ANTENNA MEASUREMENTS R.P.Haviland, W4MB LEGENDS FOR FIGURES Fig. 1. Use of Lamp Bulbs as Ammeters a) Use as wide band RF ammeter. Estimate current by brightness. Or place a second bulb with known current from a battery, rheostat and ammeter as close as possible, adjusting current for equal brightness. The RMS RF current is the same as the ammeter reading. b) Two bulbs separated by 1/4 wavelength make a SWR indicator. Equal brightness is 1:1 SWR unless line length is accidently such as to give a false equality. If the relative brightness changes on moving the 1/8 wave section to the transmitter end, the new relative brightness is the correct indication. Fig. 2. Neon Bulb Indicators A small neon bulb is a useful indicator of the presence of RF. Sensitivity can be increased by forming the leads into dipole form. Or: a) Bias the bulb with a small DC current from a battery, to just above the striking voltage. b) Use a higher adjustable voltage to measure the difference between the striking voltage with RF present and absent. This is equal to the peak RF voltage across the terminals. Fig. 3. Diode Voltmeters a) Germanium diode RF voltmeter for low voltage measurements. A 1N914 silicon diode will withstand higher voltages, with a small loss in sensitivity. b) A vacuum tube diode voltmeter for high voltage RF measurements. Also useful in transmitter development. Use appropriate safety precautions. Fig. 4. RF Ammeters a) Hot Wire ammeter, measuring the current by amount of elongation of a wire heated by the current. Liable to damage by shock, or by too high current. b) Thermocouple ammeter, which produces a small DC current by measuring the temperature of a wire heated by RF with a minature two-metal thermocouple. Liable to damage by excessive current. Very good as a calibration standard. c) Transformer ammeter, measuring the voltage induced in the secondary by the current in the one-turn primary winding. Now the most popular RF ammeter. Fig. 5. Elements of aa Signal Generator Essential elements of a RF signal generator, producing a voltage of known level at a known frequency. Frequency accuracy depends on the oscillator calibration accuracy, and on drift. An external digital meter is necessary for precise work. Amplitude accuracy depends on the accuracy of the RF voltmeter, and on the attenuator. A good design provides both the pre- and post- attenuator outputs, typically 1.0 and zero to 0.1 volt. Frequency swept generators use a low frequency form of FM modulation, linear vs time, or sine wave, both typically syncronized to the power line frequency. Power line filtering or the double shielding of the oscillator as in the best designs is not shown. Fig. 6. Transmitter Power Reducer Values for a power attenuator, to reduce transmitter levels for antenna measurements. Designed for 10 watts input, this can be constructed for 1 or 0.1 watt output to a matched load. The unit can be mounted with the resistors in oil, or they can be in a performated metal case for good air circulation. Intended for use with in input RF wattmeter: a voltmeter as in Fig. 3a can be included. Fig. 7. Simple Signal Generator Single IC signal generator for rf work, values for 14 MHz being shown. The coil must have good Q, and an all xtal design is easier. The output may be increased by replacing the 3 parallel open collector inverters with a high voltage type, the 50 ohm resistor being fed from 12-25 volts. Fig. 8. Diode Noise Generator Elements of a noise generator using the noise produced by a reverse biased diode. The circuit assumes a DC current path through the external circuit. If not present, a resistor equal to the the line impedance may be shunted across the output terminals. Intended for relative measurement, but the noise output can be calibrated by comparison to the signal from a good RF signal generator. Precision diode types are occasionally found at hamfest tables. Fig. 9. Balanced Line SWR Indicator a) This post WWII open wire line indicator shows low SWR when the bulb towards the transmitter is bright as compared to the one towards the load. Dimension shown and bulb types suitable for a transmittter of 50-100 watts output. Sensitivity is adjusted by the spacing from the main line. b) Coax version of a). Fig. 10. SWR measuring Instruments a) Transmission line directional coupler, an extension of Fig. 9. Sensitivity is much higher, due to use of the diode detector. A typical unit made for the CB band is usable for the low power range from about 0.1 to 10 watts. b) Coupler/ indicator which separates the magnetic and electric field coupling to the line. Careful construction is neded to eliminate the effects of stray coupling, see the ARRL or other handbooks for design details. Fig. 11. Series Elements for SWR to R,X Measurement Series or parallel resistance and/or reactance can be used to give R and X measurement from 2 or 3 SWR values. Use a rectangular or Smith chart, or computer program, for calculation. Typical values for 14 MHz are shown at a) resistor, b) capacitor, c) inductor. For the parallel equivalents, use the relation Xp * Xs=Zo * Zo. Fig. 12. Elements of Impedance Measurements a) Basic measuring setup, of signal generator, bridge and detector. A wide band detector is often used with a single- frequency generator, but a narrow band detector (receiver) must be used if the signal source is wide band, such as a noise generator or a swept frequency oscillator. b) Basic four arm bridge, usually designed for equality of the upper and lower arms. The standard arms may be a center tapped transformer, or two resistors, or may be more complex. The series adjustable arm type is good for dioles below resonance, as shown for the series RC unknown element. Parallel arms may also be used, as in the GR RF bridge and some noise bridges. c) Transmission line bridge. the resistive component of the unknown is determined by adjusting the relative capacitive and inductive coupling to the line ends, and the reactive component by the fractional wavelength departure from line center at balance. This is the principle of the HP RF bridge. Other design are found in the literature. Fig. 13. Transmission Lines for Measurement a) Top view of a slotted or trough transmission line, usually designed for a Zo of 50 ohms. A probe moves along the open top of the line to give the voltage variation along the line, which gives the SWR. The position of the voltage minimum gives the second measurement needed to calculate R and X. b) Cross section of a trough line. The probe end is usually smaller than the conductor diameter. Such lines can be built with 1 by 3 inch extruded aluminum, or can be sheet metal folded around a 2 by 4 for forming. The equation gives the conductor diameter and position for the design impedance. A 6 foot length is good for 144 MHz and above. c) Elements of a Lecher wire system, which may be designed for 270, 300, 450 and 600 ohm impedance. If room is available, useful at HF, for measurement, or for check of the SWR accuracy of another instrument. Fig. 14. Basic Field Strength Meter The essential elements of a field strength meter measuring the electric field. A magnetic field measurement type omits the pickup whip, and enlarges the coil into a small loop, or a ferrite "loop-stick". Low sensitivity types omit the tuned circuit, but become sensitive to stray fields. An amplifier may be added to increase sensitivity. See handbooks for design details. Fig. 15. Dipper Modifications a) Addition of a pickup loop for magnetic coupling to a digital frequency meter. The loop should be permanently mounted, together with its associated cable, after determining that the pickup level is adequate for the meter in use. The dipper is now a precision resonance indicator. b) Addition of a coupling capacitor to permit capacitive coupling to a resonant circuit, for example an antenna. The coil connection can be one or two turns of the lead around the coil pin, but an internal connection to a tip jack is better. Should be connected to a high voltage point. In an antenna this is the element end. In a trap antenna, it is the inside end of the trap resonant on the band being investigated. Traps are best checked when separated from the rest of the element. A shallow dip is reason to suspect a poor trap. This is an alternate to a) for the frequency meter. c) Coil shape to increase magnetic coupling to a linear circuit, such as an antenna element. Several turns are needed for the lower frquencies. Large coils tend to damage the coil receptacle, so mechanical support should be added for these. A pair of hooks can be built into this, to hold the dipper at a constant position to the element. Coupling should be to a high current point. In dipoles and trap dipoles, this is the element center. Using this with a) allows determination of the exact resonant frequency of the element, especially important in parasitic beams. The assembly is very useful in checking for tower, guy and boom resonances. It is also useful in TVI elimination, for searching for unwanted resonances in the transmitter and TV antenna, and as an interfering signal source.