The article describes the methods of measuring SWR and impedance of the antenna as well as the problems associated with it. At the outset, however, it should be noted that a possible person interested in construction cannot do without basic knowledge of programming, either PC or microprocessors. Hardware simplicity is redeemed by more complicated mathematical calculations.
You will read in the article
1. Basic principle
Only bridge methods are used to measure a complex quantity such as antenna impedance. These work with sufficient accuracy and, with a suitable design, can process a large range of frequencies up to GHz. A partial disadvantage is the need to be powered by a measuring frequency generator with relatively strict requirements for spectral purity and considerable power. The bridge method itself enables various types of measurements not only on antennas but also on lines and feeders. A bridge of this type is also used in the well-known device MFJ259. Connection is trivially simple:
After connecting the generator, setting the desired frequency and connecting the complex load (antenna), the bridge supplies three voltage information, which is sufficient for calculating the impedance and PSV (SWR). In the following description, we will stick to this notation:
FWR…..half the voltage of the measuring frequency generator
REF…..differential voltage in the diagonal of the bridge characterizing PSV
VZA…..voltage on the measured load of a complex nature
2. Practical implementation
Due to the non-linearity of the diodes in the area of rectification of small voltages used as detectors, it is necessary to at least partially compensate for this non-linearity and amplify the output voltages to a sufficient level for the measurement needs. The compensating amplifier of one channel is shown in the following figure:
Each of the outputs of the bridge has its own amplifier. In the practical version, we will set the same gain of each branch with resistor R2. Diode D1 should be the same type as used in the bridge. The most suitable are Shotky diodes with ZERO BIAS properties designed specifically for detecting small voltages, e.g. 1PS79B62 (Philips) suitable up to GHz, but up to 500 MHz their selection is more than rich... Triple types are very suitable, e.g. Agilent's HSMP-386L, but many manufacturers make them. Op amps are RAIL to RAIL type with one power supply, those are also blessed.
The resistors in the bridge R1, R2 and R3 should be non-inductive, e.g. 1206, but TR191 without wire terminals, which can be used up to several GHz, are also very suitable. The absolute value does not matter, the important thing is that they have the same value, which is then included in the calculation as Rx.
3. Measuring frequency generator
Any generator of the desired frequency range with sufficient power of at least 20 dBm and low distortion can be used. Common LC oscillators are suitable, up to 500 MHz it is possible to make a mixing generator without problems. The following example shows a possible solution for the short wave region. On the Internet, you can search for a detailed diagram of the KV analyzer under the name RAINBOW, from which the following diagram is:
The parts used are completely normal. You don't even need to wind up and tune all broadband, impedance transformers and low-pass filters. American company Coilcraft http://www.Coilcraft.com/ it has all of this in its production program, and it also sends it to such a banana republic for free - just fill out the form and chat a little in English. At least it's been working for me for about two years now...
An elegant generator solution for the KV region up to 50 MHz is using a DDS circuit, which is direct frequency synthesis. Involvement with the company's IO is widespread Analog Devices typ AD9851 a refined example of which is in the following image. As a rule, the circuit is controlled by a microprocessor, which is a suitable solution because, among other things, there are no problems with measuring the frequency - the programming word corresponds to the frequency, which can be easily displayed. A similar solution was once described in RŽ 3/98.
It must be said that the company also produces more powerful DDS circuits operating at much higher frequencies. These are less suitable for amateur use (many legs, small cases) and are very expensive. In addition to AD, some such circuits are also produced by the HP company, but for amateurs without much importance.
4. Own calculation
The calculation is carried out in absolute units - Volts, Ohms, Farads and Henrys - so the following formulas are compiled. We assume that the measuring device shows us real tensions of all three quantities and the generator delivers an Fx frequency of 20 dBm (100mW/2.2V/50 Ohm) spectrally pure as "the word of God...":
Standing wave ratio - PSV , it is necessary to check the REF limit values, for which there is no point in continuing the calculation, because the PSV would be too high:
PSV = (1 + P) / (1 – P)
Determination of impedance character:
- Increase the frequency of the generator Fx by a small value
- Recalculate the value of the characteristic impedance Z1 = (Rx * VZA) / (FWD – VZA)
- If Z1 > Zo then the impedance is inductive in nature L
- If Z1 < Zo then the impedance is capacitive in nature C
If you decide to build such an analyzer and use a microprocessor with voltage converters for calculation, e.g. PIC… or AWR… I recommend 10-bit types. Then set the reference range of the AD converter to 2.5V and when programming, do not forget to realize that the read bit value of the measured voltage in the register is "far from the real one..." and must be converted to a real value:
5. Elimination of measurement errors
As can be seen from the previous calculations, a significant part of the error in the impedance calculation is caused by inaccuracies in determining the PSV. This is understandable because at low PSV values, the measured REF voltage is in the area of the most non-linear part of the diode characteristic, which cannot be compensated for, and thus the error is transferred to the calculation of the real and imaginary part of the impedance. In extreme conditions, the error can exceed 15-20%, which invalidates the measurement results.
Fortunately, there is an elegant solution for determining PSV using specialized circuits of the company MAXIM-DALLAS type MAX2016. Obvod je tvorený dvoma logaritmickými detektormi pracujúcimi v rozsahu kmitočtom od LF až do 2,5 GHz a s dynamickým rozsahom až 80 dB. Podrobný popis a použitie obvodu nájde záujemca na Maxim-IC.com where sa dá stiahnuť datasheet. Jeho veľkou nevýhodou pre amatérske použitie je puzdro QFN-28 o rozmeroch 5x5mm, ktoré sa dá zaletovať len technológiu povrchovej montáže. Uvediem preto len principiálne zapojenie pre meranie PSV a s tým súvisiace výpočty hlavne pre tých, ktorý nevedia anglicky.
Smerová väzba (vytvorená napr. vedením plošného spoja) je pripojená na vstupy logaritmických detektorov. Výstup OutD je výstupom vnútorného diferenciálneho zosilňovača, ktorého vstupy merajú rozdielové napätie výstupov logaritmických detektorov. Vlastný výpočet je v princípe jednoduchý. Najskôr sa vypočítajú straty odrazom RL in dB (PSV measurement is rarely used in professional practice, the expression of reflection losses in dB prevails):
VoutD……Differential voltage converted to an absolute value in [V]
Vcenter….Average output voltage OutD, typically 1V for R1 = 0, as can be seen from the attached graph
Slope……mV/dB transfer bias, typically 25mV/dB for R1 = 0
The PSV value is simply calculated from the reflection losses expressed in dB:
PSV = (1 + P) / (1 – P)
Logaritmické detektory použiteľné pre tento účel vyrába aj Analolog Devices napr. AD8362, alebo typ AD8364 veľmi podobný MAX2016 ale v ešte „nepodarenejšom“ 32 vývodovom puzdre o rozmeroch 5x5mm. Všetky tieto obvody sa dajú použiť na meranie zisku, výkonu na reálnej záťaži, ako citlivý merač RSSI pre smerovanie a nastavovanie antén a pod.
Today, every more serious company provides its customers with extensive technical assistance, including the development of applications of the offered integrated circuits, and the best ones, as well as the FREE SAMPLES delivery service. Unfortunately, AD is not one of them, just like those that have representation in post-communist countries.
Firma DALAS-MAXIM vyrába aj Other typy logaritmických detektorov jeden z nich MAX2015 je na obrázku vľavo v zapojení ako citlivý detektor RSSI. Ide o jednokanálový typ z vysokou citlivosťou -65dBm až +5dBm (citlivosť 0.125mV/50 Ohm) v kmitočtovom rozsahu 0,1 až 2,5 GHz. Je umiestnený v SMD 8uMAX cases, which are more suitable for amateur use.
In the company's catalog we can find many other interesting circuits for radio amateurs, such as MAX2620 which is an LC oscillator up to 1050 MHz, decoupling, broadband amplifiers, etc. It's a pity that most of the most interesting ones are in miniature SMD cases, which complicates amateur use. Unfortunately, this is the world trend, and maybe in 10-15 years, only some of those born earlier will know the discreet part...
But the best part is that they usually send two pieces of each selected type for FREE, so it's really worth visiting their site even though they changed the structure in the last few months and since then I haven't been able to get a FREE SAMPLE ORDER...but maybe that's just the new, not perfectly tuned shipping service.
6. Measure or model antennas?
...that's the question. If we consider the KV band, the measurement is not even very important. Modern modeling programs such as MMANA, NEC-2, EZNEC, NEC WIN+ and others will serve us just as well as antenna analyzer. In addition, without the effort and costs of manufacturing, while the error we can count on is practically negligible, and we "do" adjustments to the antenna comfortably at home at the table. We don't have to "jump" the monkey by climbing the roofs.
The situation with modeling in the 2m and 70cm bands is similar, if not better. The accuracy of the hardware measurement decreases slightly with increasing frequency, while mathematical modeling (regardless of the MININEC, NEC2 or NEC4 algorithm used, it is always a moment method and the individual types basically differ only in the comfort of the operator) gives surprisingly accurate results up to about 500 MHz.
If we intend to model antennas other than yagi, verticals or wires - e.g. parabolas or Helical - it is more advantageous to use a program working with the NEC algorithm, which models planar or geometric surfaces as integral structures, while the older MININEC algorithm (used by MMANA) calculates these structures as individual primitives, which significantly increases the calculation time. By the way, those interested in antenna modeling will be well served by The unofficial Numerical Electromagnetic Code (NEC) Archives.
The situation at high frequencies is different. The prices of measuring devices reach astronomical heights, but even modeling programs are not exactly cheap... Among the more well-known I mention GENESYS, HFSS92, ZELAND, MAXWELL_SV9 and many others, whose possibilities exceed the needs of even the most demanding radio amateur. They can also model such structures that practically cannot be measured in any way, and many technical solutions would not be possible to implement without these programs!
And so, dear friend, you have to find the answer to the question yourself. Measure or model…?
(C)2006 Ivan Urda, OM7UR