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Re: LF: Question to Antenna Experts

To: "[email protected]" <[email protected]cksheep.org>
Subject: Re: LF: Question to Antenna Experts
From: Rik Strobbe <[email protected]>
Date: Sun, 06 Dec 2009 21:44:43 +0100
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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 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 1900 Ohm at 137kHz. With a Q of 200 (what is very good) the loss in the coil is just less than 10 Ohm, leaving a remarkable low 20 Ohm groundloss.
Very promissing for 137kHz !
The best way to match the 30 Ohm to 50 Ohm is a 1 to 1,3 transformer at the antenna feeding point. Regarding the currents you measure: indeed, with 2 A into 30 Ohm from a 50W TX you have a perpetuum mobile of the first kind, so something is wrong.
Some possibilities:
- the meter itself picks up HF (the small coil inside), causing a far to large deflection - if small voltage are rectified the diode loss cannot be neglected, 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 <[email protected]>:

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




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