Further Points on the Static Problem

Radio Record, Volume I, Issue 31, 17 February 1928, Page 3

Further Points on the Static Problem

Graphic Explanation of Various Phenomena

By

M.I.R.

E.

N continuance of a discussion relative to metheds available for the reduction of static interference with radio reception, there are several ~ €onsiderations worthy of mention in view of the fact that they are simply explained and especially because there are points easily understood by the average user of certain types of receiving apparatus which are capable of developing the particular syniptoms about to be described. As the readers. of the last two issues of this column will understand, the most effective method of reducing static, or in other words increasing the sigzal to static ratio in favour of signal, is to use an aerial capable of receiving only in the direction of the incoming signals and ta ‘be non-recep-tive in the direction of the static, static being aether waves just as the signals are, of course, if the static and the signals are both arriving from the same point of the compass, this method of sorting the one from the other fails entirely. It has been demonstrated that an 1, type aerial of special design will assist materially under certain circumstances, and in view of the fact that an aerial with a very small electrical height will be .quite effective in driving a multi-valve set, this is the most popular method where distant signals are aimed at, and the one it is advised should be attempted seriously. . THE LOOP PICK-UP. "THE simplest directional pick-up is the loop As is well known, the loop consists of a frame-work or "‘former" on which is wound a coil of wire. This wire may be wound in the form of a spider-web in such a manner that each successive turn has a different diameter, that is, one turn inside the other, or it may be wound with the wires lying side by side and each turn having the same diameter. All designs of loops ‘are made, so that the whole framework can be rotated, and the coil of wire caused to point in such a directio as to bring in the loudest signals. When rotating the loop it will be found that there are two places 180 degrees apart where the signals rise

to a maximuim, and under certain cireumstances, of these two positions, one will be found to give somewhat better .results than the other. Theoretically the two positions should give equal results, and the reason why they invariably do not is due to wellknown effects, which will be dealt with later. Without indulging in involved technicalities as to why a loop "‘points’’ towards the direction of the incoming signals, it may be stated that this action is based om the fact that a loop is responsive to the maguetic effect of the incoming ether waves, in contradistinction to the action of an orthodox aerial, which is responsive to the ‘feltctro-static’’ camponent of the waves, and is responsive at almost any angle. However, the net ftesult is as previously described, and the loop indicates two possible directions, diametrically opposite one another, from which the signals may be arriving. For navigational purposes it is mecessaty to find which is which, unless the bearings are so well-known that obviously one of them is the correct one, Ia order to find out which is which, or decide the "sense" of the signals, auxiliary apparatus is used and two readings taken, but this is of no interest from the point of view of static reduction. INTERESTING FACTS. ie is of interest in attempting a simple understanding of these phenomena, to lay the facts out graphically. If two circles of the same diameter are drawn in such a way that they are touching one another on their circumferences, it will be seen that a figure 8 has been drawn. Now, if the point where the circles touch is taken as the centre, and two lines are drawn at right-angles, one cutting the two circles in two, the points of the compass, north, south, east, and west, may be marked on thie straight lines, such that they form a standard graphical representation of a compass. ‘The circles now represent signal strength with respect to direction, and if, in the case of the graph just described, the loop is pointing ‘north and south, and responding to signals coming from a northerly or southerly direction, it will be noticed

.that nothing is received from an easterly or westerly direction. If it is desired to receive signals from either of the latter directions, then, of course, ‘the loop is pointed accordingly, and a rearrangement of the figure takes the form of two similar cireles drawn with their edges touching at the same spot as before, but their centres are now along the easterly and westerly lines. The lines of zero reception are now north and south. It follows, therefore, that rotation of the receiving loop rectuires a similar movement of the circles round the graphical points of the compass in order to describe what is happening. A loop is said to lave "Figure of Right" reception, because of this just-| described. performance. Now, it has been shown in previous discussions in this column that a@ TT aerial receives equalling well all round the compass. If a diagram of such a performance is drawn, a circle will have to be described, having its centre where the two lines and the edges of the "figure of 8" circles, all had a common point of contact. Obviously this circle denotes equal signal strength at a central point from a portable station movine in a circle round that noint. A T aerial is therefore said to have ‘Circle’ reception. , | "OVAL" RECEPIFEON, AN L aerial, by the same reasoning, will have "oyal’’ reception, but more of the oval will be towards the direction in which the down-leads come down. It.is unnecessary to point out that unless the I, aerial is of the type described in last week’s issue, that is, exceptionally long with respect to height, the oval will not be very narrow uor will it be definitely located more in one direction than the one immediately opposite, as it would have to be to represent definite directivity. I‘urthermore, if the aerial is moored in one positien the cval diagram will always be a fixed representation so far as a graphical picture is concerned. An interesting effect is now available | for description. A loop also acts as a miniature aerial because it is sticking up in the air a good distance in some. cases. Ihe receiver, batteries, and leads, |

-étc., act-as a couuterpoise or earth connection (in many cases no doubt an earth connection 1s in use). ‘This little aerial system picks up a surprising sigual strength, and it represents "circle" reception. In adding it to the graph a small circle is drawn over the ‘‘figuze of 8." It will be at once seen that there are two bulges which represent a leakage of signals (or static, which is what 1s most undesired) into the system from the two directiom which received nothing before. Also, if the graph 1s solved mathematically it will be seen that part of the little circle adds to the big circle, but the other subtracts, and the net result of this is to enlarge one of the circles forming ths figure of 8, and to diminish the size of the other. Thus it is seen that it is this effect which causes the loop to show a somewhat greater signal strength in one of the two positions at which it is receptive. COMBINING THE SIGNALS. APY ANTAGE is taken of this effect to further improve the fight against static by reducing the size of one of the circles, and this is done by deliberately combining the signals obtained from a loop and an aerial. It is necessary to "phase" the effects from the aerial in order to adjust the signal strength to be the same as that obtained from the loop in order that the size of the "circle" due to the signals from the aerial will have a radius equal to the diameter of one of the circles of the figure of 8 it is combined with. ‘This is done by inserting a naimeable resistance of several hundred ohms (this will vary according to wave-length) | in series with the aerial. The effects cs aerial and locp are combined by feeding a common sccondary attached to the receiver from the primaries, one being in the loop circuit and one in the aerial circuit. Actually the diagram of reception oLtained by combining circle with figure of 8 reception is a figure exactly the same as a heart. Such a combination is called "heart-shape’’ recep tion, The core of the heart is the centre and the heart revolves with the loop, and the pick-up of the combination is that of the loop and aerial add- ed together. By drawing a diagram of a lieart on a compass, as was done previously, it will be seen that sig-

nals (or static) can only come in from one direction, and this single direction is regulable by simply: rotating the loop. Of course, owing to the serics resistance in the acrial circuit (and this resistance, by the way, must be non-inductive) the aerial will not give its normal impetus to the receiver, but will deliver the same signal strength as the loop when "phased" correctly, and this means that the total pick-up of the system will be twice that obtainable from the loop, RESULT OF RE-RADIATION, QIGHT should not be lost of the fact mentioned in a previous article that re-radiation of signals from metallic or other objects in the vicinity of the receiver will cause loss ef definition of direction, and that the shorter the wavelength being received, the more indefinite will become ‘the direction. On standard broadcast wavelengths, however, the effect is quite marked, arid very considerable relief may be obtained from both static and induction. This effect of re-radiation is most noticeable if a loop set is in operation near to an aerial, and even if the aerial is out of commission by being ‘disconnected from a receiver or even earthed through a resistance to damp it, it will iuvariably be found that the maximum signals will be available when the loop is pointing to the aerial, and this direction may be at right angles to the true direction of the transmitter. A very simple method of getting a crude but effective ‘‘heart shape" is to join the aerial lead on to the loop either directly, through a series fixed condetiser, or by just laving the wire over the loop or receiver. There should be no earth connection from the receiver. Under these circumstances the loop will be energised by the acrial, and will give wonderful signal strength. There will still be two positions on the loop where signals will come in, but otie will not be as definite as the other. Under circumstances of bad static, reception will often be found possille under these circumstances, whereas with the aerial direct there will be nothing but clatter and on the loop alone. The strength will be too small for comfortable reception.