/~arc5/SCR-183/coilpad.JPG
(yes, yes I know it's sloppy.  I was prototyping, guys!)

Assuming you use a 100 pFd RF-rated or air variable 
cap in series with the output to feed a 50-ohm load,
you'll find the tap for best output around here:
http://home.netcom.com/~arc5/SCR-183/coiltap.jpg

You will get about 10-20% less power out than 
you would with the "correct" 3-4 MC coil, 
but then most of us don't have that one ;-)  
A working rig in exchange for 10% is a good deal.

73 DE Dave AB5S