Hi Rik and others...
After I discuss this problems with Bob OK2BVG and we found the problem. The
tower itsefs is DC isolated, but...on the tower are 4 lihgts (2x in 70m and
2x in 135m) powered by 410m of twisted-pair cable. So, this is total 820m of
wire well cupled with the antenna. This antenna has been on air for about 20
year on the frequency 1097kHz. The filters are designed for this frequency
and have a very low reactance on 136kHz. Any idea?
73 de Rich OM2TW
-----Original Message-----
From: Rik Strobbe [mailto:[email protected]]
Sent: Monday, January 19, 2004 2:58 PM
To: [email protected]
Subject: Re: LF: Test failed...
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.
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