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Message-ID: <56DF8C4BED71466A80EB2EB1D99F6788@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <MFEJIKCLCMPDFHOKMANAMEMEOEAA.paulc@snet.net> <4BAD19D0.300@telia.com>
In-Reply-To: <4BAD19D0.300@telia.com>
Date: Sat, 27 Mar 2010 12:06:27 -0000
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Subject: Re: LF: Question about ground impedance at 8.97 KHZ  Stefan.
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Dear Paul, LF Group,

For Stefan's benefit, 1' = 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 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 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 measurement 
frequency with a series inductor, then measure the relatively low resistive 
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 the 
way the antenna will actually be used.

Cheers, Jim Moritz
73 de M0BMU