Design Problems of Tuning Dials
September 1935 Radio-Craft

September 1935 Radio-Craft [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Craft, published 1929 - 1953. All copyrights are hereby acknowledged.

We tend to take for granted "standards" that have been in place and working well ever since they were instituted long ago. Some - maybe most - standards evolve over time with user preferences driving the end result; they tend to continue evolving. Examples include keyboard layout, advertising and product color selection, and test instrument front panel configurations. Other standards are driven by technology improvements. More and more often it seems, standards are being set by industry groups that want to assure interoperability and exchangeability amongst products and users. Often this kind of standard is driven by government imposed regulations. Wireless communications is a prime instance of the latter. This article is an example of a combination of standards motivators since it considers user experience preferences with scientific research to determine how best to implement a radio tuning dial. This was done nearly a century ago when a large portion of the world's population had never touched a radio dial or turned the knob of an instrument. Some of the findings were widely adopted, but looking at the huge number of radio that were produced since 1935 when this article appeared in Radio-Craft magazine shows that no recognizable standard ever was stuck to for radio dial layout.

Design Problems of Tuning Dials

By Wilhelm E. Schrage

"Short-sightedness" is not only an ocular disability - it is, according to the author, also a manufacturers' ailment!

While tremendous strides have been made in radio receiver parts during the last few years, constructors have given little attention to tuning dials. That such is the situation can easily be seen by comparing present European dials with those of American design. Our constructors seem to prefer the old orthodox "electric meter" dial in use since 1850! More attention is today given to tuning dial decoration than to actual scale design. As a matter of fact, the present decoration fad is being overemphasized so much in many instances as to occupy twice the square-inch area allotment given to the really useful part of the dial - that is, the scale!

It is high time for the radio engineer to break down the dictatorship of the cabinet designer, and demand a tuning dial which will allow the listener to discard the "microscope or magnifying glass" so often needed for selective station tuning (especially on short waves). Modern tuning dials serve to promote activity for the oculist rather than to serve as a restive agent for the eye of the radio listener. The present high-fidelity movement toward fulfilling all the wishes of the musical-trained ear, should be accompanied by equal solicitude on the part of the engineer toward the welfare of the eye.

The average radio buyer merely asks for faithful speech and music reproduction, together with a simple method of station tuning. In order to learn the kilocycle indication for tuning in the desired station it is necessary today for the listener to refer to the radio program column in a daily newspaper or magazine. However, after "finding the proper kilocycle indication, and dropping the last cipher," the listener then must go through body exercises comparable to his daily dozen in order to adjust the tuning dial. Such body and head movements, often consist of a 50 degree banding motion to the right or left, the angle depending upon the type of dial in use.

If the set dial is furnished with inclined numerals as in Fig. 1A, only the part indicated as "A" can be read without bending the head to right or left. Thus, about 50 per cent of the scale is ineffective insofar as ease of reading is concerned. Yet, surprising as it may seem, more than 22 per cent of American radio sets are equipped with this type of dial.

The same dial would be easier to operate if the numerals were horizontally arranged as shown in Fig.  1B. A scale designed in this manner may be used over the entire semi-circle. The suggested dial type must be of slightly wider diameter than those in present use; however, this demands only a cut-down of the decoration area customary at present.

Another important point concerning the dials used today, (and one which is worthy of change) is that of dial direction. The average table height is 31 ins., hence the tuning dial height averages 38 ins. above the floor, as shown in Fig. 3A. while the average eye is 64 ins. above the floor. At present it is custom to use the upper part of the circle for the tuning dial as shown in Fig. 2A, requiring the listener to stoop to a position which places the eyes on a level with the table top. Apparently the only reason for this archaic design is because of similar "electric meter" style in dials having been used by our grandfathers since 1850.

However, the use of the lower part of the circle as shown in Fig. 2B, for a tuning dial with an inclined scale will not give the desired satisfaction unless such inclination is to a certain degree accommodated to our eyes. An example of "unaccommodation" is shown in Fig. 3A, in which a radio set is standing on a table (average height 31 ins.), and with its tuning dial 7 ins. above the top. In this case if the radio receiver has sufficiently large numbers, and if the radio listener is long-sighted his eye will look upon the dial at an angle of 32 degrees. Should the listener be near-sighted (see Fig. 3B) the angle under the same conditions will be only 20 degrees. These angles of sight will be obtained if the radio listener is of average height, (eyes 64 ins. above the floor). Small deviations of a few inches more or less will not make a very great difference because of the comparatively short distance involved.

According to Fig. 4A. we have a variation in the angle of sight between 32 degrees and 20 degrees. The average angle of sight would then be:

(32° + 20°) / 2 = 26°

Upon drawing this angle in Fig. 4B. we find that the accurate inclination for our tuning scale should be 26 degrees to the table top.

However. a dial constructed according to Fig. 4B, with an inclination of 26 degrees will not fulfill its purpose if the numerals are not of a certain size. Since the radio listener does not like to carry a chair to the receiver, there to remain sitting until he has found the desired station, it is necessary for him to make some unpleasant gymnastic exercises (as shown in Fig. 3C) to tune in the desired station, because of insufficiently large numerals on the scale. This example shows the importance of providing tuning dials with large numerals.

In order to be of greatest ease to the eye the oculist has set the extreme minimum size of tuning dial numerals at 1/8-in. The numerals should be simple in design and the size of "d" and "w" (see Fig.  5B) should not be smaller than:

d = 0.13h

w = h / 2

Many dial constructors claim that due to the small size of the dial it is impossible to use numerals of this size. This problem, however. like most, can be solved. The simplest method to obtain sufficient space for use of larger numerals is that of constructing scales with a circle segment of 90 divisions, 100 divisions, or 110 divisions only as in Fig. 5A. This type of scale, with numerals printed horizontally, affords a full sight over the full segment without bending the head to the left or right. The distance between the scale divisions will become much greater, serving to give an effect of "greater" receiver selectivity. Division lines may then be employed with a thickness greater than those now in use, offering not only eye relief, but also helping to cut down adjustment expenses, and last but not least the "apparent selectivity" is greatly enhanced.

If the width "H" of the scale shown in Fig. 5A, is of sufficient size it is relatively easy to divide the dial into several parts; this will afford sufficient space for several wave ranges. Dials of the suggested type can be designed to meet any demand, and with the right inclination and proper size of the numerals it should prove to be a first-class advertising and selling argument. What has been said about the semi-circle dial applies also to dials well known under the designations "full vision," and "airplane type," since for scale readability it makes little difference whether the dial is designed in the form of a circle or a semi-circle.

Many of the full-vision dials in use today are badly constructed - over a space of a few square inches we find three or four wave ranges, so that the poor radio listener needs a guide to find his way through this labyrinth! From the point of usefulness, the dial design shown in Fig. 5A is best. However, if the cabinet designer believes at present that a full circle is desirable in order to secure some aesthetic effect. he should use a circular dial such as shown in Fig. 6. The dial shown in Fig. 7, often used today, utilizes only 180 degrees of the full circle; an expensive gearing is necessary if we wish to use the full 360 degrees. If there is a need for several wave ranges, and if numerals of a larger size are desired. more space must be given to the window.

An "Engineered" Dial

All these disadvantages can be avoided by use of what might be termed a "propeller" scale (see Fig.  6.) Two small windows are used one for the medium waves, the two together for police calls and the other one for two short-wave ranges. By use of a wave-range switch ganged with a multi-contact pilot-light switch, the correct-range windows can be respectively illuminated. The numerals are horizontal in any position of the scale, and if we use slender figures their size can be quite large without enlarging the disc diameter. Such propeller-like dials are very useful for midgets because they do not require a great deal of space, and so give the radio listener the full benefit of a good, legible dial; the dial can be made still more legible if a magnifying glass ("bullseye") is arranged in the window frame as shown in Fig. 8A. The indicator line can then be directly engraved into the glass.

If a good legible dial is desired, an important factor is the illumination of the scale especially as regards the color and material carrying the scale divisions. It is essential for the scale to be sufficiently well illuminated but not so strongly as to partially blind the eyes. One illumination system is shown in Fig. 8B. The front and the rear of the dial are illuminated. By using two small, low-voltage lamps instead of a single large one, a very pleasing effect can be obtained.

Color is another important consideration in dial design. At present most scale discs are made of celluloid or some similar material showing a brownish cast. Some manufacturers use this material for scale discs only because of low cost, yet no advertising manager would use a transparent sign of such color for he realizes the difficulty that would be encountered in reading and illuminating it. Only a white material printed with black divisions and numerals should be used for the disk. It might be useful to use blue numerals for the police-call range, and red ones for the short-wave range. The use of white, transparent figures and divisions on a black background is not be recommended, for a dial of this type has a tendency to dazzle the eye. "White on black" may, however, be used for large dials with a few divisions and numerals only - for example, in airplanes to avoid annoyance to the pilot who must look through darkness, but not for tuning dials on home radio sets. No electric meter manufacturer today would use these "negative" dials; ask any power station engineer whether he prefers a scale having a negative design and the answer will be "No!"

Dials made by stamping, after the first few hundred run off no longer show the clean-cut appearance of the earlier dials. A much better method of dial manufacturing, then, is that of photo-lithography. Scales made by the latter method have the distinctive advantage of uniform, clean and clear-cut divisions.

However, by the use of front illumination together with a magnifying glass (a third manufacturing method) the "photo-etched" type of dial is very satisfactory. These dials, which are opaque, are made of a white metal having a semi-polished surface. They are not costly if identical dials on quantity basis are needed and they give the radio cabinets a more expensive appearance.