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!)
