Thanks,
Now that I've looked at other configurations, I agree the DC input is
likely 12VDC. I think I will keep an eye out for the injector at local
garage sales. Hmmm, I wonder what the polarity of the DC is on the
coax. Probably center conduct is +.

No, there is no separate DC input connection at the antenna. I was
thinking I'd just modify the amplifier to supply external power (not
from the coax). The antenna is in the attic so it won't see real
weather. I am assuming there is a choke between the center conductor of
the coax and the power supply in the amplifier that allows the DC from
the coax to pass but stops the RF. If I disconnect ths choke from the
coax, the local amplifier power will not back feed the coax and damage
anything on the receiving side. Then I can supply the DC between the
open end of the choke and ground.

I just noticed the cover of my R-S catalog. It says, "You've got
questions. We've got answers." So then why am I still asking questions?
;-)
Ted