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Message-ID: <001b01ca736f$1f3324d0$0900a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <38A51B74B884D74083D7950AD0DD85E828AAF4@File-Server-HST.hst.e-technik.tu-darmstadt.de>
Date: Wed, 2 Dec 2009 16:47:09 -0000
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Subject: LF: Re: Results of optimising an active antenna
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Hi Stephan, well if you look at Roelof 'PA0RDTs "MiniWhip" you will find the
"whip" is  about 4 square cms area of copper pcb material. The thing you
muct be careful about is where you put your "ground" reference. The usual
method is to run a wire vertically down to ground brlow the "whip". The
volatge "collected" by the e-probe is proportional to its "height" so you
might say you are sampling two points on the incoming wave and the farther
apart you sample the bigger induced voltage you get. The difficult ting if
you are in a high noise area is to stop the coax feeder braid from piching
up the noise and injecting it into the whip "amplifier" (really and
impedance converter.)

For instance Laurence KL1X has had these on top of tower blocks, but has
found the best low noise ground reference is the frame of the balcony doors,
these presumably are connected to the reinforcing material in the structuse
and that seems to suppress noise generated inside the block rom the likes of
TV and lift motors. They dont work for everybody because it very much
depends on your noise source and how far away it is. There certainly is not
much advantage in having a long whip or wire on these antennas, but
increased height will improve the picked up signal strength. It may be with
a shorter wire there is less coupling to the feeder coax. If you can reject
the noise from there you could increase the height and get better S/N on
signals. Every location is different!!.

Good experiment....keep trying !! you will be amazed what can be a achieved
once you conquor the locally generated stuff.

Alan G3NYK



----- Original Message -----
From: "Stefan Schäfer" <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, December 02, 2009 4:01 PM
Subject: LF: Results of optimising an active antenna


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!)