Home made antenna analyzer
Antenna analyzer is for radio amateurs, which home-made antennas prefer an excellent helper. It allows “peek” to the antenna team properties, that displays individual impedance components. From these data we can determine the impedance, PSV and resonance frequency. The prices of commercially produced antenna analyzers are mostly beyond the means of our radio amateurs. Making an antenna analyzer does not have to be difficult or expensive.
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 unblocked for broadcasting outside the amateur radio bands, which will make it possible to measure antennas 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.
It is used to measure impedance components bridge method. He published this involvement already in the year 1965 Doyle Strandland W8CGT in US QST. The advantage is, that it is simple and has no adjustment elements.
It is convenient to set the voltage from the generator (TCVR-a) to above, in order to minimize the influence of the characteristics of the diodes used. If the voltage is only in hundreds of mV (= the output power is too small), it is possible to change the resistor values in the input divider, whose task is to separate (suppress) the effect of unsuitable impedances (impedances different from 50 ohm), which an unadapted antenna can represent.
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 HF technology, i.e. we will shorten the terminals of all components to a minimum and the components must be of high quality. The resistors in the input divider are the most stressed by power, therefore, they must be dimensioned accordingly, 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, 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 frequency, on 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 approx 50 ohm at each frequency, but the exact value is not necessary, nor does it affect the accuracy of the measurement. Only in case of large differences (pod 25 to nad 100 ohm) the measurement accuracy decreases. At higher frequencies (When he crosses 10 MHz) it is not advisable to use a capacity switch or a capacitor exchange system.
We connect the antenna directly to the terminals of the bridge, or through an impedance repeater (coaxial cable long 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 with Z-match on the 20m band. The antenna is connected by a coaxial cable at the working height (impedance repeater). We will switch TCVR to CW (or FM), we lock and in the switch position “Ea” we set the power so, so that the voltage is several volts. We write the voltage in the table, or directly to the macro (UFB option, for example in a laptop, immediately after the measurement we see the results). We will also measure and write down other voltages (Is, pharmacist, Ec a Ez). We unlock and retune the TCVR to the next frequency, e.g.. O 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, graph of dependence of impedance on frequency and Smith diagram.
Parts list
| R1 | 7,5 ohm (2×24 a 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, which can be curved if necessary, mica and so on. |
| C | see .table |
| D | BAT46 (or other Schottky RF diodes) |
Literature
[1] Peter Dodd G3LDO, Tom Lloyd G3TML: Measurement of antenna impedance, SPRATè.50, jar 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
