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Date: Sat, 27 Mar 2010 13:27:36 +0100
Message-ID: <38A51B74B884D74083D7950AD0DD85E82A1BA3@File-Server-HST.hst.e-technik.tu-darmstadt.de>
Thread-Topic: LF: Question about ground impedance at 8.97 KHZ  Stefan.
Thread-Index: AcrNppcEw3qAX6hzQkGTgtMRGH1gSgAAVdNF
References: <MFEJIKCLCMPDFHOKMANAMEMEOEAA.paulc@snet.net> <4BAD19D0.300@telia.com> <56DF8C4BED71466A80EB2EB1D99F6788@JimPC>
From: =?iso-8859-1?Q?Stefan_Sch=E4fer?= <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
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Subject: AW: LF: Question about ground impedance at 8.97 KHZ  Stefan.
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


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Jim,=20
=20
What do you think is the reason why you measure just 340pF on a 55m wire?=
 And which C do you measure at 137kHz on the same antenna? What do you thi=
nk is the accuracy of your measurement?
=20
300 Ohm earth+surroundings losses is very good compared to my values on th=
e hill, although i have just one small tree in a radius of >100m. What abo=
ut your ground conductivity?=20
=20
Maybe i have to try other additional earth systems on VLF...
=20
Will you prepare a TX system on the dreamers band as well? Than we could=
 repeat our CW QSO just like on 137kHz ;-)
=20
73, Stefan/DK7FC

________________________________

Von: owner-rsgb_lf_group@blacksheep.org im Auftrag von James Moritz
Gesendet: Sa 27.03.2010 13:06
An: rsgb_lf_group@blacksheep.org
Betreff: Re: LF: Question about ground impedance at 8.97 KHZ Stefan.



Dear Paul, LF Group,

For Stefan's benefit, 1' =3D 0.3048m, so we are talking about a 52m long=
 wire.

I measured my home QTH inv-L antenna (about 55m of wire in total, about 10=
m
height) at 10kHz to have a capacitance of about 340pF, in series with a
resistance of about 300ohms. This is in a location with several small tree=
s,
etc., so you could possibly expect lower resistance in an open location,=
 or
higher if you are in a forest!

As has been pointed out, it depends on the type of bridge whether you are
measuring a series or parallel RC equivalent - for the above antenna, the
parallel equivalent would be about 340pF/7.3Megohms. The magnitude of the
impedance at 9kHz would be about 52kohms

These are much higher impedances than are commonly measured using amateur=
 RF
bridges. If you look at old textbooks on AC bridge measurements, you will
see elaborate shielding/guarding measures are often required for high
impedance measurements. The basic problem is you are trying to measure a
small resistance component while balancing out a much larger, quadrature,
capacitive component. So anything that affects the balance of the bridge
(e.g. stray coupling between the components, or source or detector), can
cause large errors.  I imagine good electrostatic shielding between the
bridge components and antenna wire would be essential. A first step would=
 be
to check the bridge works accurately on some known impedances (e.g. a
low-loss capacitor of a few 100pF with a series resistor).

The way I did my measurements was to resonate the antenna at the measureme=
nt
frequency with a series inductor, then measure the relatively low resistiv=
e
impedance of the combination with a simple resistance bridge. The loss
resistance of the coil was measured by replacing the antenna with a
calibrated air-variable capacitor, measuring the resistance again, and
subtracting it from the antenna+coil resistance. This still has some
potential errors (e.g. due to the stray capacitance of the inductor to
ground) but avoids the need to resolve the R/C components, and reflects th=
e
way the antenna will actually be used.

Cheers, Jim Moritz
73 de M0BMU





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