REPORT No. K.005. Serial No. 1933/26. APPARATUS FOR RECORDING SI AL STRENGTH.

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Ellen Ramsey
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1 .. ' Research Department. REPORT No. K.005. Serial No. 1933/26... Apparatus developed by T. SOMERVILLE. Drawing Nos: K.005.1to K APPARATUS FOR RECORDING SI AL STRENGTH. SUMMARY. To enable signal stren hs to be recorded for the study of fading by standard measuri g sets, an attachment for use with same has been designed and co tructed. This attachment enables records to be made with a recor ng D.C. milliammeter having a full scale deflection of 10 ma. he system used-is similar to that already described in Report N 730-A. TECHNICAL CONSIDERATIONS. ations: ( 1) apparatus was governed by the following considershould be capable of operating with any type of asuring set, or ordinary receiver; The characteristic of the rectifier operating the recording milliammeter should be linear; The output of this rectifier should be proportional to the incoming carrier and not to the modulation. the first condition.it was decided that some form of be connected by means of plugs and jacks should be provided, a this would make reversion to the normal method of measuring easy, the attachment to be applied to any receiver fitted with The requirements of (2) are satisfied by using full wave diode rectification, the valve having a characteristic whioh is straight exoept for a

2 -~small bend at the bottom. A fortunate coincidence is that the straight portion when produced passes through the origin. Requirement (3) is the most difficult to satisfy. Considerable amplification of the output of the S. T. &. C. superheterodyne measuring set is necessary in order to provide sufficient current to operate the recording milliammeter. A. D.C. amplifier is a possible solution, but was rejected in favour of a heterodyne method. A beating oscillator is used to beat the intermediate frequency on the grid of the second detector of the measuring set, the difference frequency being about leoo cycles, and the output of the second detector is then amplified before final rectification in the full wave rectifier. This method has the advantage that amplification is easily married out nt loco cycles, nnd it is nlso possible to obtnin grent selectivity nnd cut the sidebnnds by tuning the nmplifier to this frequency. However, the selectivity of the mensuring set, with which the nppnratus is normnlly used, is so grent thnt no tuning of the looo-cycle amplifier is necessary. When used with a receiver which is not a superheterodyne the beating oscillator is arranged to beat the oarrier at the detector, this being followed by the usual amplifier and rectifier. In this case, tuning of the 1000-cycle amplifier is essential to increase the overall selectivity. THE BEATING OSCILLATOR. The amplifier (Fig.2) consists of an L85 valve coupled by a transformer to the output of the receiver detector valve. The method of coupling will depend on the particular receiver in use, and will have to be varied accordingly. The choke and condenser (shown dotted), together with the inductance of the transformer form a circuit resonant at 1000 cycles to make

3 -3- the amplifier highly selective at that frequency. No values are given, for here again the particular. receiver will have to be taken into consideration. As previously stated, this resonant circuit is not used with the superheterodyne receiver with which the apparatus is normally used. The amplifier is coupled to the full-wave rectifying valve (mains type UU60/250) by a 1 : 1 ratio transformer with a centre-tapped secondary, and the recorder is connected as shown. A resistance of 1.0 ot~ is included in the filament circuit of the rectifying valve to drop the 6 V. supply to 4 volts. Fig. 3 shows the circuit actually used. Provision is made for listening actoss the transformer secondary by fitting a jack for telephones, because it has been found that the pitch of the note is a good indication of any changes,in tuning. Two other jacks are provided - one for the recorder and the other to allow a milliammeter to be used in series with it. Calibration is carried out with the milliammeter only in circuit, and the recorder is plugged up when recording commences. To prevent the addition of the recorder resistance from upsetting the calibration a resistance of equal value is connected across the jack to be in circuit when the recorder plug is out. CALIBRATION. The normal method is to inject into the receiver, across a noninductive resistance in the aerial circuit, a signal from a local signal generator equal to that induced by the carrier being measured, after the incoming signal has been removed by rotating the loop to the minimum position. The current output from the local signal generator necessary to produce this

4 -4- equal signal is measured by means of a thermocouple and galvanometer, and by multiplying it by the known value of the resistance the voltage across this resistance is obtained. Division of this voltage by the effective height of the aerial gives the signal strength. Expressed algebraically this is where E I - I r mv/m. h current through resistance in ma; r value in ohms of aerial circuit resistance; h affective height of aerial in metres (which is calculated). To set up the apparatus for recording, this formula is used in the form I E h...-- ma., r and I is calculated assuming a value of, say, 10 mv/metre. After the receiver is tuned to the station to be recorded, the loop is rotated to the minimum position and a current I (soe above formula) from the local signal generator, corresponding to a signal of 10 mv/metre, is injected into the resistance on the wavelength of the station in question. The gain of the receiver is now adjusted to give full scale deflection of the recording milliammeter, using the milliammeter in series with it. The local signal is then switched off and the loop returned to the maximum position, when the recorder has only to be plugged up and the clock started for a record to be made. A calibration curve of the recorder is shown in Fig. 4. It will be seen that as it is straight, and the straight portion if produced goes threugh the origin, calibration at one point is sufficient to allow direct readings to be made from the record. Due to the bend at 1.5 ma., it is usual

5 -5- in lining up for recording to choose the range of the calibration, taking account of the signal expected, so that most of the record lies between 1.5 and 10 ma. A calibration of 10 mv/metre for full scale deflection of the recorder has been found most satisfactory because it is usually sufficient to cover the peaks while recording the troughs in a satisfactory manner, but it has to be varied in accordance with circumstances. CURRENT SUPPLY. As this apparatus has to be carried in a van, weighty H.T. batterios have been replaced by M-L converters driven by L.T. batteries which also provide filament supplies for the apparatus. The output of these maohines is well smoothed, but it is necessary to fit additional condensers across the input to eliminate commutator ripple. FURTHER DEVELOPMENT OF THE APPARATUS: MEASURING IN MOTION. It has been noted at various times, and particularly while testing the Droitwich site, that it is often difficult to obtain reliable readings on sites at which there were no recognised causes of interference such as trees, telegraph wires, power lines, bridges etc. Moving the van a few yards is often sufficient completely to alter the reading, so that it became essential to take a number of readings round each point, reject abnormally high or low values and average the remainder. This increases very greatly the time required for measuring. To surmount these difficulties the recording apparatus was arranged to enable readings to be made with the van in motion. The conditions are similar to those experienced when recording, but

6 -6- in this case the recording milliammeter is unnecessary, its place being taken by a meter on the dashboard of the car. Before attempting to do measurements in motion, a different aerial had to be substituted for the loop, whioh is unsatisfactory on account of its directional properties. The aerial used is a vertical one between the top of the set and the roof of the van, and it is coupled to the receiver by a coupling coil which plugs into the loop socket. The primary of the coupling coil has two windings (representing the two halves of the loop), over which is wound a secondary winding connected to the aerial at one end, and to the frame of the set and chassis at the other. To use this vertical aerial a value must be found for its effective height. This is done by comparing it with that of the loop, because calculation is difficult, and the method of arriving at this value is as follows: The field strength E is measured with the loop in the usual way, and then a measurement is made with the other aerial, which gives a value for I in the formula I r E = h mv/m. As E has been measured with the loop and r is known, h can be oalculated. This value of h is then used to calibrate th~ apparatus as for recording, with the following exception. It is impossible to remove the incoming signal before injecting the calibrating signal, in the manner available with the loop, viz., by rotation, because the aerial is omnidirectional. The incoming signal can be removed by shutting down the transmitter after the galvanometer deflection has been noted, but this method is seldom convenient, so the signal is removed by

7 -7- de-tuning the receiver 10 k.c. The local signal is injected at this frequency, and after the calibration is completed the receiver is re-tuned to the station. The results are quite satisfactory, no noticeable error being introduced by de-tuning. Another difficulty which had to be overcome with a receiver in a vehicle in motion is the effect of mechanical vibration on the tuning. This was serious, but was cured by bracing the receiver internally and packing the valves with rubber. It was also necessary to arrange for all tuning adjust- ments to be clamped. SUPPRESSION OF ENGINE INTERFERENCE. Engine interference was very serious, the chief cause of noise being radiation from the H.T. ignition system. 3uppressor resistances were fitted in the leads at the plugs, and a resistance inserted in the H.T. lead from the coil t,o the distributor. With the car concerned, this cured the interference. DASHBOARD CONTROL. To enable the apparatus to be switched off and on from the dash of the car without stopping and entering the van, a relay unit controlled by a switch on the dash has been fitted. METHOD OF OPERATION. The apparatus is calibrated as previously described by the de-tuning method, the range of calibration being governed by the signal expected, and the operator then takes his seat beside the driver. When approaching the point at which a reading is desired, he switches on by means of the dashboard switch, and observes the variation in signal strength as he approaches and moves away

8 -.'.~ -8- from the point, observing simultaneously the local topography, and notes the average value of field strengths as the value round the point. high or low values are immediately apparent, and are rejected. Any abnormally This method effects a considerable saving in measuring time. This method of measuring was thoroughly tested out and used throughout the testing of sites for the Irish Regional Station, with entirely satisfactory results (1V~~ ~~ ~r,( S I<R J;-. TS!CHF

9 ... ' TO t:2eceivei< :., (WV''----Q' 3OOV+ L5:5B. ~----o.~. 06v+ ~ ~ ~~COMMO~- FIG.i. '..,' BEATING OSCILLATOR. B.B.C. I<ESEAI2CH OE~. D-aN. /4 /1 33 ~EPOC2\_ AP~D. K

10 , ooV+.. t: 1 " \.)U~O/2SO.TO I2ECEIVE~ o(hllo ~ECOI2DE ~-+~------~------~~-----COMMO~- Ci8.- FIG. 2. AMPLIFIER - J<ECTI~JE~~ SCHEMAT1C,. B.8. C. I<ESE,4,I:CCH DEPi icepol2i. ~ K.OOS.2.

11 ~ o300v+ I : I r~t=io=~ ]OT=LePHo",es. INPUT CI2CUIT TO uu.so!z5q ~2EC0I20EI2. L-~ ~ ~~&V+ ReSISlANCEEQUAI.. TO rzec:.oizdei2.. FIG. 3.. e. e. C.: ~ESeAI<CH OEI=>,... AMPL\F'E~- RECT'F'E~. OI2'N. '({K/33,eEPor;?"',. A'Pp'o,?<ifJ'~K. 0'05. 3.

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RADIO AMATEUR EXAM GENERAL CLASS

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