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!)
