On Sun, 22 Apr 2018 03:56:22 -0400, George Cornelius wrote:
+AD4 In article +ADw-pb4t82+ACQ-nrk+ACQ-5+AEA-dont-email.me+AD4, Frank +ADw-analogdial+AEA-mail.com+AD4
+AD4 writes:
+AD4APg On Tue, 17 Apr 2018 04:17:33 -0400, George Cornelius wrote:
+AD4
+AD4APgA+ Also, I wrote that you might need an antenna tuner for an external
+AD4APgA+ antenna, but this just seems to clip on directly, so that's a good
+AD4APgA+ sign.
+AD4APgA+ Otherwise I was going to guess that the receiver had inductive antenna
+AD4APgA+ impedance compensation, varying by band, and building just a series
+AD4APgA+ capacitor substitution box that went by decade from 50pf to 50nf might
+AD4APgA+ compensate for that in order to work with an antenna of design
+AD4APgA+ impedance of, say, 50-300 ohms.
+AD4
+AD4APg Most portables have an untuned high impedance connection to the whip
+AD4APg antenna. Basically just coupled to the gate of a FET.
+AD4
+AD4 Unaware of that. Untuned input stages? I'm not sure I have ever looked
+AD4 at the circuit diagram for any shortwave receiver that was not tube
+AD4 based, so I'll accept that as something quite possible.
The way it's typically done on digital tune radios is the 1st IF is
typically on the order of 50 Mhz. The internal oscillator need to cover
from 50 MHz to 20 MHz for the radio to receive from 0 Hz to 30 MHz. They
don't really go down to 0 Hz, but that's the principle.
The rub is that this sort of arrangement requires a VERY stable
oscillator. Pretty much impossible with a LC oscillator.
The images will all be above the IF frequency and can be easily filtered
with a low pass filter.
+AD4
+AD4 I know FET's are marvelous for input stages. But beyond that everything
+AD4 I say about the input to this receiver is based upon pure wild-assed
+AD4 guess.
+AD4
+AD4 Please note, though, that the users' manual explains that the internal
+AD4 loop antenna is involved, as well as the whip, below 7200 khz. So they
+AD4 (a) have a bit of band switching and (b) do most likely have a tuned
+AD4 input stage, at least below 7200 .
The internal loop doesn't have to be tuned. I'm pretty sure the internal
loop for my DX 440 isn't tuned. It feeds into it's own amplifier.
+AD4APg A high input impedance input is good enough for a rod antenna. The
+AD4APg antenna is electrically short at SW frequencies and the ground half of
+AD4APg the antenna is whatever capacitive coupling the radio can get to the
+AD4APg rest of the world.
+AD4
+AD4 Yes, and you do tend to have capacitive coupling to the AC line,
+AD4 which in turn, has all sorts of coupling to ground.
Done right, it should work just as well unplugged.
+AD4
+AD4 Anyway, I only know what theory says. I am told that typically one does
+AD4 not bother with impedance matching for shortwave listening+ADs but theory
+AD4 is that for best benefit from an external antenna you want a conjugate
+AD4 match. That means the resistive part of the impedance should match the
+AD4 resistive part of the load, and any reactance at the source should be
+AD4 balanced out - cancelled - by equal and opposite reactance at the load.
+AD4
+AD4 Now I could easily understand a 10:1 energy loss (3:1 impedance matching
+AD4 error) not being too much of an issue, but once you get to 100:1 losses
+AD4 and worse, it would seem that some kind of antenna tuning would be in
+AD4 order.
My DX 440 has a big drop off in sensitivity when it automatically
switches from the internal loop to the external whip at something like
1620 kHz. The whip works well enough at normal SW reception frequencies.
Now that I think of it, the DX 375 might be using the internal loop also
as an impedance matching device for the high impedance whip.
+AD4
+AD4APg An antenna tuner would be helpful because the untuned input stage is
+AD4APg going to overload first on the strongest signal, which is likely a
+AD4APg local BCB station.
+AD4
+AD4 Yes, of course. You're really getting killed with an untuned input
+AD4 stage if there are powerful sources nearby.
+AD4
+AD4 George