Amateur Bands .sx Amateurs are allowed to transmit continuous-wave telegraphy and telephony on small bands of frequencies which are usually referred to as the 1.75 3.5 , 7 , 14 , 28 and 56 megacycle bands .sx The 1.75 M.C. band is just below the medium broadcast waves and is useful for local British and European contacts .sx The 3.5 M.C. band is useful for local British , European and occasionally long-distance work ; amateurs call long-distance work " DX .sx " The 7 M.C. band is good for both fairly local work and DX ; the 14 M.C. band is mainly DX .sx 28 M.C. has provided surprising DX in the past but is extremely erratic ; it might be made more use of for local " round the town " contacts .sx 56 M.C. is now being largely developed for short-range telephony .sx The bands mentioned above are in harmonic relationship , i.e. , the frequencies are multiples of the lowest frequency .sx This is of great advantage in the design of crystal-controlled transmitters as will be seen later .sx Production of Oscillations .sx Having chosen the band of waves on which he intends to transmit the amateur must equip himself with apparatus capable of producing an alternating current of the desired frequency in his aerial circuit ; such " transmitters " in amateur work are invariably valve circuits , and the frequency is decided in practically every circuit by the size of the coil and condenser used in a tuned circuit connected to the valve .sx It is easy enough to produce high-frequency currents in a simple tuned circuit such as is shown in Fig. 5A .sx When the switch is to the right the condenser charges up from the battery and some energy is stored in it .sx Throwing the switch to the left disconnects the battery and allows the condenser to discharge through the inductance , and an oscillatory current will flow in the condenser-inductance circuit until the energy is all dissipated in resistance losses .sx The frequency of the oscillation is very closely where L and C are Henries and Farads respectively .sx The oscillatory current is a die-away oscillation called a " damped oscillation " because the resistance damps it out by dissipating its limited energy ; Fig. 5n is a diagram of such an oscillation .sx If the energy were not lost in the resistance of the circuit , or if sufficient additional energy were continuously supplied from somewhere , the current variations would not die away but would rise and fall to the same peak values and continue doing this as long as we wished .sx This would then be a " Continuous oscillation , " and the type that we must obtain for transmitting work .sx Suppose too much energy is supplied from this " somewhere " ; then the oscillations will build up in size and will continue doing so until the energy lost in resistance is equal to the energy supplied .sx Negative Resistance .sx By connecting an oscillatory circuit to a valve in the right manner we are able to supply the necessary energy for any oscillation to be ( a ) prolonged , or ( b ) built up in value .sx The supply of additional energy in this way produces the same effect as reducing the resistance of a circuit , and for this reason the feed-back is often referred to as " negative resistance .sx " There is an intermediate case between ( a ) and ( b ) , where any oscillation is maintained at its original size without decaying or growing ; this is the limiting case between ( a ) and ( b ) and cannot be obtained in practice .sx Reaction .sx Case ( a ) is very useful in receivers for increasing signal strength and improving selectivity ; it is known as " reaction , " and is the case where the feed-back of energy is kept below a critical value known as " oscillating point .sx " When the reaction is increased beyond this critical value we get case ( b ) where any oscillation is built up in value .sx This is known as a state of oscillation .sx The oscillations will grow very rapidly until limited by the conditions of operation of the valve , and for a given valve may be taken as proportional to the normal plate voltage .sx The switching on of the filament or plate supply is sufficient to provide the little initial disturbance which is necessary to set the valve oscillating .sx Valve Oscillator .sx Fig. 6 shows a simple case of reaction ; the plate and grid circuits are coupled by mutual inductance , and if this coupling is great enough the tuned grid circuit will have an oscillatory current set up in it .sx If an aerial circuit is coupled to L the oscillations will produce in it an oscillatory current of the same frequency ; the current will be a maximum when the impedance of the aerial circuit is a minimum, .sx and this state of affairs occurs when the reactance is zero , i.e. , when the circuit is tuned to that particular frequency .sx A high-frequency ammeter ( either hot-wire or thermo-junction type ) placed in the aerial circuit will have a maximum reading when the aerial is properly tuned .sx Aerial Current .sx Fig. 7 shows how the aerial current will rise to a maximum for the proper setting of the aerial condenser .sx If we could reduce the aerial resistance the current would increase .sx The resistance of the aerial circuit is composed of useful and wasteful resistance .sx Radiation Resistance .sx The latter comprises the " ohmic " resistance of the aerial , resistance introduced by eddy current losses , leakage , and all those things which produce only heating with the power they absorb .sx Useful resistance is called " Radiation Resistance , " and is an imaginary resistance which may be considered to account for the energy which is radiated into space .sx Suppose we knew that 5 watts of energy were being radiated when our aerial ammeter read 0.25 amperes .sx Then , since the power absorbed by a resistance is its value in ohms multiplied by the square of the current it carries , the radiation resistance of our aerial must be or 80 ohms .sx With the same aerial carrying 0.3 amperes the power radiated would be 80 ( 0.3)2 , or 7.2 watts .sx The radiation resistance measured at the centre of a horizontal half-wave aerial is about 80 ohms .sx The value for a Marconi aerial may be calculated approximately by formula .sx It will be obvious from the foregoing that most energy is radiated when the aerial current is a maximum .sx Let us leave aerials for the moment and return to the apparatus which is to energise the aerial .sx Oscillator Frequency .sx The frequency of the wave radiated from a simple oscillator as shown in Fig. 6 is fixed mainly by the frequency of tune of the grid circuit of the valve .sx For this frequency we had the formula , or , if you feel happier with wave-lengths , you have the formula for wave-length ( in this case the values of L and C are in microhenries and microfarads , but for the first case the values are in henries and farads) .sx Whether you prefer to work in frequency or wave-length it must be clear that the product LC is the deciding factor .sx When an aerial coil is coupled rather tightly to the oscillator tuned circuit , and both are tuned to the same frequency , the oscillator circuit will be found to have two frequencies of resonance ; i.e. , two frequencies will give the necessary " zero reactance " state , and the valve may therefore oscillate at one or other of these frequencies .sx On switching on it may start up on either frequency or , during keying , it may decide to jump to the other frequency .sx This makes satisfactory reception well-nigh impossible , and the aerial coupling should be loosened until the valve shows no tendency to behave in this manner .sx Frequency Stability .sx Any variations in the aerial circuit , for example , a swaying aerial , alter the capacity and hence the frequency of tune of the aerial circuit and the oscillator circuit which is coupled to it .sx The frequency of the signal radiated is unsteady and it may be quite unreadable at the receiver , besides being a nuisance to other amateurs .sx Vibration near the transmitter will produce the same effect , and all transmitters should be constructed as rigidly as possible and not situated just beside the key where vibration may occur .sx Another source of unstable frequency is the warming up of the valve during transmission .sx This causes the electrodes to expand and change the inter-electrode capacity which is in shunt with the tuned circuit .sx The slow change of capacity is equivalent to rotating very slowly the tuning dial of the oscillator and the wave will " creep .sx " A listener will have to follow your signals with his tuning and then , when you start up again after an interval which cools the valve , he will have to search for you again .sx One method of masking such capacity changes is to use a fairly large value of tuning capacity so that the changes in inter-electrode capacity will have much less effect .sx This method is popularly known as a " high C " circuit .sx High " C. " .sx When we make C large we must use a small value of L to keep the frequency right .sx The oscillatory current in a tuned circuit depends in value upon the ratio C/L , i.e. , the greater the tuning capacity for any given frequency the greater will be the oscillatory current .sx The disadvantage of this is that the watts of power wasted in the resistance of .sx the circuit depend on the square of the oscillatory current .sx To minimise this wastage , which produces only heat , we must use low resistance circuits and keep the oscillatory current low .sx That means a low C/L ratio which is exactly the opposite of what we need for stability of frequency .sx We must therefore compromise between efficiency and a steady note ; and a steady note is much the more important .sx A condenser of .sx 00025 microfarad used near its maximum value should be big enough if everything else is rigid .sx Anode Tap .sx There is no advantage in using a " high C " circuit when the frequency is determined by a separate oscillator , usually controlled by a quartz crystal , and in such cases we are able to choose a value of L/C ratio on efficiency grounds alone .sx The L/C ratio can , in effect , be varied by what is known as the " variable anode tap .sx " Fig. 8 ( a ) shows a tuned circuit connected to the plate of a valve .sx The opposition of this circuit to alternating currents at its resonant frequency is given by L/CR ohms , which obviously depends on the L/C ratio .sx To get the maximum H.F. output power from a valve the circuit into which it works should have the same opposition as the internal resistance of the valve under oscillating conditions .sx To match a circuit to a valve the L/C ratio must be varied , and the variable anode tap allows this to be done easily .sx When the anode tap is moved down the coil , as in Fig. 8 ( b ) , part of the inductance may be considered in the condenser side of the circuit , as in ( c) .sx The reactance of this inductance will subtract from the capacitive reactance of the condenser and give as a result a lower value of capacitive reactance for this side .sx This is equivalent to a bigger condenser value ; so by moving the anode tap down the coil we , in effect , increase the capacity and decrease the inductance .sx This keeps the product LC the same , and hence the frequency , but varies the L/C ratio .sx Self-Excited Oscillators .sx Self-excited oscillators are so called because the voltage variation of the grid is produced by the oscillatory currents in the plate circuit of the valve .sx This type of oscillator is inexpensive and may be adjusted to give good quality transmission on low power if every care is taken to obtain a stable note .sx It should not be used for anything but very low power work or ultra-high frequency transmissions .sx The circuit shown in Fig. 6 is not recommended as a practical transmitting circuit for reasons which need not be dealt with here .sx Self-excited circuits fall into two general categories depending on whether the grid excitation is obtained by ( a ) inductive coupling , or ( b ) capacitive coupling with the plate circuit .sx Each of these types may be sub-divided into several classes differing only in detail but not in general principle .sx There are many transmitter circuits and the beginner may be more than a little confused and uncertain as to which is best .sx