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AMATEUR RADIO BY GARRY CRATT, VK2YBX Build this relative field strength meter It is often said that simple is best. This simple relative field strength meter can be built in less than an hour for just a few dollars. One simple yet quite useful instrument that should be part of every amateur radio "shack" is a field strength meter. Such a device can be used to check the operation of a transmitter, regardless of the frequency band. Simple relative signal strength indications can also be used to evaluate the efficiency of antennas and the output stage of transmitters. Our unit, by necessity, is broadband. Most published designs are quite insensitive, relying solely on the rectifi- . cation of RF using a small signal diode. The resultant DC voltage is then filtered and used to drive a DC meter. Fig.1 shows this typical arrangement. To overcome this shortcoming, we have added an LM358 operational amplifier, which is used to amplify D2 DCNIDEO OUTPUT Fig.1: basic circuit for a field strength meter. The filtered output appears across C2 & can be coupled to a DMM. the rectified RF voltage. Also, we decided that because most amateur operators have a digital multimeter, we would use this as the indicator. Fig.2 shows the final circuit. Of course, virtually any op amp capable of single supply operation could be used. The RF field applied to the antenna induces a voltage across the terminating resistor Rl. This voltage is rectified by diode Dl, which is a germanium or Schottky diode. Resistor R2 and capacitor Cl filter the resulting DC voltage, while Cl also bypasses The circuit is assembled onto a small PC board & this is plugged into a digital multimeter via two banana plugs to obtain the relative signal strength readout. 86 SILICON CHIP pin 3 of the op amp, thus preventing RF energy from being coupled into the input. The DC amplifier we chose is an LM358, with an adjustable gain ofup to 101. This is set by VRl, a 100kQ potentiometer. If additional sensitivity is needed, then the bias arrangement shown in Fig.3 can be used. Here, a small amount of bias is used to almost turn the diode" on", thus overcoming most of the 0.25V drop characteristic of germanium diodes. The output of the op amp is fed via current limiting resistor R4 directly to the input of the DMM. By selecting the 200mV scale, the multimeter can be used as quite a good indicator. In fact, it is possible to drive the meter to read several volts and if the DMM is autoranging, then so much the better. Power for the circuit is derived from a 9V battery. Diode DZ has been included to ensure that the incorrect polarity can never be accidentally applied to the op amp. Construction Our prototype was built on a small PC board (code SC06110921) but there's no reason why you cannot assemble yours on a small piece of preetched circuit board (available from most electronics stores). Fig.4 shows the parts layout on the PC board. The ideal mounting arrangement would be in a metal box, large enough to accommodate the 9V battery and an on/off switch. The output plugs should protrude through one end of the case and be positioned so that they plug straight into the "banana" input sockets of the DMM. The only critical component is Cl, which should be located as close to the antenna terminal as possible. Apart from that, the usual precautions when ANTENNA Fig.2: the final circuit uses D1 to detect the incoming RF signal. The detected signal is then filtered and amplified using op amp IC1. D1 protects the op amp against reverse connection to the battery. w 1 D1 c2 OA~5. or s1m. _047 c, .oo,I 7 .,. 7 Protect your valuable issues Silicon Chip Binders OUTPUT TO DMM VR1 100k RJ 1k .,. RELATIVE FIELD STRENGTH METER ··-+ . M G~ ~ LI ANTENNA fl 02$ .041W +9V-----... TO 1 ANTENNA- -i 1 -.111.:_ OUP~TTO DMM ------• ..._.,. E::!).001 .- {il'D- ..ffiD. .~ Fig.4: install the parts on the board as shown here & be careful not to confuse diodes D1 & D2. R1 Fig.3: this modified circuit provides greater sensitivity by applying forward bias to the detector diode. working at VHF and UHF should be observed; ie, all component leads should be kept as short as possible. The entire circuit draws less than 2mA when fully driven, so battery life will not be a problem. The antenna should ideally be telescopic, so that it can be adjusted in length (corresponding to a quarter wavelength) to suit the freque!lcy of interest. Alternatively, you can use an insulated PARTS LIST 1 PC board, code SC06110921, 51 x 30mm 2 banana plugs 1 battery snap connector 1 9V battery Semiconductors 1 LM358 op amp (IC1) 1 OA95 germanium diode (D1) 1 1N914 silicon diode (02) 1 100kQ trim pot (VR 1) Capacitors 1 .001µF ceramic capacitor (C1) 1 .047µF ceramic capacitor (C2) Resistors (0.25W, 1%) 1 12kQ (R1) 1 1kQ (R3) 1 4.7kQ (R2) 1 1.5kQ (R4) These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a distinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. * High quality * Hold up to 14 issues * 80mm internal width * SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A 11.95 plus $3 p&p each (NZ $6 p&p). Send your order to: Fig.5: this is the full-size pattern for the PC board. wire lead for the antenna (again cut to the appropriate length). The length of the antenna can be easily calculated using the formula L = 7140/F, where L is the calculated length in centimetres and F is the frequency in MHz. For example, if F = 400MHz, then L = 7140/400 = 17.85cm. At frequencies lower than UHF, the quarter wavelength must be significantly increased to compensate for the n egligible groundplane afforded by the case and the meter body. A larger metal case would partly overcpme this but this may not always be convenient. Our prototype showed good sensitivity and detected a 5-watt VHF handheld transceiver at a distance of 30 metres. We also checked the unit using both cordless and cellular telephones, with good results. SC Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form l ----------Enclosed is my cheque/money order for $_ __ or please debit my O Bankcard O Visa O Mastercard Card No: Card Expiry Date _ _/_ _ Signature _ _ _ __ _ __ _ Name _ _ __ _ _ __ __ Address_ _ _ _ _ __ _ __ _ _ _ _ _ _ _ P/code _ __ .. _ _ _ _ _ _ _ _ _ _ _ ,1 SEPTEMBER1992 87