Hello Peter, Clemens, Jim, LF,
Before a few days i have bought a new copper tube with 18mm diameter
and 1mm thickness. I formed a loop with 1m diameter. This time i put 2
copper plates into the tube and hammered it at this point where it was
fixed later on a non conducting material block. Then i directly
soldered the caps on this plate which was quite easy done with a big
soldering iron. I need slightly more C, somewhat above 500 nF. Actually
the caps are the same types, so the problem must (mainly) have been the
short wires between copper tube and caps in the last version.
A photo is here: http://dl.dropbox.com/u/19882028/LF/LF%20RX%20Loop.JPG
This time i'm achieving a Q of 176 in a first attempt although i
want to use even more suitable caps later. In this measurement the
center frequency was 137.72 kHz, the 3 dB points are at 137.31 and
138.09 kHz which sounds very good to me. Later i want to add a switch
with some additional caps to make QSY to the CW region on LF which
should be no problem
This design is really robost and thus most suitable for a /p rig! It
even works indoors , in contrast to the E field antenna.
As Jim i found no better NPN Transistors like the ZTX690B for this
loop preamp and they are available at RS so i will exactly build this
amp and am very optimistic about the results!
One more question: Does it make sense (regarding good SNR) to further
try to improve the Q or does it just make frequency adjustment
complicated? If the dynamic range of my soundcard can handle the signal
of DCF39 and DCF49 and DLF and i should have no problem, right? The
background noise should be limited by the band noise only of course. Is
there a straight dependency of gain and Q?
Thanks for further opinions and advice.
73, Stefan/DK7FC
Am 17.06.2011 15:47, schrieb pws:
Hi Stefan,
You wrote:
...
I found the Q of the
loop is just 8! Is this normal? Which Q do others achieve, with a
similar antenna for 137 kHz?
...
Confirming your findings. In the late 90th I built a 2m-loop from 1"
coax ("Flexwell") for nulling out Loran-C splatter. I remember a
bandwidth of ~12-15 kHz => Q ~= 10 with 8.2uH/165nF.
http://www.qsl.net/df3lp/projects/lfloop/index.html
...
It is fascinating to null out s signal, which i have done the first
time now. I can reduce the signal of DCF39 by 25 dB by turning the
loop...
That's somewhat different to what could be found here. I observed very
sharp Nulls well >40dB below main lobes. May be due do the orthodox
"screened" design including the usual gap. But as you suggested that's only
true for pure groundwave reception. At VLF I'm observing the deepest null
of DHO38 (~200km; noon) at ~30deg. of elevation.
Good luck for tomorrow,
Peter, df3lp
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