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= 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?
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
OE3GHB Antenna1.jpg
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