The most usual type of aerial is the so called Marconi aerial which consists of a length of wire, one end insulated and the other end attached to the aerial terminal of the transmitter. A second length of wire joins the earth terminal of the transmitter to a connector entering the ground . In this type of aerial the main losses are due to the resistance of the earth connection and every effort must be made to obtain as good an earth connection as possible.
The rated power of a transmitter is the power it will deliver to a suitable aerial but the power delivered to the aerial is NOT the power radiated by the aerial, which is always less and unless the aerial-earth system is efficient may be very considerably less. The remainder of the power is dissipated as heat in neighbouring objects such as walls, etc and in the ground.
CURRENT DISTRIBUTION IN AERIALS.
If a long wire, insulated at one end, has the other end attached to the aerial terminal of the transmitter, an alternating current is produced in the wire, the amplitude of which varies along the wire. For a long wire the current reaches a maximum at a distance of a ¼ wavelength along the wire and then decreases.
Since the power radiated is proportional to the square of the current, it is clearly desirable to have at least one current maximum occur somewhere along the aerial. the shortest aerial which can be considered reasonably efficient is a quarter wave aerial.
|| 6 Mc/s
|| 12 Mc/s
|| 16 Mc/s
| WAVE LENGHT
|| 100 metres
|| 50 metres
|| 25 metres
|| 20 metres
| ¼ WAVE LENTH
|| 25 metres
|| 12½ metres
|| 6¼ metres
|| 5 metres
The resistance of the earth connection usually varies from about 10 ohms, obtained when the earth wire is soldered to a main water pipe near the ground to about 100 ohms, obtained from a moderate earth connection.
| Earth resistance in ohms
| Radiated power as a percentage
|| ¼ wave
| of the power in the aerial.
|| ½ wave
|| 1/16 wave
The amount of power that can be afforded to be wasted when using suitcase sets is small, it is therefore never any use trying to use a shorter aerial than ½ wave and this only in conjunction with a very good earth.
ERECTING AN AERIAL.
It is not usually practicable to erect a vertical ¼ wave aerial although this would be very efficient, but at least this length of wire and more if possible should be erected with a long vertical or rising portion and the top bent in some way towards the horizontal as in an inverted L. The exact length of wire is not critical as the transmitter is matched to the aerial in use during the tuning operations. The whole should be left well away from earthed objects as buildings, cliff sides, surrounding trees, etc and the end not attached to the transmitter should be insulated. In dry weather the rubber covering of the wire will be sufficient insulation, but in wet weather it would be better to use an insulator. An old bottle neck may be used for this purpose. If it is impossible to use an outdoor aerial, great care must be used to erect the most efficient indoor aerial possible. At least a ¼ wave length of wire should be used and this arranged high in the house, possibly in zig-zag fashion in the space amongst the rafters under the roof. Should circumstances restrict activities to one room the aerial wire should be arranged in zig-zag fashion across the room, about a foor below the ceiling, spacing the wires as possible, paying special attention to the fact that no part of the should run parallel to metal girders, electric wires, water pipes or spouting, nor should the wire be doubled back to itself at any point.
An efficient earth is most important. the ideal would be to solder a short length of wire to a large sheet of copper buried in moist earth near to the transmitter and to attach the free end of the wire to the earth terminal of the transmitter. Failing this, a copper earth tube, a large coil of barbed wire, an old oil drum well scraped, or some such metal receptical could be buried instead, but it is most important that where it is attached to the earth wire should be clean metal, a good electrical contact preferable soldered should be made and that the ground should be moist.
If indoors, a waterpipe may be convenient. Choose a cold water pipe near to the ground if possible, rather than a hot pipe which may be loosely attached to dry walls in several places before finally making a good earth connection. Scrape the pipe clean before attaching the earth wire.
If no pipes are available a length of wire in zig-zag fashion or a piece of wire netting may be placed underneath the floor covering and attached to the transmitter by a short earth wire.
An efficient counterpoise earth may be made by arranging a wire of about the same length as the aerial wire, and insulated from earthed objects underneath the aerial wire and 2 or 3 feet above the ground. If indoors the counterpoise earth should be on the floor, perhaps under the carpet and well separated from indoor aerial wire.
AERIALS FOR USE WITH SUITCASE EQUIPMENT AND IN PARTICULAR 3/II
The dipole aerial possesses considerable advantage over open wire types of aerial, particularly since it does not require an earth for satisfactory operation.
The aerial should be erected at least ½ wave length but preferably 1/3 wave high for good results, at distances up to 1000 miles. It should be ½ wave high for the best results at distances over 1000 miles. It is normally arranged so that the length of the aerial is at right angles to the desired direction of transmission. It does not matter greatly if one end is appreciable higher than the other. If it is not possible, for physical reasons to arrange the aerial at right angles to the direction of transmission and if one end is higher than the other, it is desirable to make the lower end towards the homestation. In other words, the aerial should slope down towards the homestation.
The main difficulty with dipoles on the lower frequencies is that sufficient span is seldom available to erect the necessary length of wire, e.g. 66 ft at 7 Mc/s. This difficulty can be over come without appreciable reducing the performance of the aerial by folding the wire in the manner illustrated below.
The dimensions A, B and C are not critical and if it is borne in mind that best results will be obtained when length A is approximately 1/3 wave length or longer and that dimension B should not be less than 1/3 of dimension C, then satisfactory results are more or less assured.
The aerial should be kept fairly symmetrical and the previous remarks about sloping apply
Dipoles may be folded more than once as illustrated in figure 5.
It is important to keep the current carrying part A-A, of the wire straight and so far as possible in the "clear" and it should be recommended that the extremities of the aerial are voltage points and should be fairly well insulated.
The transmission line presents little difficulty since ordinary lighting flex will give fairly good results compared with high grade transmission lines. For a twisted pair transmission line it is essential to use link coupling to the transmitter. This is readily done on the 3/I and 3/II transmitters, since these have exposed tank coils/
The table below shows the number of link turns for different frequencies.
TABLE OF LINK TURNS FOR TRANSMITTER 3/I
| Frequency (Mc/s)
|| Number of turns
| 3 - 4.4
|| L 1
| 4.5 - 5.8
| 5.8 - 8
|| L 3 or L4
| 7 - 9
|| L 4
|| L 6
TABLE OF LINK TURNS FOR TRANSMITTER 3/II.
| 3 - 5
|| L 1A
| 5 - 8
|| L 2B
| 8 -12
|| L 3B
| 12 - 16
|| L 4B
In case of coil L1A the windings are over the turns at the extreme left (earth) end of the coil and with coils L2B, L3B and L4B they start at the tapping near pin 2.
The following method will assist setting up when conditions are not well known.
First of all, one link turn may be experimentally wound round the earth en of the tank coil (L.H. side). The aerial matching condensers in the case of the 3/I and 3/II should be turned to maximum capacity, dial number 0 for 3/I and 10 for 3/II and then left alone. When the transmitter is tuned to resonance, if the number of link turns is too small then the amplifier cathode current will drop to a lower level than normal. The number of turns may then be increased until when tuned to resonance, the cathode current is correct (65-70 mA). In case of the 3/I, the aerial ammeter may be put in series with the transmission line to indicate maximum feeder current. When the feeder current is so high that the meter reading exceeds full scale, a short length of very thin wire connected across the meter will effectively shunt it so that a convenient scale reading is obtained. (3/II = A MkII)
END FED AERIALS.
Performance similar to that of a dipole may be obtained from an end-fed aerial arranged as shown in Fig.3. The length A should be ½ wave length long or sligthly shorter (0,95 of ½ wave length) if straight. It may be folded as previously described in which case the folding should be done at the high voltage ends. It may only be necessary to fold back the distant end of the aerial, but if the span available is restricted it may also be folded near the feeder,
The current carrying portion, which is approximately ¼ wave length long and in the middle of length A, should be kept straight. The feeder shown consists of a ¼ wave length of wire which goes to a link wrapped round the tank coil, the other end of which to second ¼ wave length of wire held parallel to the feeder, and insulated at the top end. This effectively eliminates the use of an earth.
The dimension C should be 0,98 of ¼ wave length as nearly as possible. The dimension B is not critical. It should be noted that the open ends of the wires as well as the knee of the aerial at the junction of A and C are voltage points and should well be insulated.
The remarks about direction and sloping concerning the dipole apply equally well to this aerial.
Matching is done in the same way.
The aerial shown in Fig.5 consists of an open wire of a total length approximately 3/4 wave length (A, B and C), using a counter-poise D. A counterpoise is preferable to an earth in this case since the low impedance of the aerial at the transmitter end would involve considerable loss when used with an average earth.
The length D should be ¼ wave length of wire suspended a foot or so above earth and since the far end of the wire is a voltage point, it should be well insulated. The maximum current point in the aerial will occur approximately ¼ wave length from the far end and this should be arranged to be as high as possible and well clear of trees, etc.
The method of folding shown in figure 5 is not essential and the aerial may be folded more or less as convenient. the remarks regarding direction, sloping, etc, concerning dipoles apply equally well in this case
It frequently happens when using high frequencies that a full wave length aerial can be conveniently erected. It should be noted that a full wave length aerial is markedly more directional than shorter aerials and has four main lobes. It should not, if at all possible, be erected so that its length is at right angles to the direction of transmission but should preferably be arranged so that it lies in a plane approximately 45° to the desired direction of transmission. Although the propagation from the ends is low, that is the transmission is weaker in the directions along the length of the aerial, the drop in power is not so great as in direction at right angles to the aerial, especially at distances under 500 miles. (see Fig.6)
AERIALS AT LOW FREQUENCIES.
At very low frequencies the length of even ¼ wave length of aerial becomes inconvenient to use. This may be partly offset by folding the aerial many times starting from the distant end, with a view to getting the current point, which occurs ¼ wave length from the distant end, up into a useful position. Thus when assuming at 3 Mc/s a ¼ wave length of aerial is approximately 80 ft. and if a span of 40 ft. at a height of 20 ft. is available it will be seen that only 60 ft. of straight wire can be used and the current point would actually occur inside the transmitter. In order to bring it out and put it at the top of the aerial, 20 ft. high, a total length of wire of 100ft. should be used folded up from the distant end so that approximately 80 ft. is used up in the horizontal span. This aerial would then be very much more effective, although occupying only the same space as 60 ft. of straight wire.
It is desirable to use a counterpoise with the aerial because of its low impedance at the transmission end. It may be convenient in some cases to use several counterpoise wires of 20 and 30 ft. connected to the transmitter earth terminal and radiating in different directions from the transmitter. Any earthed part of the transmitter, including associated battery and Power Pack, may be regarded as an earth terminal.
It should always be remembered that it is the current in the aerial that does the work and the effectivenss of the current depends on its height.