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Message-ID: <5.1.0.14.0.20040119140000.03714bf0@u0019445.kuleuven.be>
Date: Mon, 19 Jan 2004 14:57:53 +0100
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
In-reply-to: <C75A388845B4B54B9CF5E2ADB589B0E3031AEFDE@btss.siemens-pse. sk>
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Subject: Re: LF: Test failed...
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Hello Rich,

According to MMAMA the antenna capacitance should be something like 2.7nF, 
thus a loadingcoil of +/- 500uH would be needed.
I assume that at least one of the variometers covered this range.

It is a wild guess, but maybe the power is "DC grounded" using a coil from 
the antenna base to ground. If this coil is dimensioned for a much higher 
frequency (MF) it can cause rather strange effects on LF.
Eg. :
Assume the original use of the tower was a lambda/4 antenna at MF, for a 
135 m tower that would be +/- 550 kHz.
Antenna impedance (at 550kHz) would be +/- 36 Ohm, so "good engineering" 
would require a DC path to ground (for safety) with a Z of at least 10 
times the antenna impedance = 360 Ohm. At 550 kHz this would mean a +/- 100 
uH inductor.
So, instead of seeing the antenna as a 2.7nF capacitor to ground it should 
be seen as a L-C parallel circuit (100 uH // 2.7 nF).
Or in terms of reactances (at 136 kHz) : +j85 Ohm // -j433 Ohm = +j106 Ohm 
(= +/- 124 uH at 136 kHz).
To get the antenna tuned (brought to resonance) you would need a series 
capacitor (not inductor !) with a reactance of -j106 Ohm at 136 kHz = +/- 
11 nF.
This kind of tuning will also cause impedance transformation :
If the resistive component (radiation resistance + loss) is 20 Ohm it will 
be transformed to 1.2 Ohm and the "system Q" will rise from 22 to 108 
(making tuning rather critical).

73, Rik  ON7YD

>We will try tu tune the antenna with your help, if anybody can help me. 
>I'll be very happy.
>I'll give you all necessary datas:
>
>Vertical is 135m high with 170cm in diameter (no cap hat), there are 120
>radials in range from the base and 50m, after that are 240 radials between
>range 50-130m. Each radial is from 6mm Cu wire about 30cm under the ground
>level. Ground conductivity is 14,3mS.