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Date: Mon, 7 Dec 2009 10:24:13 +0100
Message-ID: <38A51B74B884D74083D7950AD0DD85E828AB57@File-Server-HST.hst.e-technik.tu-darmstadt.de>
In-Reply-To: <1260128134.2455.23.camel@gerhard-desktop>
Thread-Topic: Question to Antenna Experts
Thread-Index: Acp2rAuoamzvRayKSrSal3Ap/cD0MwAceUdQ
From: =?iso-8859-1?Q?Stefan_Sch=E4fer?= <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
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Subject: LF: AW: Question to Antenna Experts
Sender: owner-rsgb_lf_group@blacksheep.org
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Reply-To: rsgb_lf_group@blacksheep.org


Hallo Gerhard, ich nochmal...
Wegen der Anpassung auf 30 Ohm:
Es gibt mehrere M=F6glichkeiten, ich nenne mal 3:
-Eine Anpassung mit einem dicken Ferritkern (z.B. Schaltung als Spartrafo)
-Anpassung durch eine Anzapfung an der Ladespule: Wenn die Spule unten an=
 Erde angeschlossen ist und oben der Draht und die Antenne resonant ist (Z=
=3DR), dann findest du bei einer bestimmten Windung einen Punkt, bei dem=
 U/I 50 Ohm betr=E4gt. Das ermittelt man am besten experimentell.
-Du kannst eine zweite Spule mit weniger Windungen bauen und an diese dein=
en TX anschlie=DFen. Dann stellst du die Spule in die Ladespule und koppel=
st die Leistung induktiv ein. Vorteil: Galvanische Trennung, also keine Ve=
rbindung zwischen Sender und LF-Erde.
Ich habe schon alle drei Varianten ausprobiert und es funktioniert einwand=
frei.
Die einfachste M=F6glichkeit ist nat=FCrlich, wenn du einen Kreiswiderstan=
d von 50 Ohm hast, den Sender direkt unten an die Spule und die Abschirmun=
g des Koaxkabels auf die LF-Erde legen. Dann musst du aber wegen statische=
n Aufladungen aufpassen, da die LF-Antenne nicht geerdet ist (kann man mit=
 einem 10k Widerstand gegen Erde vermeiden)...
Gru=DF,=20
Stefan
-----Urspr=FCngliche Nachricht-----
Von: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacks=
heep.org] Im Auftrag von Gerhard Hickl
Gesendet: Sonntag, 6. Dezember 2009 20:36
An: rsgb_lf_group@blacksheep.org
Betreff: LF: Question to Antenna Experts

Hello folks!

As some of you may have noticed, I became active on LF. My TX makes
about 50W at 13,8V and the Antenna is a Marconi with multiple topload
wires.

Basically it consists of two 12m Fiber poles in a distance of abt. 50m
(North to South). After about 20m from the feeding point, a 20m wire is
connected in a nearly right angle. The end of this wire is in about 10m
height to a tree. From the 2nd pole, a wire of 25m leads back to the
roof of the house in an acute angle. End is also abt. 10m above ground.
The end of both wires are at least 10m away from the tree and the roof
to avoid losses due high voltage.

The wire diameter is 3mm in the vertical section (11m) and 0,8mm for the
first 20m, where the right angled wire diverges. From there all other
wires are 0,6mm. The cable to the shack is RG58 abt. 25m long.

The feeding point is behind the house and the loading coil is about
2,2mH made with insulated 3mm copper-wire wound on a plastic flower-pot
of abt. 45cm diameter. It is placed on the roof of the a little shed.

I use "Mini-VNA" to measure this antenna and I added a picture of todays
measurement. As you can see, the SWR is abt. 1,8 at best because Z=3D abt.
30.

I built a current transformer to measure the antenna current and
calibrated it with the dummy. I have 140V p-p on the 50R and this is
should produce a current of 1A. I used a Iron-Powder core, primary one
turn, secondary is 14 turns and it shows a voltage of 1V after
rectifying. Now, that I have it outside on the feeding point, it shows
2V which would means 2A....Thant seems a little bit high to me. Where
does this error come from? I want to optimize the antenna regarding
Impedance because it's only abt. 30R instead of 50R. Where should I
start? More wires in the ground? Does this reduce Impedance or not? More
topload? Thicker wires?=20

You see, a lot of questions from somebody who is new to LF.

Thank everybody who will take the time an read this long mail in
advance.

Best regards es 73

OE3GHB

Gerhard