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January 15, 2010, 08:09:03 PM
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The conventional resistors, inductors and capacitors exhibit their respective properties upto 30 MHz only; but above these frequencies, resistors have inductive reactance inductors have capacitive reactance and capacitors have inductive reactance due to the respective geometries of construction. Special techniques have, therefore, to be adopted if the resistor, inductor and capacitor are to exhibit purely their respective characteristics. Even the loads which are normally supposed to have no impedance, present and inductance(of the order of 0.0125) to 0.025 micro henry per inch) and assume higher resistance also on account of the "Skin effect". This applies even to the short leads of components like resistors, condensers, coils and the pin valves. A wire lead with resistance of only 10 ohms at 3 MHz becomes a choke with 1000 ohms reactance at 300 MHz. One method to overcome this defect is to choose the individual components and adopt such wiring methods as to reduce the undesirable stray inductance, capacitance or resistance.


Carbon composition type resistors of values upto 10K ohms do not exhibit any reactance upto 100 MHz. At higher frequencies, the effect of stray shunt capacitance increases and hence, the impedance decreases. The stray inductance of this type can be neglected upto about 100 MHz except when it is to be used as attenuators. An equivalent circuit of a resistor at VHF/UHF is shown in the Figs. The cracked carbon type is satisfactory upto several hundred MHz when mounting and connections are properly made. The wire wound type, even if non inductively wound, tends to become capacitive at higher frequencies. Some important features of resistors for use in VHF and UHF range are given below.

1. The dimensions should be as small as possible. A long cylindrical resistor with small diameter has better frequency characteristics than a short cylindrical one with large diameter.

2. Sudden linear changes of shape should be avoided.

3. They should be film type.

4. All connections am trade as short as possible.
At VHF & UHF frequencies as inductor can be represented as shown in Fig. 93. There is capacitance between the input leads as well as between adjacent turns as well as between any turn and gnound. Besides this, there is also a higher resistance of the leads due to skin, effect.

The condensers tend to become lossy with the increase in frequency.  An equivalent circuit of a condenser at VHF/UHF is shown in Fig. 94.  The power dissipation is high in the series resistor R.  This is normally due to increased impedance of the lead or due to contact resistance and skin effect.  The performance of electrolytic condensers is very poor at VHF and UHF  bands.

Variable condensers are required for tuning different circuits.  Some of the special requimments are:-

a) The vanes should be thicker and rigidlyfixed to prevent microphony.
b) The performance have contact forks which bear against slip rings on the rotor, must be very good to avoid noise, The materials mast be non-cormsive and should be shielded fmm dust.
c) The inductance of the earth ncturn circuit should be minimum. It should, therefox, be rigidly mounted, and not float-mounted,
d) They should be electrically screened to reduce stray couplings which may give rise to oscillatory conditions.

In the power stage, where a high performance of the variable tuned circuit is required, butterfly capacitors are used, in these, the same metallic surface act both as capacitor and inductor and hence, they can be used as compact variable tuned circuits. Fig. 95 shows a butterfly condenser. The stator plates are supported by a rectangular band which represents the inductor. When the notor is fully meshed, both the capacitance and inductance are maximum and the resonant freqqency is, therefore, minimum. When the rotor is fully out ofmesh,both the inductance and capacitance are minimum and the resonant frequency is maximum. Since the rotor requires no electrical connection, the necessity of conventional contact forks with its attendant faults is avoided.

The VHF transreceivers are generally employed for spur links orPore commonly in mobile communication. At present fully solid state portable transreceivers are available indigenously, as for example, GV 650, RP 3001, LVP 213 etc. However, certain portable transreceivers employ vacuum tubes only in the output stages, e.g., GH650. The remaining stages using transistors.
Another method is to use the. "Special quality" or "Ruggedized" valves which are expensive. In these types special precautions are taken during manufacture to ensure that the, valves will:-
a) have a life of at least 10,000-hrs. under normal operating voltages;
b) be highly reliable (e.g. only 0.15% special quality tubes will fail, say in 1000 hours)
c) work to very close  tolerances
d) have a greater immunity to shock or vibration.
e) be free from cathode interface,which affects the functioning of normal tubes when they have to work for long periods under standby operation, with filament power only supplied.
f) have filaments which will withstand numerous On and off cycles.
The performance of transistors falls off much more severely with rise in frequency than that of valves. Special types of transistors are being used in VHF.  The transit time of current carriers (holes or electrons) is one of the limiting factors. This is not due to the distance the carriers have to travel, but because of the characteristics conduction by diffusion which is a relatively slow rocess. This results in phase shift and. a loss. of current gain. The inter- electrode capacitance at both the junctions have also a shuting effect on the input and output, circuits introducing phase shift and feedback.  The collector junction capacitance, in particular, can introduce an effect similar to grid-anode capacitance of a.triode valve (viz., tendency to set up oscillatory conditions). This can be reduced by increasing the collector voltage or by neutralisation.

The cables, in VHF equipment are used not only for the conventional purposes of conveying signal,from one point to another, but also as quarter wave.impedance transformers (by cutting them to critical lengths)and attenuators. Twin wire with solid polythene dielectric at low voltages are used normally as such cables. The standard impedances are 50, 75, 150, 300 and 600 ohms. The most commonly used type is the co axial cable. It radiates the least power and picks up interfering signals to the least degree. The usual impedances are 40   50 ohms and 70   80 ohms, so that the diameter remains reasonably small.

The size of the cable to be chosen depends upon the voltage or power to be handled and the permissible attenuation, which is quoted in dBs per 100 feet of the cable. The voltage and power ratings increase while the attenuation factor decreases linearly with the diameter. The weight and cost, however, increase in proportion to the square of the diameter. But the cost of the plugs and sockets also play an important pan in the overall cost. The centre conductor may be single or strandard. In the case of former,  the losses are slightly less and voltage rating slightly high, it is also less likely to fracture the entry into the connector. On the other hand, the later can be flexed easily without the conductor.

The characteristic impedance of open wire and co axial cable are calculated when the physical dimensions and dielectric constant 'K’ known.

In an infinite transmission line no reflection takes place. A finite transmission line can be converted into an infinite transmission line by terminating it with a calculated value of impedance. This is called the characteristic impedance.

Terminations and Reflections: When a signal is travelling in any particular direction along a line its voltages and currents assume a ratio known as the characteristic impedance.

However, when the signal reaches the other end of the line where the load is connected  

(i) if the load connected happens to be exactly the same as the characteristic impedance. The signal happily moves into the load and it completely used up.

(ii) if the load is not exactly the same as the characteristic impedance, some of the signal turns back towards the source.

The amount that turns back depends on the mismatch.
« Last Edit: January 01, 1970, 04:00:00 AM by Guest »


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