Antenna analyzer it is an excellent helper for radio amateurs who prefer home-made antennas. It allows "insight" into the properties of the antenna by displaying the individual components of the impedance. From these data we can determine the impedance, PSV aj rezonančnú frekvenciu. Ceny komerčne vyrábaných anténnych analyzátorov sú väčšinou nad možnosti našich rádioamatérov. Zhotoviť anténny analyzátor pritom nemusí byť vec zložitá ani nákladná.
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The principle of the antenna analyzer
The basis is a VF generator with controllable output power, which can also be a regular transceiver. It is advantageous if it is also unlocked for broadcasting outside the amateur radio bands, which will allow antennas to be measured in a wider range of frequencies. A simple measuring device connected to a millivoltmeter is a necessary accessory. We will process the measured values on a computer.
Na meranie zložiek impedancie sa využíva mostíková metóda. This connection was already published in 1965 by Doyle Strandland W8CGT in the American QST. The advantage is that it is simple and has no adjustment elements.
It is advisable to set the voltage from the generator (TCVR-a) higher in order to minimize the influence of the characteristics of the diodes used. If the voltage is only in the hundreds of mV (= the output power is too small), it is possible to change the values of the resistors in the input divider, the task of which is to separate (suppress) the action of inappropriate impedances (impedances different from 50 ohm) that an unadapted antenna can present.
A millivoltmeter or digital multimeter with a high input resistance must be used to measure voltages. Not every multimeter is suitable for working in the RF field near the antenna, especially cheaper multimeters show meaningless values.
Construction requirements
The entire structure of the bridge must meet the conditions of VF technology, i.e. the terminals of all components will be shortened to a minimum and the components must be of high quality. The resistors in the input divider are the most stressed by power, so they must be sized appropriately, but at the same time they must have a minimum self-inductance. Therefore, the parallel connection of two resistors of a relatively unconventional value is indicated. It is suitable if we wall off this divider from the rest of the bridge, for example with a cuprextite partition. It is enough to dimension the resistor R to one-fifth of the input power. We will use high-quality capacitors, especially capacitor C, which is in the branch of the bridge. Its capacity is determined by the frequency at which we measure.
| Frequency | 3.5 MHz | 7 MHz | 14 MHz | 21 MHz | 28 MHz | 50 MHz | 144 MHz |
| Capacity | 1000 pF | 560 pF | 390 pF | 180 pF | 100 pF | 49 pF | 22 pF |
The reactance of the capacitor is approximately 50 ohms at each frequency, but the exact value is not necessary, nor does it affect the accuracy of the measurement. Only in the case of large differences (below 25 and above 100 ohm) does the measurement accuracy decrease. At higher frequencies (above 10 MHz), it is not advisable to use a capacity switch or a capacitor replacement system.
We connect the antenna directly to the terminals of the bridge or via an impedance repeater (coaxial cable length Lambda/2 x shortening factor).
Processing of measured results
Among the advantages of Joe's macro, in addition to increasing accuracy, is the graphical display of the result, including in a Smith chart.
The macro is available on Joe's site http://www.QSL.net/n8xpv/index.html. Working with it is relatively simple.
A practical example of antenna measurement G5RV so Z-matchom na 20m pásme. Anténa je v pracovnej výške pripojená koaxiálnym káblom (opakovač impedancie). TCVR prepneme na CW (alebo FM), we lock and in the "Ea" position of the switch, we set the power so that the voltage is a few volts. We write the voltage in the table or directly in the macro (UFB option, for example in the notebook, we see the results immediately after the measurement). We will also measure and write down other voltages (Er, Ecz, Ec and Ez). We unlock and retune the TCVR to another frequency, e.g. 50 kHz higher and we repeat the whole procedure. The result is a table of R and jX (also with a sign), possible inaccuracy resulting from the measurement method, PSV, a graph of the dependence of impedance on frequency and a Smith diagram.
Parts list
| R1 | 7.5 ohm (2×24 and 1×21 ohm in parallel, see text) |
| R2 | 21 ohm (see .text) |
| R3 – R7 | 1M |
| R | 50 ohm (2×100 ohm in parallel) |
| C1 – C10 | 1000pF styroflex, ceramics, mica, etc. |
| C | see .table |
| D | BAT46 (or other Schottky RF diodes) |
Literature
[1] Peter Dodd G3LDO, Tom Lloyd G3TML: Measurement of antenna impedance, SPRAT No. 50, spring 1987
[2] http://web.ukonline.co.uk/g3ldo/
[3] http://www.qsl.net/n8xpv/index.html
[4] Jan Bocek OK2BNG, Ing. Tomáš Klimčík: Electrotechnical measurement (11), Measurement of complex antenna impedance, RŽ 2/2002
[5] Peter Dodd G3LDO: Antennas, RadCom 7/2005