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
<[email protected]>
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