US 2171256 A
Description (OCR text may contain errors)
E B. MOU LLlNi RADIO AERIAL Filed May 4, 1937 1240/4 mva rowm J L 2 INVENTOR ERIC BALL/0L MOULL/N BY 07% ATTORNEY Patented Aug. 29, 1939 UNlTED STATES RADIO AERIAL Eric Balliol Moullin, Oxford, England, assignor to Radio Corporation of America, a corporation of Delaware Application May 4, 1937, Serial No. 140,584 In Great Britain May 6, 1936 10 Claims.
This invention relates to radio aerials, and has for its object to provide improved aerials suitable for use for broadcast transmission, and in other cases where a direct ray service is required, and it is therefore of importance, from the point of view of power economy and of securing minimum interference, to minimize high angle radiation as far as possible.
It is Well known that an ordinaryvertical aerial becomes more efficient as its height is increased, this increased efficiency resulting from the diminution of high angle radiation which accompanies increase of the height at which the current node occurs. The efficiency or performance of an aerial (from this point of View) may be measured by means of a parameter K, termed height factor, and which is defined by the expression:
where P is the mean power radiated to produce a given field strength at an assigned place in the equatorial plane;
c is the velocity of light; A is the wave length; i is the root mean square current at a height 2 above the ground in the vertical portion of total height h; and K is the height factor and is a function of hr. Clearly if the height of aerial is so chosen that K is at a minimum, since P is proportional to K, the power required and the indirect ray communication are at minimum values. The general expression for the height factor of a vertical aerial having any current distribution is given by:--
that an aerial somewhat higher than and having the antinode of current as high up as possible is likely to produce a minimum of high angle radiation.
It will now be seen that by making the current distribution curve in an aerial system depart a T366 0 t 1 2 4) materially from the sinusoidal formfor example by so arranging matters that there is substantially uniform current in the upper portions of an aerial system and substantially zero current in the lower portionsit is possible materially to reduce the height factor. Mere changes in the geometrical form of an aerial conductor, e. g., variation of the diameter of an aerial wire over the length thereof are insufficient in practice to produce any material result in this direction; for example in those known aerial systems of the kind wherein a cigar shaped tower constitutes the aerial, the current distribution, though not strictly sinusoidal, is not sufiiciently difierent from the sinusoidal to produce a value of height factor materially below that appropriate to the sinusoidal distribution case. It would appear, in fact, that the natural current distribution curve in known aerials is sinusoidal or nearly so.
The present invention seeks to provide arrangements wherein the height factor is substantially below that for a known aerial with sinusoidal or approximately sinusoidal current distribution and this object is achieved by employing in combination what is in effect a plurality of aerials of different electrical lengths and carrying currents of such phase relation that the net or resultant current distribution curve, although the resultant of a plurality of component sinusoidal or approximately sinusoidal curves, departs materially from the sinusoidal when considered as a continuous curve along the whole length of the aerial. For example, one form of combination aerial system in accordance with this invention comprises a main vertical or approximately vertical aerial or aerial system of desired height in' combination with a subsidiary vertical or approximately vertical aerial or aerial system of less height and which is energized (either directly or by induction) in or substantially in antiphase.
The invention is illustrated in and further explained in connection with the accompanying drawing in which Figures 1 to 3 are current distribution curves illustrating the principle of my invention, while Figure 4 illustrates an embodiment of my invention and Figure 5 illustrates a modification thereof.
In one way of carrying out the invention, there is employed a combination system which produces the equivalent of a current distribution curve as shown in. conventional manner in Figure 1. This curve, it will be noted, is in effect the combination of an ordinary half wave distribution curve (as shown in the Figure 2) with the distribution curve for a quarter wave aerial having an antiphase current. Such a combination system might consist as shown diagrammatically in Figure 4 of a half wave aerial wire hung vertically or nearly vertically from a suitable mast 2 and a second vertical or nearly vertical wire 3 suspended by insulators at from a cross tree 4 on the mast and extending up for half the height of the half wave wire I. These two aerials are fed with antiphase currents of substantially equal magnitudes either direetlythis however may be in practice inconvenientor (as is the case represented in Figure 4) by radiation or induction from the main aerial which is fed from a transmitter (not shown) over a feeder represented by the line 8. It is important to note that where radiation or inductive feeding is employed, it is essential to tune the shorter aerial 3 accurately in order to obtain the required'antiphase energization and also to ensure that the current in the said shorter aerial shall be as nearly as possible equal to that in the main aerial and, in Figure 4, thesubsidiary aerial 3 is shown connected to earth at its lower end through an inductance s in series with an adjustable condenser .8 to constitute a variable tuning impedance.
The invention is thus quite distinct from those known aerial systems of the kind wherein simple Faraday cages are arranged to surround a radiating wire in any attempt to annul radiation from some desired part of its length, usually the bottom.
In a modification of the above described arrangement illustrated in Figure 5 a half wave tower la insulated at its base (at 9) is used as the half wave aerial. About half way up this tower is provided a surrounding insulated ring if! from which hang (say) six equally spaced wires 3 arranged round the tower and each earthed at the foot. These wires are accurately adjusted, i. e., tuned, either in geometrical length or as shown electrically (by providing adjustable impedance combinations 1' 8 like the combination l, 8, of Figure 4) so that there will be an induced quarter wave distribution in antiphase with the half wave distribution in the main aerial (the tower) induced in the cage constituted by the hanging wires. In Figure 5 1 some of the wires 3' are shown brokenaway to avoid complication of the figure (only two wires 3 are shown in full) and, as in Figure 4, the dots 5 represent insulators. The net result will be substantially equivalent to a distribution curve as shown in Figure 1. This curve is, as will be apparent, a great improvement over the curve of Figure 2 while it will be noted that the tower, with its dependent wires, forms a selfcontained and sensibly coincident system.
Numerical computation of the above equation giving the general expression for the height factor K has shown that whereas K is 0.4133 for a distribution curve as shown in Figure 2, the value of K drops to 0.32 for a curve as shown in Figure 1 and to 0.33 for a curve as shown in Figure 3. Thus the provision of the properly tuned and fed cage round a half wave aerial results in a saving of about 25% of the power and in eifect makes the half wave aerial nearly the equivalent of a %A aerial.
Although the invention has been particularly described with reference to its application to hal: wave aerials it will, of course, be appreciated that it is not limited to its application to this type of aerial.
Having thus described my invention, what I claim is:
1. An aerial arrangement comprising a vertical half wave antenna and means for increasing the effective height of the current distribution curve of said antenna, said means including a subsidiary vertical antenna extending parallel to said half wave antenna for substantially half its height and spaced therefrom.
2. An aerial arrangement comprising a half wave antenna tower and means for increasing the effective height of the current distribution curve of said antenna, said means including a subsidiary vertical antenna supported from substantially the center of said half wave antenna. and extending in spaced parallel relationship therefrom to the ground.
3. An arrangement as claimed in claim 1 wherein the main aerial or aerial system is constituted by a tower which is insulated at its base.
4. An arrangement as claimed in claim 2 wherein the subsidiary aerial is constituted by a cage of wires disposed about the main aerial.
5. An arrangement as claimed in claim 1 wherein the subsidiary aerial is fed by radiation from the main aerial system.
6. An arrangement as claimed in claim 2 wherein the subsidiary aerial is constituted by a cage of wires disposed about the main aerial and means for varying the tuning of said subsidiary aerial.
'7. An arrangement as claimed in claim 2 wherein the subsidiary aerial is fed by radiation from. the main aerial system.
8. An aerial arrangement comprising a half wave antenna tower and means for reducing the high vertical angle radiation therefrom, said means including a subsidiary vertical antenna supported from substantially the center of said half wave antenna and extending in spaced parallel relationship therefrom to the ground.
9. An arrangement as claimed in claim 8 wherein the subsidiary aerial is constituted by a cage of wires disposed about the main aerial and means for varying the tuning of said subsidiary aerial.
10. An arrangement as claimed in claim 8 wherein the subsidiary aerial is fed by radiation from the main aerial system.
ERIC BALLIOL MOULLIN.