In article , Frank writes:
On Tue, 17 Apr 2018 04:17:33 -0400, George Cornelius wrote:

Also, I wrote that you might need an antenna tuner for an external
antenna, but this just seems to clip on directly, so that's a good sign.
Otherwise I was going to guess that the receiver had inductive antenna
impedance compensation, varying by band, and building just a series
capacitor substitution box that went by decade from 50pf to 50nf might
compensate for that in order to work with an antenna of design impedance
of, say, 50-300 ohms.

Most portables have an untuned high impedance connection to the whip
antenna. Basically just coupled to the gate of a FET.

Unaware of that. Untuned input stages? I'm not sure I
have ever looked at the circuit diagram for any shortwave
receiver that was not tube based, so I'll accept that as
something quite possible.

I know FET's are marvelous for input stages. But beyond that
everything I say about the input to this receiver is based upon
pure wild-assed guess.

Please note, though, that the users' manual explains that the
internal loop antenna is involved, as well as the whip, below 7200
khz. So they (a) have a bit of band switching and (b) do most
likely have a tuned input stage, at least below 7200 .

A high input impedance input is good enough for a rod antenna. The
antenna is electrically short at SW frequencies and the ground half of
the antenna is whatever capacitive coupling the radio can get to the rest
of the world.

Yes, and you do tend to have capacitive coupling to the AC line,
which in turn, has all sorts of coupling to ground.

Anyway, I only know what theory says. I am told that typically one does
not bother with impedance matching for shortwave listening; but
theory is that for best benefit from an external antenna you want
a conjugate match. That means the resistive part of the impedance
should match the resistive part of the load, and any reactance at
the source should be balanced out - cancelled - by equal and opposite
reactance at the load.

Now I could easily understand a 10:1 energy loss (3:1 impedance
matching error) not being too much of an issue, but once you get
to 100:1 losses and worse, it would seem that some kind of antenna
tuning would be in order.

An antenna tuner would be helpful because the untuned input stage is
going to overload first on the strongest signal, which is likely a local
BCB station.

Yes, of course. You're really getting killed with an untuned input
stage if there are powerful sources nearby.

George