On Sunday, April 22, 2018 at 6:10:49 AM UTC-5, Frank wrote:
On Sun, 22 Apr 2018 03:56:22 -0400, George Cornelius wrote:
In article <pb4t82$nrk$5...@dont-email.me>, Frank <analo...@mail.com> 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.
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.
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 .
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.
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.
Done right, it should work just as well unplugged.
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.
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.
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
Wow, all of you guys are amazing. I don't understand most of what you guys are explaining, however, I have the same issue as Derek and am going to try to find the parts and instructions on what you are talking about doing. Thanks all of you! VickyH.
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