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Relative Field Strength Meter With this low cost field strength meter you can adjust antennas to resonance, align transmitters for maximum output and calculate the half-power points of circuits. It will even let you measure the relative strengths of audio signals. By RALPH HOLLAND quency. A convenient way to perform measurements and make adjustments to an antenna system is to place the Field Strength Meter where a noticeable reading is obtained and vary the frequency of the transmitter. The relative strengths can be plotted against frequency and used as an indication of the adjustments that need to be performed to obtain the desired resonant frequency. In the case of a dipole antenna, the elements can be shortened if the resonant frequency (maximum field strength) occurs at too low a frequency. Conversely, the elements can be lengthened if the resonant frequency is too high. Fig.2 shows a typical frequency response plot from an antenna, for a frequency range either side of resonance. Point B on the curve is When most people think of tuning antennas, they think of using an SWR meter. But there is a better way - use a field strength meter. It can give a better indication of resonance, provided the meter is sited correctly. Fig.1 shows a typical test situation. The field strength meter is placed in direct sight of and at least two wavelengths away from the antenna. An effort should be made to keep the meter away from the transmitter and feed system (coax) in order to avoid false indications which may be caused by stray radiation when standing waves are present in the antenna feed system. If a transmitter is operated into an antenna, the field strength surrounding the antenna will be at a maximum when the antenna system is resonant at the transmitting fre- □ 0 0 the resonant frequency. The range of frequencies between A and C on the curve is the - 3dB (half power) bandwidth. The simple field strength meter lets you do these frequency plots quite easily, provided you know or can measure the transmitter frequency. Most transmitters these days use phase-locked loop circuitry and so (J/) 0 A Fig.1: typical test set-up. The field strength meter should be at least two wavelengths away from the transmitting antenna. 20 SILICON CHIP B C FREQUENCY Fig.2: typical frequency response plot from an antenna. Point B on the curve is the resonant frequency. TO TRANSMITTER TO 52!! DUMMY LOAD 4. ~:--===::::::::::::::;-~ .11.. Fig.3: directional coupler arrangement for directly monitoring a transmitter output. TO TRANSMITTER TO DUMMY <at> ~ Fig.4: non-directional resistive coupler. The transmitter is typically adjusted for maximum output in the centre of the operating hand. for any channel setting on a CB or amateur transmitter, you will know the frequency very precisely. There's no need for a frequency meter. Transmitter adjustment To adjust a transmitter for maximum power, the field strength meter can be coupled to its output. The coupling can be via the normal transmitter antenna and radiated field, or preferably by operating the transmitter into a non-reactive dummy load and coupling system. Since adjustments can take some time, q.ummy load operation is highly desirable as it will eliminate interference to other users of the test frequency. There are two straightforward techniques of coupling to the output of a transmitter. Fig.3 employs a directional coupler while Fig.4 shows a non-directional resistive coupler. Before making transmitter adjustments, you need to know the design bandwidth and operating characteristics of the transmitter. The adjustments are a compromise between several parameters and they often interact with each other. A typical compromise is to adjust the transmitter for maximum output in the centre of the operating band. This will be indicated by the maximum field strength on the meter. If a newly constructed transmitter is to be adjusted, it is often necessary to align the transmitter right through the power amplifer chain. To do this, the field strength meter can be coupled with a loop or directly via a capacitor, to make ad- 680() S1 ANTENNA 1 0-:-1 .01 ZERO . .I + VR1 10k LIN 4.7k .01 8 EOC .,. VIEWED FROM BELOW SIMPLE FIELD STRENGTH METER Fig.5: the circuit uses diodes D1 & D2 and their associated 10k!2 resistors in bridge configuration. The bridge output drives differential amplifier stage Q1 & Q2, which in turn drives the meter. justments to the lower power sections. The input impedance of the meter is around 5k0 which is reasonably high, but any initial adjustments should be re-done with the meter coupled to successively higher power stages as the meter will still present substantial loading on the tuned circuits. PARTS LIST 1 PC board, code 04103901, 95 x 50mm 1 50µA meter movement 1 plastic utility box, 1 60 x 95 x 56mm (OSE Cat. H-2851 or similar) 1 1.5V O cell 1 1.5V O cell holder (Jaycar Cat. PH-9218 or similar) 2 knobs to suit pots 1 RCA panel socket 1 RCA plug 2 1N914 silicon diodes (01, 02) 2 BC549 NPN transistors (01, 02) 4 1on ¼ W 5 % resistors 2 6800 ¼ W 1 % resistors 1 1 OkO linear potentiometer (VR1) 1 1 kO linear potentiometer with switch (VR2, S 1 ) 1 1µF metallised polyester capacitor (greencap) 3 .01 µF metallised polyester or ceramic capacitors Miscellaneous Short length of coax cable, hookup wire, solder, adhesive (to secure O cell holder). Design parameters To be a general purpose instrument, a field strength meter should be as broadband as possible. I have measured the lower 3dB cutoff point as 35Hz and have operated the meter. at over 400MHz, so that means the circuit is pretty broadband! The circuit should also be sensitive. Fitted with a suitable pickup, the unit decribed here will respond to a 1 watt 144MHz transmitter feeding a quarter wave antenna from over 10 meters away. The prototype field strength APRIL 1990 21 Fig.6: the parts layout & wiring diagram. An RCA panel-mounting socket is used for the output connector and is wired to the PC board via a short length of coaxial cable. current through QZ is reduced by an equivalent amount. Thus, the average voltage at the collector of Ql will be higher than that at the collector of 02. This causes current to pass through potentiometer VRZ and the associated meter Ml, and thus provide an indication of the input signal amplitude. When pot VRZ is adjusted so that its wiper connects directly to the collector of QZ, the circuit is in its most sensitive condition. Winding the pot back the other way reduces the sensitivity so that the circuit can handle quite large signals. The current drain of the circuit is around 1 milliamp, so a D-size cell should last for several years. Construction Most of the components · are mounted on a small printed circuit board measuring 95 x 50mm (coded 04103901). This has been designed to ensure that stray capacitance and inductance is balanced on both sides of the bridge. The board is also designed to mount in the slots of the specified plastic utility case. All component leads should be dressed and kept as short as possible, according to good practice for The PC board slots into a plastic utility case & has been designed to ensure that stray capacitance and inductance is balanced on both sides of the bridge. meter was mounted in a standard plastic instrument case, together with a small meter and two knobs - one for sensitivity and the other for zeroing the meter. It runs from a 1.5V battery. Circuit operation The circuit is shown in Fig.5. It is effectively a bridge circuit consisting of diodes Dl and DZ and the four associated lOkO resistors. The bridge is initially balanced by potentiometer VRl and the degree of unbalance is indicated by the meter Ml which is driven by a differential amplifier consisting of transistors Ql and QZ. It works as follows. Dl and DZ are high speed silicon 22 SILICON CHIP diodes and these are biased on slightly at about 30 microamps, which has the effect of improving their sensitivity. When the bridge is balanced, by setting VR3, the voltage at the collectors of Ql and QZ will be exactly equal and so no current will flow through the lkO potentiometer VRZ or the 50µA meter, Ml. When a signal is picked up by the antenna, it is coupled via the lµF capacitor to the anode of Dl. This diode then conducts on positive cycles of the input waveform and this raises the average voltage at the base of Ql. This causes the current through Ql to increase and since it is a differential amplifier with a common emitter resistor, the This is the full-size pattern for the PC board. ... The battery holder must be positioned so that it sits between the pots & the meter when the lid is closed. RF circuitry. No special order is necessary to assemble the components onto the board but take care to orient the transistors and diodes correctly. You will have to cut a suitable hole for the meter in the lid of the case, as well as drill holes for the two potentiometers and the RCA panel socket. The D cell holder for the battery can be glued to the bottom of the case before all the wiring is completed. Note that the battery holder will have to be positioned in the case so that it sits between the meter and the pots when the lid is installed. Using the meter To use the field strength meter, you will need to connect an antenna to the RCA socket. This can be a short length of stiff copper wire soldered to an RCA plug. Rotate the sensitivity control to about half setting and then adjust the zero control to zero the meter. Now key your transmitter and check that the meter deflects. Maximum pickup will be obtained when the field strength meter's antenna is parallel to the transmitter anten- + 1.5V Fig.7: these two simple circuits can be used to select matched diodes & transistors. The transistors should be selected for matched collector/emitter voltages. na; ie, both oriented in the same direction. Remember though, when an antenna is being tested for resonance, the field strength meter should be at least two wavelengths away (at the operating frequency). You can maximise the pickup of the field strength meter by cutting its antenna pickup lead so that it is resonant at the frequency of interest. If you like, you can make up several plug-in antennas, each with a different resonant frequency. Minimising drift If you want to minimise drift in the circuit you can select the transistors and diodes so that they are matched. The circuits of Fig.7 will enable you to do this. When selecting diodes, pick those which have the closest possible match in their voltage drop, which will be about 400mV. Similarly, when picking the transistors, pick a pair which have the closest possible collector emitter voltage drop when measured in the suggested circuit of Fig.7. ~ APRIL 1990 23