The impedance meter is a versatile test instrument, whose capability can be utilised in various ways. Here are some of its applications:
To determine the characteristic im- pedance or the terminating load imped- ance of a transmission cable: Differ-
ent cables pose different impedances to RF passing through it. This imped- ance is independent of the signal fre- quency, if the load matching is proper. This fact is very important for the selection of cable for proper signal transmission. It helps to avoid reflec- tion of signal from the antenna or the input circuit back into the cable or vice versa. If the impedance of source, transmission line and load is proper, no signal reflection or absorption takes place in the transmission line and, above all, a transmission line of any suitable length may be used.
To use, connect one end of the cable to the bridge at Rx position by the short- est length of leads. To the other end of the cable, connect a known resistor of the order of 200 ohms.
Feed low-amplitude RF signal of any frequency from the signal genera- tor. Adjust the bridge pot. to get the null point, i.e. zero on the VU meter. As the reading may not be exactly zero
due to stray effects, take the minimum of meter as null point.
Let’s say the pot. is at 150 ohms at null. When you vary the signal genera- tor frequency, the VU meter will show variance in null position. Change the terminating load resistance to 150 ohms. Vary the frequency from signal generator again. The meter now will not move from its minimum position. (You might have to change a bit of resistance further but generally you get the matching in the very first instance.) Now at this position your imped- ance meter reads 150 ohms with any length of the cable and at almost any frequency, with a load of 150 ohms itself, and your cable is posing practi- cally zero resistance to the signal. This 150 ohms is the characteristic imped- ance of the cable, i.e. this cable will match exactly into 150-ohm load.
It will be found that a flat TV cable
gives the free null point with 300-ohm impedance while the round coaxial cable has 75-ohm impedance. Imped- ance matching of cable is important because then any suitable length of the.
cable (not necessarily an integer mul- tiple of half wavelength) may be used with the matched load.
Antenna and feeder matching:
Theoretically, a half-wave dipole an- tenna, fed at the centre, by a half-wave- length sized feedline cable (or an inte- ger multiple of the half wavelength) gives a proper antenna system with the minimum impedance. But, practically, a bit less is required for proper match- ing.
To ascertain this fact, connect the input of the antenna system (free end of feedline cable) to the impedance meter at Rx position. Feed low-ampli- tude RF and rotate the bridge pot. to get the null. The bridge reading gives the antenna system
impedance directly. As an example, 27MHz frequency an- tenna system should require 5.555 metres of feeder cable (of any characteristic imped- ance) feeding into a dipole with each arm
of 2.777 metres. But with the above- mentioned theoretical lengths, nearly 150-ohm antenna system was obtained at 27 MHz. With 5.10 metres (100- ohm type) flexible cable feeder and 2.70 metres each of balanced 16 SWG
wire dipoles, an antenna system of only 25-ohm impedance was obtained.
A final test was done by feeding in exactly 27.045 MHz (fixed) signal from a small transmitter of a toy car, and the above said impedances were confirmed.
Parallel and series tuned circuits:
Just connect the tunable circuit to the meter (in Rx position) by short leads. Let it be a series LC circuit. As we know, series circuit is an acceptor type of circuit and will provide the mini- mum (but not zero!) impedance at reso- nance to the signal frequency, which is given by the relationship
1 f =
2LC
Our aim is to tune the circuit so that it provides a minimum impedance at a particular frequency. Feed in the de- sired frequency low-amplitude signal. Say we get a null point at 150-ohm position. Move the pot. knob to a lesser impedance position, say 100 ohms. Re- tune the circuit, by changing the in- ductance or capacitance to come to the null position again. Come down fur- ther on the impedance scale and re- tune, and so on, till you get the mini- mum possible impedance. Increase the signal from the signal generator for a more pronounced null and more clear tuning.
If the natural frequency of the al- ready tuned circuit is to be established, connect as usual and vary the frequency from the generator till you have the minimum possible reading on the im- pedance scale. A final reading should always be taken with elevated signal feed.
Further, we know a parallel tuned circuit is a rejector circuit, as it pro- vides the maximum impedance to the
COIL DATA
Ciol Turns Former Descrp. Band Limits No. (1-2):(2-3)
1 AM osc. coil 10 mm IFT 0.4 to 1.2 MHz 2 15:150 6mm, Full core 0.55 to 1.8 MHz 3 10:60 6mm, Full core 1.6 to 5.5 MHz 4 06:20 6mm, Halfcore 4 to 15 MHz 5 05:8.5 6mm, Halfcore 10 to 38 MHz 1(i) 10:40 6mm, Full core 1.8 to 6.0 MHz 2(i) 06:15 6mm, Half core 5.4 to 18 MHz
Fig. 5: PCB layout for the AF-RF signal generator.
gave an impedance of 2000 ohms when the same very components were put in parallel, at proper resonance.
In rejection type circuits, the VU meter does not read zero exactly, due to reflected feed signals.
Tuning the output filter circuit: To
the output end of the filter circuit, con- nect a properly selected load resistor of the same value, as that of the an- tenna system or the load to be con- nected. It should be of suitable wattage (generally 50 ohms or else as required by the circuit design). Connect the in- put end of the filter circuit to the im- pedance meter (at Rx position). Apply low-amplitude RF signal of the desired frequency and obtain null. Move the knob to the desired impedance posi- tion, the same as that of the terminat- ing load (50-ohm position or else), and tune the filter circuit (through core or capacitor, as the case may be) till you get your meter to the null position again. Increase the signal level for an exact and final tuning. Now, your filter cir- cuit poses the minimum possible resis- tance at the desired frequency and the stipulated operating load, and hence has the best filtering efficiency.
Some more interesting applications of the impedance meter are suggested below, but you may try others yourself.
Measuring L and C: A bridge can
measure (in fact compare) inductances and capacitances. But our present meter has diminished accuracy due to the presence of filtering capacitors C3 and C4, following the basic bridge circuit. The way out is: replace 100-ohm resis- tor Rc with a fixed inductor. (For ex- ample, an ordinary radio IFT, without capacitor, has an inductance of around
PARTS LIST (for Fig. 4) Semiconductors:
IC1 — 555 of any series D1 — OA79 detector diode D2 — 9.1V zener diode TI,T2 — BFW10 (BEL) T3 — BF959 (BEL)
Resistors (all 1/4watt, ±5% carbon unless stated otherwise): R1,R2 — 100-kilohm R3,R4,R5 — 470-ohm R6 — 27-kilohm R7 — 330-ohm R8 — 10-ohm R9 — 1-kilohm R10 — 2.2-kilohm R11 — 220-ohm R12 — 47-ohm VR1 — 1-kilohm pot. Capacitors: C1 — 2J air gang C2 — 100pF ceramic disc C3, C4, C11 — 0.1μF ceramic disc C5 — 5pF ceramic disc C6,C7,C8,C12 — 0.01μF ceramic disc C9,C10 — 47μF, 25V electrolytic C13 — 0.47μF ceramic disc C14 — 4.7μF, 25V electrolytic Miscellaneous:
Sl — 1-pole, 2-way switch Coil 1 (L1) — One of the 5 coils (please
refer text)
L2 — 300 turns of fine copper wire on 6 or 8mm dia former.
— Output jacks etc.
600 μH.)
Now, you can calibrate your meter for various inductances and capaci- tances at Rx position, as you did for resistances. Of course, the scale will be direct for inductances and inverse for capacitors.
As mentioned earlier, the VU meter may not read exactly zero in this case too.
Check yourself the functioning of balun: The input of a TV set has 75-
ohm input impedance. But we gener- ally use 300-ohm antenna, with a 300- ohm flat cable. Sometimes, 75-ohm co- axial cable is used. Let us see how a balun converts impedances in such cases.
Connect 75-ohm end of the balun to the impedance meter. To the other (300-ohm) end of the balun, connect a 300-ohm resistor directly or with a piece of (300-ohm) flat cable. You will see that your bridge nulls at 75-ohm position, although the terminating re- sistor is of 300 ohms. It means the balun has converted the impedance from 300 ohms to 75 ohms.
Conversely, connect the 300-ohm end of the balun to the meter. Now, you will see that you have to connect 75- ohm resistor to the other end of the balun, directly or through a piece of 75-ohm coaxial cable to get the null position on the impedance meter at 300- ohm position, thus showing that the balun has converted the impedance from 75 to 300 ohms. tuned frequency. So, you have to go in
for the maximum possible impedance in this case. Rest of the procedure re- mains the same.
A 27MHz tuned circuit, with in- ductor and capacitor in series, and the secondary open, was tuned by core, and the impedance was found to be 5 ohms at resonance. The same circuit