Message-ID: <2000060621400768199@zetnet.co.uk>
Date: Tue, 6 Jun 2000 21:40:07 +0100
To: rsgb_lf_group@blacksheep.org
From: "Peter Dodd" <g3ldo@zetnet.co.uk>
Subject: LF: Re: Antenna Z measurements at LF
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 Measurement of Antenna System Impedance at LF

Although a suitable matching transformer can be arrived at 
empirically the design can be simplified if the antenna feed 
impedance is known. An impedance value will also give you some idea 
of how good your ground system is, once you have estimated the coil 
loss. There may be other uses.

The method of measuring impedance I use is called the 3 Meter 
Impedance Measuring Method (3m Z Bridge) and I have used it for the 
last 20 years on HF. It is described in more detail in 'The Antenna 
Experimenter's Guide'  (from RSGB Bookshop).
The 3-meter impedance measurement technique is very simple. It 
compares the unknown impedance with a fixed standard impedance, (a 50 
ohm resistor and a .01uF capacitor). An excitation source (signal 
generator) applies an RF voltage to the standards and diode 
voltmeters, selected by a switch allow digital voltmeter readings to 
be made at various points. Impedance is indicated by the ratio of 
three voltmeter readings. One additional reading allows in-place 
calibration of the reference capacitor and the second permits several 
solutions for the unknown impedance, thus giving an indication of the 
random errors that may be present in the data.
Impedance values are derived from the voltage data by calculation. 
However, this may be simplified by using a graphic method (using 
graph paper and a pair of compasses and a ruler) or a computer. The 
BASIC (do you remember that stuff?) source code is available at the 
RSGB Web site <www.rsgb.org>  If you download it as a text file it 
will run in Q-Basic - I dont know about Visual Basic.
If you are using the standard Marconi antenna measure the feedpoint 
impedance from the bottom of the loading coil and earth.

The following measured impedances in the range 136.4 to 138kHz 
indicate some errors just below 137kHz, which I have not yet identified. 

G3LDO_LF_Antenna
 
                    Results        +/-Errors
Freq MHz   Res      jX       Res    jX
----------------------------------------
 .1364       37.9   - 29.4      2      2.1
 .1366       37.4   - 22.8      4.6    5.3
 .1368       33.5   - 22.2     5.1    5.7
 .1370       33.4   - 17.3      5.8    7.4
 .1372       35.6   - 8.8       2.8    3.5
 .1374       36.4   -0 1        1.7    2
 .1376       36.9   +0 5       1.1    1.3
 .1378       36.5   + 13.3    1      1.3
 .1380       38.1   + 21       1.1    1.8

The above measurements were made in the late afternoon when the 
ground was relatively dry. In the morning the resistance was above 
40ohms and resonance was around 136.8kHz. 

A full description will appear in the forthcoming LF Book.


-- 
Regards, Peter, G3LDO

<g3ldo@zetnet.co.uk>