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• , tang of a small file until the capacitors could be secured with a press-tight fit and finally secured with a dab of "super-glue." This arrangement has pro ved quite satisfactory but purists might prefer to solder the capacito rs to a small rectangle of tin plate and bolt the assembly to the side of the aluminium box. The wiring-up of the meters and the dual potentiometer should present no difficulty. Sett ing-up O nce the un it is assembled, you must balance the bridge. Connect the tra nsmitter 10 one of the rear sockets via a short length of coaxial cable having the same impedance as the T X output. A further length of the same cable co nnected to rne other socket should be terminated by a non-inductive dummy load o f the same impedance. You can make this up from one o r more carbon resistors to obtain the resistance and wattage required. Now apply a rad io-frequency (RF) ca rrier at the highest used freq uency. One meter should indicate a hig h, and the other a low , reading . Adjust the trimmer capacitor on the side of the bridge showi ng a low read ing (reflected power) to obtain the lowest posslble reading . Now, re ...erse the connections to the sockets and make the same adjustment with the trimmer for the second meter. Repeat this procedure once or twice until finally both meters , whe n indicating reflected power, read ze ro , and the br idge is then balanced. During this process, adjust the potentiometer so that .....hic he...er meter is indicating forward po.....er is set at full-scale deflection. C alib ration-SW R You can use either meter for forward or reflected power indication , dependi ng on .....hich socket is used for input or output . Fo r SW R readings, both meters arc used and that showing forward po .....cr need s only to indicate FSO . For reflected po.....er, op inions differ on the need for detailed calibration . The most important marking is at a point exactly halfway across the scale , which represents an SWR of 3: I . Any SWR in excess of that may be detrimental to the transmitter. An SW R of 2: I or less is acceptable, especially with low po.....er. Therefore , you need only a center marking to indicate maximum permissible SWR. The aim is to get the reading down to as near to zero as possible . Calibrallon-c.Power It's easy to get forward power readings since the circuit pro ...ides a reasonably uniform indicatio n of RF po.....er, irrcspecti...c of frequency , o ...er its range ofoperation. Calibration modes, unfortunately , require an e xternal means of measuring RF power for comparison purposes. Those .....ho have access to RF measurement can set the forward meter to FSD .....hen the desired maximum RF power passes through it into a durnmy load. Note the setting of the pointer on the control knob by making a mark on the front panel . W ith the control at th is setting, feed successively lowe r RF powers through 46 73AmateurRadiO . June,1989 the un it and mark the meter scale accordingly. Operation Th e bridge should still be co nnected to the transmitter. Connect the output to an antenna system. Apply power and check the meters. Adjust the tuner for minimum SWR ( I : I if possible). Take care that the forward meter reads FSO, but that it isn 't pegging against the endstop, as that ca n damage the meter. If you are using an antenna system without a tuner, and the antenna is cut forthe frequency for the input signal, you may want to check the bandwidth of you r anten na. Do this by checking the SWR at diffe rent frequencies across a ha nd. This will often sho w you ho w to alter the antenna d imension s for a d ifferent resonant frequency . Lea...ing the bridge permanently in line lets you monitor the effect of the antenna system on the transmitter output. Bear in mind, however, thai a low SWR indication does not necessarily mean an antenna is performing wel l. A dummy load, for example, presents a near I : I SWR through a matching feeder, and yet is virtually non -radiating. Too, it measures SWR only at the point in the feedline where the bridge is located-not at the antenna itself. An SWR bridge is a ...aluable pa rt of every radio operator's station. It is a useful tool when constructing anten nas and exercises an essential control function whe n they are in use. Because of its simplicity it is an ideal project for home construction, especially for beginners, and .....ith good quality components it will last for years. QRP operation itself offers eno rmous scope for home construction and experimentation, and a unit such as this should be an integral part of e...ery QRP statio n. !w"wu41 n, 50~"" I n, J. 1 O,odn ~nde 'l..t r-' w '" I I .. Figure 4. Simple diode matching circuit. Match the two resistors with an ohmmeter. As the voltage i$ increased b)' rotating the potentiometer, the meter shouldnot deflect rrwre than I ~ from its no-curre11l setting. It may be necessary to test several diodes 10 obtain a matched pair. TRACK BREAK S AT : A4, 14 , 2 4 B4 , 24 C4 . 24 H" K' 0 . 14 . 18 Figure 3. Componens tocanons on the veroboard. Drill the mounting holes \ 'n ) ' carefully to avoiddamaging the board. Reslslora Carbon film V.W5% 330 2 Polenllometerl Oual-ganglKi 22'" , CapacltOf1l CeramiC ,-, n ...~, ~'l L> n, " 2 2 COMPONENTS - r n '" '" , ~ ~, " """ R3 06.1 04.5 C3 C1,2 I I- n op m ~ 'm' I Figure 5. Single meter version oflhe S WR bridge. u 21 23 2!t 27 Warning Th e unit as described is suitable only for low power operatio n. The circuit is capable ofoperation up to about 100 Watts. Taken from Practical wireless , October 1983. DI '.... FeeD-through '.' 2 Polystyrene 330pF 1 Miniature trimmer 2-22pf 2 SemlconduetOf1l [);ode QA91 Mi_Uaneoul R1,2 (Matched, see text) 01 .2 1Matched, see texl) Mater 1()()pA FS0(2); RF Choke 1.SmH (1); Toroidal COfe T68-2 (1); Veroboard 0.1 inch matrix 24 holes x 10 tfllcks; Metal box 73 .51 x25mm: Phono sockets (2); Pointer knob; Enamelled copper wire 28 s.w_g. (1.3m); Insulated wire 24 sow_g. (l00mm). rw Awm l
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• TH-75A WO 2m170cm Dual Band HT
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