Message-ID: <20091206214443.uaqrb36tcwgogso8@webmail6.kuleuven.be>
Date: Sun, 06 Dec 2009 21:44:43 +0100
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
References: <1260128134.2455.23.camel@gerhard-desktop>
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Hello Gerhard,

my 2 penny worth:

You are the first one I hear complaining about a to low antenna resistance=
 ;-)
30 Ohm is a very good value at 137 kHz (I was strugling with 130-170 Ohm).
 From this 30 Ohm only a fraction is radiation resistance, 99% or more =20
is loss in the loading coil and ground. So the lower the better.
You mention a 2,2mH loading coil that represents a reactance of about =20
1900 Ohm at 137kHz. With a Q of 200 (what is very good) the loss in =20
the coil is just less than 10 Ohm, leaving a remarkable low 20 Ohm =20
groundloss.
Very promissing for 137kHz !
The best way to match the 30 Ohm to 50 Ohm is a 1 to 1,3 transformer =20
at the antenna feeding point.
Regarding the currents you measure: indeed, with 2 A into 30 Ohm from =20
a 50W TX you have a perpetuum mobile of the first kind, so something =20
is wrong.
Some possibilities:
- the meter itself picks up HF (the small coil inside), causing a far =20
to large deflection
- if small voltage are rectified the diode loss cannot be neglected, =20
certainly not with a Si diode:
1Veff RF will produce only 0.3 Vdc
The double DC voltage will be produced by 1.3Veff RF, not by 2Veff.

73, Rik  ON7YD - OR7T


Quoting Gerhard Hickl <gerhard.hickl@aon.at>:

> 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 ab=
t.
> 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?
>
> 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
>
>