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From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Sat, 27 Mar 2010 13:19:28 -0000
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Subject: Re: LF: Question about ground impedance at 8.97 KHZ  Stefan.
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That sounds about right for a sloping 100m wire.....an oft quoted figu=
re was
5pF pepr metre for the vertical section and 6 pF /m for the horizonata=
l
top-load.

Alan G3NYK

----- Original Message -----=20
From: "Stefan Sch=E4fer" <schaefer@hst.tu-darmstadt.de>
To: <rsgb_lf_group@blacksheep.org>
Sent: Saturday, March 27, 2010 12:31 PM
Subject: AW: LF: Question about ground impedance at 8.97 KHZ Stefan.


Addition: When i replace the wire by a fixed capacitor of 570pF (470+1=
00) i
come to resonance at about the same frequency. So my antenna acts as=
 a 580pF
C on <9 just like on 137kHz.

If you have a inv-L, the C' (in pF/m) should be even higher?

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 lo=
ng wire.

I measured my home QTH inv-L antenna (about 55m of wire in total, abou=
t 10m
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=
 trees,
etc., so you could possibly expect lower resistance in an open locatio=
n, 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 amat=
eur 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, quadratu=
re,
capacitive component. So anything that affects the balance of the brid=
ge
(e.g. stray coupling between the components, or source or detector),=
 can
cause large errors.  I imagine good electrostatic shielding between th=
e
bridge components and antenna wire would be essential. A first step wo=
uld 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 measu=
rement
frequency with a series inductor, then measure the relatively low resi=
stive
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 reflect=
s the
way the antenna will actually be used.

Cheers, Jim Moritz
73 de M0BMU