Wavelength Calibration

Radio Record, Volume IV, Issue 47, 5 June 1931, Page 31

Wavelength Calibration

Every Set its own Wavemeter

(An address by

F. W.

Sellens

to Short-wave Club.y

AFTER the owner of a short-wave receiving set has mastered the art of tuning and has heard several ef the most powerful stations, he is naturally anxious to know where to tune in for the others that he knows are on the air, but has but very little idea which coil to use or where to find them. Or again a station is heard, but the call-sign is not heard clearly enough to get it in full, and the operator is. left wondering who it is. If he had a means of arriv-

ing at the waveléngth of the stranger ‘he mystery would most likely be solved. The most satisfactory means of obtaining this information is by a correctly calibrated wavemeter, but the average short-wave. listener cannot afford an instrument of this descrip{ion, or would rather spend the money in improvements to his receiver, A wavemeter in its simplest form amounts to no more than a _ tuned circuit which has been calibrated in wavelengths, so that the wavelength to .every different setting of the tuntng condenser is known. With such an instrument available. if a station is tuned in on the receiver one has only to tune the wavemeter fo it, note the setting of the wave-

meter dial, and read off the wavelengtn of the station being received. Inversely, if one want: to hear, say, GSSW, which transmits on 25.55 metras, he has only to set the wavemeter to this and tune the set to the wavemeter. It is then also tuned to the required station. For those who like experimenting, and in consequence frequently alter, Improve ar rebuild their receivers. a wavemeter is almost a necessity. Those, however, who use their sets primarily as a source of entertainment and are satisfied to leave them unaltered for long periods. can take advantage of the fact that the ‘essentials of a wavemeter are incorporated

in every set, and can make their set act as its own wavemeter. To proceed with the construction of the calibration charts, one: will be needed for each coil, it will be necessary to procure several sheets of squared or graph paper. ‘The best for the purpose is ruled in tenths of an inch, each tenth line being ruled with a heavier line, thus facilitating the reading of the finished chart. . The sheets should be marked off along the foot in degrees, corresponding with the condenser dial scale,.which may read from 0 to 100 or 0-to 180, and along the left-hand vertical edge a scale of wavelengths, each chart according to the range in wavelengths of the coil which will have to be found by trial. , : Having the blank charts:.ready, we may now tune in the various stations of which the wavelength is known. Hach is tuned to obtain the fullest volume,. keeping reaction just short of oscillation. As each station is tuned in a note should be made of the dia! reading and the coil used. However, it would take quite a long ’time in the ordinary course of events to tune in enough stations sufficiently separated te be able’ to produce a reliable chart. Another means is open to those who live near enough to a broadcast station to be able to tune in its harmonics. In Wellington we have an excellent station in 2YA for this purpose. The first thing to do is to work out the harmonics of the broadcasf station 2YA works on 720 kilocycles, the harmonics being multiples of this, that is 1440, 2160, 2880 and-so on. It is a simple matter to convert kilocycles to metres; it is done by dividing kilocycles into 300,000. Seven hundred and twenty kilocycles (2YA frequency), therefore, is 416.66 metres. Working out the harmonics down to as low as your ‘smallest edil will tune; a list of 2YA’s harmonics. down to 10.16 metres is published herewith. "THE next operation is to tune in each harmonic, starting with the largest coil and working down to the smallest, making a note of each dial setting, with the coil used. It is ne cessary to take as much care in tun ing in the harmonics as one would a shortwave station. A good way toa get stronger signals on. the . higher harmonics-lower wavelengths-is to tune in a crystal set to 2YA and then

use the phones on the shortwave set. You will be better able to decide the exact dial reading this way. a After having prepared a list-of hax monies of 2YA for each coil, and as many shortwave stations of known wavelength as possible, we can then, start on our calibration charts, ‘ The approximate range of each call will be known by this time, so we will be able to mark off the vertical left.

hand edge in wavelengths for the first coil, the highest at the top, leaving ten squares for fractions of a metre, and then the next wavelength (i.e, a wavelength in even metre on every tenth line which, on the square paper recome mended, is an extra heavy line), and so on to the bottom of the chart. We then make a dot at the junction of the dial reading at the foot of the chart, and the wavelength marked on the edge for each station and harmonic, and then draw a curve through these dots, this will give you a calibration of this coil, The same procedure is followed for each coil. It is advisable to draw the lines lightly at first, and log as many stations as possible before finally ink ing in the curves, as quite likely an odd station will not be quite at its allotted’ wavelength, but I have found short-wave stations’ to be fairly reli« able in this respect, more so than some broadcast stations. Listeners in Wellington can depend on the harmonics of 2YA. I have made a number of charts at different times on account of so much experimenting with various coils, using their harmonics as a basis, and find they fit in pers fectly with the wavelengths of crystalcontrolled shortwave stations, giving a good. reliable calibration chart. With a carefully-drawn curve for each coil, you will be able to determine the wavelength, to within a very small fr..ction, of any station heard, by noting where the dial reading intersects. the curve, and glance to ‘the left,. where you will have the wavelength i. metres and tenths. On the other hand, if ‘itis desired to tune in a station of khown wavelength, this.is located on the chart, and the vertical line intersecting the curve will give you the dial reading. .