Additional Information and Computer Programs for The Antenna Experimenter's Guide.
Updated 18th March 2002
(Chapter 6, polar diagram measuring program to be released later)
Chapter 2, The 3 Meter Impedance Measuring Method (3m Z Bridge)
The 3-meter impedance measurement technique compares the unknown impedance with a fixed standard impedance, and the ratio is indicated by three voltmeter readings. This makes it one of the simplest of impedance measuring instruments; it uses no variable reference components and it is self-calibrating.
In this variation of the method, five readings are made. One of the additional readings 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. The voltage readings are converted into impedance values with downloadable software, see below.
The fixed standard impedance comprises a resistor and capacitor. An RF excitation voltage, at the measurement frequency, is applied to Z via R and C. The voltages across R and C are measured together with the input voltage Ea, the voltage across Z and the voltage across Z, plus C. The excitation level is adjusted to a specific level; all the other voltages are then measured.
The RF voltages are measured using diode probes, selected by a switch. These probes measure peak volts and require a high impedance voltmeter; a digital voltmeter is ideal. For greatest accuracy the value of R and the value of C need to be appropriate to the range of impedance and frequency respectively, of the measurements being made. The impedance range of most amateur LF antennas is around 40 to 90ohms. The original value of R used in the design of the 3-M box R was 50ohms because the method was used to measure impedances in 50ohm systems, however this still turns out to be suitable for most amateur radio purposes. The reactance of C can also be 50ohms. In practice a reactance value for C of between 25ohms and 100ohms, will give reasonable results.
Three-M circuit diagram. Note that the switch positions Ec and Ez are correct in this diagram.
In the circuit the value of C is show as 20nF, which is appropriate for 136kHz. For other bands suitable capacitor sizes are 2000pF for 1.8MHz, 1000pF for 3.5MHz, 560pF for 7MHz, 390pF for 14MHz, 180pF for 21MHz, 100pF for 28MHz, 49pF for 50MHz and 22pF for 144MHz.
A low power transmitter with a variable power output was originally used as excitation source. An attenuator was used at the input so the transmitter was isolated from the variations of unknown impedance as the transmitter frequency is varied during a series of measurements. This attenuator can be dispensed if a signal generator with a small broad-band RF amplifier is used as the excitation source. It is very important that the harmonic output from the excitation source is kept as low as possible.
You may have noticed that in the procedure for using the 3M box described in the book calls for the excitation level to be increased until the voltmeter reads 5 volts across the reference resistor, eg Er = 5 volts. The reason for this is mainly historical. In the original article QST article by W8CGD [Jun 1965] a graphic method was used for extracting the impedance from the voltage data, and because the impedance measurements were related to a 50ohm system, 50ohms became the standard for the fixed reference components and the divisions on the graph paper. You don't have to use 5volts, after all it takes a relatively high level of excitation power to achieve this. Provided all the voltages are in the correct ratio, theoretically the results should be the same.
However, all the original computer programs were just software implementations of the graphic method and they will only work with an Er measurement of.5, 5 or 50. To save rewriting the programs used to plot tabular or graphic signatures of antennas or components relative to frequency, the main measurement program normalizes the data for a value of 50.
Download the programs by clicking Zmeasure and Moredata. Put the zip files into a separate folder/directory (say ZCALC) and activate them. Run MENU and the four programs should all run together. A number of antenna files have been included to give you something to play with when trying out the Table and Graph (items 3 and 4 from the menu).
Extracting Impedance from 3 M Data When the program 1 from the menu is run you will be prompted for items of data from your 3-m measurements. The program prints the solution as soon as the last item of data is entered. In addition the program will report errors, which will give an indication of data reliability.
The program will amend any data resulting in a small geometric non-intersection. It will also inform you of any change in any item of data required to effect this intersection. The program will make 10 attempts to correct a non-intersecting error. If the data cannot be corrected during these attempts the program will report this and not give a solution.
In practice any excitation level appeared to give good results but errors increased sharply if any one of the measured voltages fell below one volt. This level may depend on the type of diode being used but the program will give a good indication of the errors if the excitation level is too low.
Some readers are in the process of making interesting developments to the 3-meter impedance measuring method. I have been asked for the source code of some of my programs. These are provided free by e-mailing me direct - see address on the home page.
Calibrating Impedance Bridges.
While it is easy to calibrate impedance bridges using several resistance standards, reactance is more difficult. A solution to this problem can be found on Checking your RF Bridge Calibration