[Lf] [Fwd: LF: Re: Antenna Z measurements at LF]

[Andre' Kesteloot]

Peter Dodd wrote:

>  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 at zetnet.co.uk>