Hi!

Is there a reflection in the feeder from mis-match?

I am not sure how I would know. There is a matching transformer way up
at the top of the antenna that goes from its connections to the
coaxial cable. This in turn feeds into the input of the amplifier.

Coming out of the signal amp is another length of coaxial cable, this
one 25 feet long. This goes to a grounding block and then on into the
building with another twenty five feet or so of coax, where it
connects to the "power" unit.

I'm not sure if the grounding block is in the "right" place but it is
connected and properly grounded. I don't know that it would make a
difference. I tried temporarily removing it and noticed no change.

The "power" unit has a built in splitter. A stereo receiver is
connected to one end with approximately six feet of coaxial cable. The
other output goes to an HD Radio tuner. Both have 75 ohm connections.

Has anyone previously tried to use the cable to feed two sets
and installed a hidden junction box somewhere along the line?

Definitely no. I ran all of the cable for this, so there is nothing in
it that I would not know about.

*Is the cable connected in the correct way to the units, with the
screen properly clamped?

I believe I've done this correctly, by following the printed
instructions.

*Is the cable of the correct characteristic impedance for the
pre-amplifiers (50-ohm or 75-ohm)?

The instructions that came with the amplifier don't talk about this in
any great detail. The obvious thing to do (to me at least) was to use
75 ohm cable. I would have to think that if some special cable were
required, the instructions would talk of it.

William