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• Number 13 on you, Feedbllck urd QRP SWR Bridge This simple project lets you monitoryour antenna system's SWR. by Tony Smith G4FAI L OW power (Q RP) operating enjoys an enduring popularity in amateur rad io. Much ofthe equipment is relatively simple to home-brew. and there isn't the need for many ofthe precautions required whe n operati ng at higher power levels. Opinion on what constitutes low power varies between different countr ies. and between different operators in those cou ntries. The G-QRP C lub's definitio n of less than 5 Wans input. and the (American) QRP Amaleur Radio Club International' s maximum of 5 Watts output. are the power levels referred to in this article. Transmitters need to "see" a specified load , usually 50 10 75 ohms . at their output. Whether the a ntenna in use is already matched to the requ ired impedance , or whether the match is obtai ned through an ante nna tuning unit (ATU). a standing wave ratio (SWR) bridge lets you monitor and adjust the effect of the load on the transmitter (Figure I ). (Many modem transmitters automatically reduce their output power if the antenna system presents a mismatch.) The bridge is thus a usefu l device to help obtain optimum perfonnance at all times. When a transmitter transfers power t o a fe ed e r l ine o f t he correct impedance. and the feeder terminates at an antenna also presenting the correct impedance , the antenna accepts and rad iates all ofthe power coming to it. When the antenna has the wrong impedance, e.g. because the feeder is not correctly terminated, a portion of r the power is reflected back down the feeder in the form of standing waves. The ratio between the forward power and the reflected power is the standing wave rat io, and the funct ion of an SWR bridge is to indicate that ratio at the point where the bridge is located in the feede r line. The Circuit The design shown in Figure 2 is a simple unit for QRP operation on all authorized frequencies up to 30 MHz , based on a toroidal transformer TI . The secondary winding of T I sam ples a small amount of RF power (both forward and reflected) which is d ivided by the bridge circuit and rectified by diodes 01 and D2. Forward and reflected readings are obtained simultaneously on the two meters M I and M2, and the bridge is matched and balanced at the required load impedance by CI 44 73 Amateur Radio • June, 1989 and C2 . See Figure 5 for an alternative. less expensive , single meter version. The bridge also measures forward power. Although it should not be regarded as a laboratory instrument, the bridge is sufficiently accurate for all practical hamming purposes. The project is housed in an easily constructed wood/hardboard case, partly to keep the cost down and partly to give the builder the satisfaction o f creating a completel y " home-brewed" unit. If you want to use a metal case, feed-through capacitors C4 and C5 are then unnecessary. Construcnon There are any number of ways to fabricate a case for the bridge. I built my case out of wood , using nails and glue to hold it together. I suggest d rilling the holes for the nails, slightly undersize, to keep the wood from splitting. The front panel is secured by panel pins and glue, and the top and rea r panels are secured by woodscrews to facilitate access and setti ng-up. In the prototype. the nails and panel pins " - ,., - A t>rl(lg ~ 0' ,., - t> ,,(lg ~ n l ~nna - lunIng uni t Figure 2. Circuit diagram of the QRP SWR bridge, " Ant ~nna were punched below the surface level o f the case and all gaps, holes, and irregularities made good with filler and rubbed down. The case. plus rear panel. was painted inside and out with matt black paint, and the top cover with black gloss. The fron t and sides were covered with Fablon" after the meter holes had been cut out. The holes for the meters were cut by marking the position of the meters on the front panel and drilling a se ries of small holes round the inside ofthe circle. The meter holes were then finished off with a half-round file. Exact details and measurements for meter and potentiometer mounting will depend on the particular meters obtained for the project. You can use virtually any meter having a . n l ~ nn a Figure 1. SWR bridge placement- keep the feeder length between the transmitter and bridge as short as possible. .... ------------ -------- - - -- - - -, • " 14S"G • ~<
The HF5B "Butterfly"'· A Compact 2 Element Beam for 20-15-12·10 Meter. Operale As A OJ-Pole on 11 Meter. Butternut Verticals Bunernuf s HF vertears use hoghesl-Q lun"...g Circuits (nOllossy Iraps') 10 outpenorm all mulhbar'ld des.qns 0' comparable size' Performance Above and Beyond AR2002 Continuous Coverage Professional Scanning Manilar Recelvtr 25 - 550 MHz/800 - 1300 MHz Model HF6V _80 .&0. 30. 2(l1!> and 10 ~ au_ uc I>;io n
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