From: vickyhoward@sbcglobal.net   
      
   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 , 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.   
   > 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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