Hi Mike, Bernd, LF,
Thank you very much for your transmission yesterday in
the evening! I could do very useful tests and improve my little rx-antenna
significantly.
I'm not sure if anyone who uses a short E-field active
antenna as a rx antenna knows about the dependence of wire length and the
height above ground so I want to give a report of the results of my tests
yesterday in the dark. Perhaps some others could improve their antenna by
these explanations?
(First I tried to
sent the pictures into the mail but the mail size became more than 100kB and
so the mail wasn?t reflected, as it seems. So I do modify the mail cancelling
the Pictures. I put them on my page at qrz.com and those who are interested in
the results of my improvements can find them there (at the lower end of the
site)?)
For the first test I tried the active antenna with a
wire of 1,4m. The lower end of the wire was abt 2m above gnd (observed signal
of G3XDV)(Argo without AGC and RX with fast AGC). There was almost nothing to
see. Then, I reduced the wire length to 80cm while the hight above gnd keeping
constant. Results were much better as can be seen.
Next: 40cm wire length: even much better!
Next: 30cm: signal gets lower but qrm also. SNR
slightly better.
Next: 25cm is the best, as I think.
Next: 20cm Signal becomes worse. So, the optimum seems
to be at around 25?30cm!
Next: 20cm in a height of 4m: Signal comes up but
noise also. No significant increase of SNR
The signals of Ossi/OE5ODL were audible in all the
tests and vy gd to cpi. But the signal was too strong to see any differences
between the S/N ratios (a Picture is also available).
My Conclusion: It seems that a short receiving antenna
can bee seen as a capacitive divider out of the capacity between far field and
antenna and the capacity between antenna and ground. If the wire length is to
much, the input stage becomes nonlinear/goes into saturation. If the height
above ground is increased the signal comes up, but not the signal/noise ratio(surely there will be
a benefit if such an antenna is placed in a region with heavy local qrm. Then,
the height should be increased and the wire length can be decreased). So, one cannot say ?the more the better?
talking about the wire length!
It?s exciting, we can receive our ?QRP?-Signals
(compared to HF) over a distance of 100s or 1000s of km with a wire that is
1/10000 Lambda! In comparison, in
the 80m band that would be an antenna of 8mm (!)
;-)
With this improvement I get new hope for receiving
anything out of the city, where my home QTH is?
I hope this report isn?t nerving because of the long
text (and
pictures) and size. I
try to stay always below 100kB. Perhaps some RXs can be improved or Lowfers
gets motivated to try such an antenna?
73, Stefan/DK7FC
PS: Mike, what's your locator? I want to check the
distance and take a view to your QTH (on http://f6fvy.free.fr/qthLocator/fullScreen.php
everyone can type the searched QTH-Locator and watch the QTH of the received
stn). So one can see the distance and the wave travelling path and if the stn
in directly at the beach or in the mountains and so on. Vy
fine!
PPS: I forward this message to Bernd, the constructor
of the preamp. I think he doesn?t know that the antenna gets even better when
reducing the wire length! (tnx Bernd!)
