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Simple antenna instrumentation and measurements



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It is very difficult to get an antenna working properly without making some simple measurements. Although it would be nice to make pattern measurements, those are beyond the reach of almost all of us. On the other hand, the 'I worked [or heard]a guy on the other side of the world' type of measurement tells us little or nothing. During the peak of the sunspot cycle a breath of hot air on the antenna can be picked up on all continents.

There are some things that can be measured, however. For example, the voltage standing wave ratio (VSWR) and the resonant frequency of the antenna are readily accessible. It is also possible to measure the impedance of the antenna feedpoint. Hams can measure the VSWR either with a special VSWR meter (often built into transmitters or antenna tuning units), or by using a radio frequency (RF) wattmeter.



By stepping through the band and testing the VSWR at various frequencies, one can draw a VSWR curve (Ill. 1) that shows how the antenna performs across the band. The resonant frequency is the point where the VSWR dips to a minimum. You can use the resonant frequency to figure out whether the antenna is too long (resonant frequency lower than the hoped for design frequency), or too short (resonant frequency above the design frequency).

But resonant frequency and VSWR curves are not the entire story because they don't tell us anything about the impedance presented by the antenna. One can't get the VSWR to be 1:1 unless the antenna impedance and transmission line impedance are the same. For example, a dipole has a nominal textbook impedance of 73-ohms, so makes a very good match to 75-ohm coaxial cable. But the actual impedance of a real dipole may vary from a few ohms to more than 100 ohms. For example, if your antenna exhibits a feedpoint impedance of (say) 25-ohms, using 75-ohm coaxial cable to feed it produces a VSWR = 75/25 = 3:1. Not too great. Measuring the feedpoint impedance is therefore quite important to making the antenna work properly.

There are any number of instruments on the market that will aid in making antenna measurements. Some of them are quite reasonably priced (or can be built), while others are beyond the reach of all but the most ardent and well-endowed enthusiasts. In this chapter we will look at RF power meters, VSWR meters, dip meters, noise bridges, impedance bridges, and a newer breed of more universal instrument called the SWR analyzer.

We will also look at a few accessories that are useful in the measurement or adjustment of radio antennas (e.g. coaxial switches, attenuators, and dummy loads).



RF POWER METERS AND VSWR METERS

The RF power meter is designed to indicate the actual value of RF power (in watts). VSWR meters are similar, but are either calibrated in VSWR units or have a selectable scale that's so calibrated. Some RF wattmeters measure the voltage on the transmission line, while others measure the current flowing in the line. If things are done right, then these approaches work nicely.

But sometimes those methods can be fooled. A better system is to measure both the current and the voltage and to display their product. Although this task sounds like a formidable instrumentation job, it's actually the way one of the most popular RF power meters works. The Bird Electronics-Model 43 (Ill. 2) is one of the oldest and most popular professional-grade instruments. It uses a specially designed internal transmission line segment with sampling loops inserted in the form of plug-in 'elements' (the round object with an arrow in Ill. 2). The elements are customized for the RF power level and frequency range. The arrow on the element indicates the direction of the power flow. When the arrow points to the antenna feedline, the instrument is measuring the forward RF power, and if pointed toward the transmitter it's measuring the reflected RF power.

Although the Model 43 type of meter will provide the RF power measurements more accurately under a wider range of conditions, a ham radio style VSWR meter/RF power meter will work well (and measure accurately) under most ham conditions. In addition, these instruments will provide a measure of the VSWR (with an RF wattmeter you need to calculate the VSWR). Ill. 3 shows two typical ham-style VSWR/RF power meters.

The instrument in Ill. 3A is small and simple, but quite effective. It will measure forward and reflected power over the entire high-frequency (HF) band, as well as the VSWR. To measure the VSWR, set the full scale/SWR switch (marked 'FS' and 'SWR') to the FS position, key the transmitter, and adjust the knob to achieve a full-scale deflection of the meter pointer. When the FS/SWR switch is set to the SWR position, it will read the VSWR. These instructions, by the way, ?t a large number of different instruments, by many manufacturers, in this same class.

The instrument in Ill. 3B is for the very high/ultrahigh frequency (VHF/UHF) bands. It does not have to be set as does the meter above. The reason is that it uses the 'crossed-needles' method of displaying forward and reverse power. Lines of intersection with the two scales form a species of nomograph that reads out the VSWR from observation of the point where the needles cross. This instrument also includes a peak reading function.

Most passive RF power meters inherently measure the average RF power because of the inertia of the meter movement. But with active circuitry, i.e. circuits that require direct-current power to operate, it's possible to sample and hold the peaks of the RF waveform. This meter then becomes more useful for measuring continuous-wave and amplitude-modulated waveforms (including SSB). Some authorities claim that the peak reading meter is preferred for SSB operation - and I concur.

DIP METERS

The dip meter works because a resonant circuit will draw power from any nearby field oscillating at its resonant frequency. An antenna that's cut to a specific frequency is resonant, so acts like an LC resonant circuit. At one time, these instruments were called grid dip meters because they were vacuum tube ('valve') operated. The metering monitored the grid current to detect the sudden decrease when the instrument was coupled to an external resonant circuit. Today, some people still use the archaic term

'grid dipper' even though dip meters have not had grids for nearly 30 years.

Ill. 4A shows a typical dip meter. It consists of an LC oscillator in which the inductor is protruding outside the box so that it can be coupled to the circuit under test. A large dial is calibrated for frequency, and a meter monitors the signal level. Coils for additional bands are stored in the carrying case.

The frequency dial of the dip meter bears some additional comment. It typically covers a very wide range of frequencies, yet the dip we need is very sharp. The calibration is none too accurate. As a result, it's wise to find the dip, and then use a receiver to measure the operating frequency while the dip meter coil is still coupled to the circuit being tested. This operation is a neat trick unless you are possessed with good manual dexterity. It is necessary to measure the frequency while the instrument is coupled because this type of LC oscillator changes frequency markedly when the loading of the coil changes.

The proper method for coupling the dip meter to an antenna transmission line is shown in Ill. 4B. Construct a 'gimmick', i.e. a small coil of one to three turns, of sufficient diameter to slip over the coil protruding from the dip meter. Either slip the dipper coil inside the gimmick, or bring it into close proximity. Energy is transferred from the dipper to the gimmick and thus to the antenna. When the dipper is tuned to the resonant frequency of the antenna, a sudden drain is put on the dipper, and the signal level drops sharply. The frequency of this dip tells you the resonant frequency of the antenna.

The dipper must be tuned very slowly - very, very slowly - or you will miss the dipping action of the meter. Tuning too fast is the principal reason why many newcomers fail to make the dipper work. To make matters more confusing, the meter needle will drift all over the scale as the instrument is tuned. This occurs because it's natural for LC-tuned oscillators to produce different signal level at different frequencies (generally, but not always, decreasing as frequency increases). The effect to watch for is a very sharp drop in the meter reading.

ANTENNA IMPEDANCE (OR 'RESISTANCE') BRIDGES

While it's necessary to measure the VSWR of an antenna to see how well it's working, the VSWR alone is insufficient to optimize performance. If you don't know the feedpoint impedance at resonance (minimum VSWR), then you can't do much to correct a problem. Several instruments are available that will measure the feedpoint resistance of the antenna (it is called 'impedance' in some user manuals, but it's really only the resistive component of impedance). The instruments in Ill. 5 are examples of antenna resistance meters. The instrument in Ill. 5A is based on the Wheatstone bridge circuit. It uses a variable resistor (connected to the thumbwheel resistance dial) as one leg of the bridge, and the antenna resistance another leg (fixed resistors form the remaining two legs of the classic Wheatstone bridge). This model is passive, and requires an external signal source. There is an internal amplifier that allows you to use a signal generator, or the amplifier can be bypassed if you wish to drive it with a transmitter signal.

The instrument in Ill. 5B is a little different. It is basically a sensitive VSWR meter with a built-in signal generator. Knowing the system impedance it can thereby calculate the resistance from knowledge of the VSWR.

RF NOISE BRIDGES

The noise bridge ( Ill. 6A) is an interesting antenna instrument that's used in conjunction with a radio receiver to measure the antenna feedpoint resistance, approximate the reactance component of the impedance, and find the resonant frequency. These instruments use white noise ('hiss' is what it sounds like on the receiver speaker) to generate a wide spectrum of radio frequencies. When the receiver is used to monitor the noise, a null is noted at the resonant frequency.

Ill. 6B shows how the noise bridge is connected between the antenna transmission line and the antenna input connector of the receiver.

The line between the bridge and the receiver can be any length, but it's wise to keep it short. One reader of my columns wrote in to take exception to the 'short as possible' requirement because it's not strictly necessary in a perfect world. My own experience says differently, especially at higher frequencies where the line is a significant fraction of a wavelength. A meter or so should not affect any HF measurement, however.

The line to the antenna should either be as short as possible (preferably zero length), but in the real world, where one has to be more practical than theoretically pure, it's sufficient to make the transmission line an integer multiple of an electrical half wavelength long (the physical length is shorter than the free space half wavelength because of the velocity factor). The impedance hanging on the far end of the half wavelength _N line is repeated every half wavelength. A consequence of this effect is that the impedance measured at the transmitter or receiver end is the same as at the antenna end.

To use the noise bridge, set the resistance (R) and reactance (X) controls to mid-scale. Adjust the receiver frequency to the expected resonant frequency. At this point you can do either of two different measurement schemes.

Method 1

First, you can set the R-control of the noise bridge to the desired antenna impedance, and then tune the receiver until a dip in the noise occurs (as heard in the output or noted on the S-meter). For example, suppose you have a dipole cut for 11.75MHz, and installed such that the expected feed point impedance should be around 70 ohms. It is fed with a half wavelength of polyfoam dielectric 75 ohm coaxial cable (V = 0:80). The cable length is =11:75 = 10:2m. The noise bridge X-control is set to 0, and the resistance control is set to the letter that represents 70 ohms (given in the calibration manual). This resistance setting can also be determined experimentally by connecting a 70 ohm resistor across the antenna terminal and adjusting the R-control for the minimum noise. When the noise bridge is set, then adjust the receiver for minimum noise. The frequency at which the dip occurs is the actual resonant frequency. I used this method to investigate a vertical cut for 21.25MHz, only to find the actual resonance was 19.2MHz, which explained the high VSWR.

Method 2

The other method is to set the receiver to the design resonant frequency of the antenna. You then adjust the R-control for a dip in the noise level. The value of the resistance is the feedpoint impedance of the antenna. You can either match that impedance, or adjust the antenna to bring the actual resonant frequency closer to the design frequency.

VSWR ANALYZERS

The basic premise in this section is that the instruments used must be accessible to people who don't have a ham radio or professional radio opera tor's license. Some of the instruments discussed above meet that requirement, but a relatively new breed of instrument called the SWR analyzer provides a lot of capability to the short-wave listener, scanner opera tor, and ham radio operator alike. It uses a low-power RF signal generator and some clever circuitry to measure the VSWR of the antenna. One model also measures the feedpoint resistance.

The simple version shown in Ill. 7 is for the low VHF band (up to and including the 6m ham band). It is a hand-held, battery-powered instrument. The meter reads the VSWR of the antenna at the frequency set by the TUNE dial.

A somewhat more sophisticated instrument, the MFJ Enterprises Model MFJ-259, is shown in Ill. 8. The front panel has two meters, SWR and RESISTANCE. The SWR meter is calibrated up to 3:1, with a little uncalibrated scale to indicate higher SWRs. The RESISTANCE meter is calibrated from 0 to 500 ohms, which is consistent with the SWR range.

Two controls on the front panel are TUNE and FREQUENCY (MHz) (a bandswitch). The MFJ-259 has a digital frequency meter to measure the operating frequency of the internal oscillator. This frequency counter can also be used to measure the frequency of external signal sources (DO NOT connect the counter to the output of a transmitter - the instrument will be destroyed). The top end of the MFJ-259 has a number of controls and connectors. An SO-239 'UHF' style coaxial connector is provided for the antenna connection. A BNC coaxial connector is provided to apply an external signal to the frequency counter, while a pushbutton INPUT switch is available to switch the counter from internal to external signal sources.

Another pushbutton switch is used to set the gate timing of the counter (a red LED on the front panel blinks every time the gate is triggered). The tuning is from 1.8 to 174MHz, while the counter will measure up to 200MHz.

The MFJ-259 will work from an external 12V DC source, or from an internal battery pack consisting of eight size-AA standard cells. MFJ recommends that either alkaline or rechargeable batteries, rather than ordinary zinc-carbon cells, be used in order to reduce the possibility of leakage that could damage the instrument (this is good practice in all battery-powered instruments). I have a homebrewed battery pack that uses eight size-D nickel cadmium batteries (4 A-h rating) that can be recharged from a 12V DC power supply, and it works well with the MFJ-259.

Unlike many lesser SWR meters, this instrument is not fooled by antennas that have impedances consisting of both resistance and reactance elements. An example in the manual demonstrates an impedance of 25+j25 ohms (i.e. R is 25 ohms and reactance, X, is also 25 ohms). When connected to a 50 ohm load one might be tempted to think the VSWR is 1:1, and some cheaper meters will so indicate. But the actual SWR is 2.6:1, which is what the MFJ-259 will read.

The resistance measurement assumes a resistive load (i.e. the measurement is made at the resonant frequency of the antenna), and is referenced to 50 ohms. The VSWR and resistance measurements should be consistent with each other. If the VSWR is 2:1, then the resistance should be either 100 ohms (100/50 = 2:1) or 25 ohms (50/25 = 2:1). If the resistance is not consistent with the VSWR reading, then you should assume that the impedance has a significant reactive component, and take steps to tune it out.

In addition to antenna measurements, the MFJ-259 is equipped to mea sure a wide variety of other things as well. It will measure the velocity factor of transmission line, help in tuning or adjusting matching stubs or matching networks, measure capacitance or inductance, and measure the resonant frequency of LC networks.

Ill. 9 shows an MFJ-249 meter (similar to the MFJ-259, but with out the resistance measurement) equipped with the MFJ-66 dip meter adapter. It can be used to make the MFJ-249 or MFJ-259 work in the same manner as a dip meter. Using this adapter allows you to measure the resonant frequency of tank circuits using the dipper approach, as well as to measure things such as the coefficient of coupling between two LC circuits, transformers, and other radio circuits.

DUMMY LOADS (ARTIFICIAL AERIAL)

The dummy load, or artificial aerial as it's also called, is a resistor used to simulate an antenna when adjusting a transmitter or other radio apparatus.

These devices consist of a non-inductive, 50 ohm resistor inside a shielded enclosure.

The dummy load offers several benefits over regular antennas. First, it's a constant resistance over the entire frequency range, and , second, it does not present any appreciable reactance. Third, and perhaps most important, the dummy load provides the ability to conduct tests off the air where you will not cause television interference, broadcast interference, or interference to other users of the test frequency that you select. Besides, it's illegal and just plain rude to radiate when you don't have to! One use for dummy loads is to adjust antenna tuners. You can connect the dummy load to the antenna output of the tuner, and then adjust the controls for best VSWR at a number of frequencies. By recording the knob settings, you can 'rough in' the antenna tuner when changing frequency.

Several types of dummy load are shown below. The version in Ill. 10 is a small 5W HF model intended for measuring the output of CB transmitters. I have used this particular dummy load for adjusting a wide range of RF projects and instruments over the years. To make accurate power measurements, the dummy load is placed at the ANT connector of the RF power meter, and the transmitter connected to the other end. The power reading is not obscured by transmission line effects or the reactance that a real antenna might present.

The dummy loads shown in Ill. 11 are intended for ham radio power levels. The model in Ill. 11A is air cooled, and operates at RF power levels up to 1500W, over a range of 1-650MHz. The version in Ill. 11B is oil cooled, and will handle up to 1000W of RF power.

COAXIAL SWITCHES

A coaxial switch (Ill. 12) is used to allow a receiver or ham radio set to use any of several antennas (models with up to 16 ports are available, but this one is a two-port model). The common connector is for the receiver or transmitter, while the two antennas are connector to the 'A' and 'B' ports, respectively. Alternatively, one can turn the switch around backwards (it is bidirectional, after all), and use the same antenna on two different receivers or transmitters.

The use of the coaxial switch in antenna measurement is in comparing the antenna being tested with either another antenna or a dummy load. The kind of off-the-air checks that amateurs and short-wave listeners can make are notoriously inaccurate, but can be made a lot more useful by making comparisons with known antennas. For example, a friend of mine, the late Johnnie H. Thorne (K4NFU/5) had an antenna farm in Texas (and it did seem that he grew antennas, judging from the number he had). He kept a standard dipole, optimally installed and cut for 20m, and made all of his test designs for the same frequency. He would compare new designs to the dipole by switching back and forth while monitoring the signal strength on the receiver S-meter. He could also compare two different antennas by comparing them against each other or against the dipole.

STEP ATTENUATORS

The step attenuator (Ill. 13) is a precision instrument that provides highly accurate levels of attenuation in steps of 1 dB, 2 dB, 3 dB, 5 dB, 10 dBV, and one or more 20 dB settings. You can use these to calibrate instruments, or measure signal levels. One use is to make comparison measurements of two signal sources, two antennas, etc.

For example, suppose you want to measure the gain of a new antenna relative to a dipole (assuming that both are cut for the same frequency). You would pick or provide a distant signal on the resonant frequency of the antenna. The attenuator is inserted into the feedline path of the test antenna (Ill. 14). The signal strength is then measured with the dipole in the circuit (see the coaxial switch discussion above), and the reading of the S-meter noted. If you have an RF sensitivity control, then adjust it to a convenient point on the meter dial, like S-9 or some other marked spot.

Next, cut to the new antenna and note the signal strength. If it increased, then select the various levels of attenuation until the new signal level is the same as the initial signal level. The set level of attenuation is equivalent to the gain difference between the antennas.

If the new signal level is lower than the initial signal level, then the new antenna has less gain than the old antenna. Put the attenuator in line with the comparison antenna, rather than the test antenna.

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Updated: Tuesday, 2012-04-24 18:38 PST