British Standard 204 gives the following definitions:
Aerial (Antenna U.S.A.) That part of a radio system which is designed to radiate electro-magnetic waves into free space. This does not include the transmission line or waveguide to the radiator.
Directional Aerial An aerial designed to radiate more strongly in some directions than in others.
Beam 1. The radiation within a lobe of a directional system.
2. The region of space illuminated by this radiation.
Thus those who use the word "Beams" meaning "Beam Aerials" would seem to be out on a limb but they have my sympathy as one who has experienced trouble by disagreeing with the B.S. pundits. I prefer to define an aerial as a device which converts electrical energy into electro-magnetic energy. Electrical energy is under full control while being generated within a transmitter or while entrusted to a feeder but, once converted to a radiating wave, it is not under control at all.
At the point of conversion (the aerial) there is one last chance to influence the behaviour of the electro-magnetic wave in that we can have a say in which direction that wave will propagate and that brings us to the directional aerial. Theory and practice however are eternally uneasy bed-fellows.
To most Readers I am sure there is little new in this but, while almost all know the advantages of a directional aerial, few seem aware of its disadvantages or that many of the advantages are illusory.
This most simple of the aerials is also the most simple form of directional-aerial; it radiates its energy in two major lobes " ... one each side of the half-wavelength radiator ... " so all the Books say. In fact it is not true. Its major (and only) lobe is doughnut-shaped and it surrounds the radiating conductor for the whole of its length. The result is that energy is radiated with equal strength in all directions in a plane that is pierced by the conductor.
Thus a horizontal dipole sends a large part of its energy to irradiate the sky at those high-altitude angles where it is unlikely to return; because the ground acts as a reflector this "lost" energy can be greater than is at first apparent. There is an advantage of course in that the one aerial simultaneously covers all angles of launch and so serves for all distances.
A dipole produces zero radiation off the ends of the conducting radiator and so it is surprising that so often we get 5/9 reports from a Contact whose Station is in line with the aerial. Practical dipoles are usually fed from a coax feeder and, as a consequence, are not dipoles at all.
For a tongue-in-cheek explanation imagine yourself to be a packet of energy flowing along a coaxial feeder as an e-m field supported by the voltage between inner and outer conductors and by the currents which flow on the inner and on the inside surface of the outer. Arrived at the aerial-feedpoint the current from the inner is sent into one limb of the aerial and the current on the outer is sent into the other limb. At the far ends of the dipole there is nowhere for current to flow and so it bounces back to the feedpoint.
Current which flowed from the inner is returned to that inner but that which flowed from the outer can return either down the inside surface of the outer (whence it came) or down the outside surface. It is this last current flowing on the outside of the feeder, which explains why a coax should not be connected directly to a dipole unless the coax/dipole combination is required to operate as a top-loaded vertical aerial.
As a purely practical point it is a good idea to connect a coax in this manner if it produces results without causing interference.
For some mysterious reason there are those who insist on calling this aerial a "magnetic loop"; as used in paging systems (i.e. at low frequencies) the magnetic loop has the extremely useful property of limiting its radiation to a distance of a few feet unless the Receiver is within the loop. To be of use in radio communication a loop aerial must launch an electra-magnetic wave.
Once again the main (and only) lobe is doughnut-shaped and surrounds the length of the conductor. There is strong and equal radiation in all directions that lie in the plane of the loop and there is negligible radiation along the axis of the loop.
Used with the loop in a vertical plane (its axis horizontal) this aerial shows directional properties around the horizon and the two sharp nulls can be useful in removing interfering signals while receiving. One of its earliest uses was in direction-finding equipment. However, unless there is a specific need to limit radiation in a particular direction, there is no advantage whatever in using a vertical loop for transmitting. A loop does not concentrate available power in one direction and so it is not a directional (beam) aerial. The best manner in which to use a loop for transmitting is undoubtedly horizontal and it will function very well in all directions at any height starting from zero.
In the first half-hour in which we tested an experimental horizontal loop suspended at head-height from the shack roof we contacted Denmark (reported signal-strength 1), Berlin (5) and Greece (9).
Essential formulae for designing loops often give dimensions in terms of wavelength but the reason for this may appear obscure because a loop does not support a standing wave. The essential is that the loop is resonated at the operating frequency so as to maximise loop-current. To this end loops are often constructed with very-low values of series resistance by using materials such as copper pipe. The disadvantage of this technique is that the resulting very-high Q-value means that the bandwidth is very restricted and the aerial must be tuned accurately at each frequency in use.
It should be remembered that the advantage of maximising loop-current can easily be lost by failure to achieve the extremely-critical tuning adjustment and, in practice, there is an optimum Q-value.
This device, highly acclaimed by television engineers and Amateurs, is probably the most misunderstood aerial of them all but that does not make it the least useful. As defined by B.S. 204 it is an aerial consisting of one primary radiator and two or more secondary radiators ... which constitute an end-fire array ... Although specifically excluded by that definition the H-aerial is a logical step in the development of a yagi configuration.
The yagi uses a single driven element and, no matter how many, all the others are parasitic. This means that they draw their power from the driven element by irradiation and then they re-radiate. In theory backward radiation is eliminated by use of a reflector which may take the form of either another dipole element or a reflective sheet or grid. So far the aerial is just an H-aerial and possesses a cardioid radiation pattern.
The other elements are all placed in front of the primary radiator and their re-radiation fields add-to and subtract-from that of the driven element so as to reinforce radiation (and reception) in the desired forward direction and to cancel it in all other directions. These functions operate both in azimuth and altitude. The result is a narrow beam-like lobe which contains (nearly) all the radiated power.
Because all the radiation is concentrated in that single beam the aerial provides forward signal-gain; it does not amplify the signal but merely concentrates in one direction the power that otherwise would be broadcast. The more elements (directors) in front of the driven element so the greater is that concentration. Similarly, in reception, the aerial gathers irradiating power to give maximum signal response from the "forward" direction. Delightfully simple though it may seem that alas is not the whole story.
Interaction between the driven element and the parasitic elements causes the driving-point impedance of the dipole to fall with each added element (it is driving them all in parallel). To compensate for this the dipole is often replaced by a folded dipole; like the loop which it closely resembles this offers a driving-point impedance around 300 ohms and so the final impedance is closer to the 50 or 75 ohms that is required to match a coaxial feeder. The folded dipole also improves the bandwidth of the overall device but it still provides a balanced load and , as with the simple dipole, it does not function as required when simply tacked-on to a coax feeder.
In theory the more elements used to construct a yagi the narrower does that forward lobe become and the greater is the power which it launches in the forward direction (for a given input power) . Unfortunately, as the elements increase in number, so the bandwidth decreases; despite correction provided by the folded dipole the aerial is very difficult to compensate and can be brought within a given tolerance only for a few spot frequencies. This is why, when you buy a new television aerial, it is necessary to state the area in which it is to be used; should you move house there is little point in taking the television aerial with you.
Look at a yagi and you will see that the reflector is longer than the driven element and that each director is shorter but also that they become progressively shorter with increasing distance from the driven dipole. The object is to make the reflector inductive and the directors capacitive; combined with their relative spacings along the aerial boom this is part of the design to ensure correct phasing of their many re-radiation fields. At least that is the theory but what happens in practice ?
If the relative phasing depends on the complex impedance (R + jX) of each element and on the element-spacings it is evident that the overall impedance of the aerial is frequency-sensitive; hence the narrow bandwidth . I recently spent three days re-aligning the band-pass filters in a Racal RAI7 receiver which somebody had tried to peak. Each filter consists of eight LC circuits placed in a straight line and coupled only by their mutual fields which makes them an exact analogy of a yagi.
In my youth I was involved in an attempt to build signal-strength-measurement yagi-aerials in brass to avoid the corrosion problems of using aluminium aerials connected to copper feeders close to the sea ; the task bordered on the impossible. Any adjustment to a resonator alters the complex impedance reflected into all its neighbours, it changes the Q-factors of the circuits and it changes their coupling factors; tuning adjustments to any one circuit detunes all the others. In fact a Yagi aerial is a very complex piece of equipment and is very easily ruined.
[ Most of the techniques available for adjusting the LC filters described above are impossible to apply to the rod-resonators of a Yagi aerial ].
On a more mundane plane the highly-directive properties for which yagis are prized are also their greatest disadvantage. Once the number of elements exceeds three ( dipole, reflector and director) it becomes ever more crucial to aim the aerial with great accuracy both in azimuth and in altitude. However accurately this may be done there is a problem in that, when the wind blows, a yagi with a long boom will wobble and dance and produce an effect I have christened "wind-fading". I achieved an enormous improvement in our television reception by taking down the installed 14-element yagi, removing the front half and re-mounting the remainder so that it was mechanically balanced about the support pole
This eliminated the wind-fading partly because the wind-wobble became very small but mainly because the front lobe was broadened and so the direction of propagation was kept within that beam-width despite the wobble. Subjectively there was an increase in the average signal strength.
From the Amateur point of view the yagi is a trickster. It does not produce that clean forward narrow lobe which theory and advertisements predict and the forward lobe is not necessarily in line with the aerial boom. It produces also what are euphemistically called back-lobes and side-lobes; i.e. the reflector does not do a perfect job and the directors fail to get the relative phasings correct. Indeed even in theory the phasings can be correct only at a few spot frequencies. Side lobes can sometimes rival (even exceed) the forward lobe and back lobes are far from being insignificant as evidenced by that oft-heard amazing tale "He was coming in 5-over-9 and that was off the back of the beam ". More likely he was being received on the outside of the coax feeder. Test a Yagi that is wrongly connected to a coax feeder and you may find that much of its directional properties have disappeared
The front-to-back ratio of a yagi expresses the relative amplitudes of the (desired) forward lobe and of that back or side lobe which is closest to the reverse direction. It is a measure of the aerial's failure to do its designated job and is often confused with the forward gain. An aerial which produces back or side lobes cannot be pushing all the available power into the forward lobe and so the forward gain must suffer.
A problem that arises from this is that of knowing whether the aerial is aimed directly at the would-be contact or whether that contact is sitting in a side lobe. Of course it is possible to pop outside with a compass but that will not resolve anything unless the Great-circle bearing is known and the reception path does not involve a reflection? And perhaps the side-lobe is greater than the forward lobe?
One advantage of a directional aerial has been mentioned already that of losing a strong interfering signal in a null ; while a yagi can be very good at this it can also be very frustrating — it has many nulls, both sharp and shallow, but most of its lobes are sharp too.
The greatest act of self-kidding is that theoretical forward gain claimed as being up to 19-dB. If the aerial is relatively new and free of corrosion, if it has been correctly attached to the feeder and so avoids currents on the outside of the feeder, if it matches the feeder impedance at the frequency in use, if none of the elements are bent or broken or missing or corroded, if the aerial is correctly aligned on its target in both azimuth and altitude and if the receiver too is in good shape then that 19-dB may be realised. But such a gain represents around two S-points only. It is possible that more contacts can be achieved with a round-the-horizon aerial added to which a loop aerial is undoubtedly much quieter.
Amateur radio, even on vhf, began with dipoles and long wires and often involved the use of separate aerials for transmitting and receiving. During World-war II (so I am told) a Mr. Yagi tried to make a death-ray and since then his cult following appear to use money rather than sense, to lose as many contacts as they gain, to wipe out some rigs while remaining unaware of others and to use good engineering skills to erect unwieldy sagging doubtful arrays of bad engineering.
In the immediate post-war period the term "TV Aerials" meant an H-aerial and there were many who believed that the new-fangled television signals were H-shaped. With the move to shorter wavelengths the extended fish skeleton became fashionable. While it is true that the shorter uhf dipoles pick-up less signal and require boosting the proliferation of directors has more to do with profit than with engineering necessity.
In practical engineering there are no absolutes and parameters must be specified in terms of ranges over which tolerances must be met. Applied to the aerial this term Bandwidth refers to the range of frequencies over which the driving-point impedance matches the characteristic-impedance of the feeder within a specified limit. For example a match to within 10% may be specified over the range 3.5 to 3.8 MHz which would produce a s.w.r. of 1.1:1. The bandwidth is the same whether the device is used for transmitting or receiving but the results are different.
It is believed by many that an aerial mismatch does not matter so much when their cheque book can stand the shock of an aerial-tuning unit (properly called an aerial transducer unit) but they confuse the baby with the bath water. A good match is essential between a transmitter and its load (the feeder plus aerial) if the maximum amount of power is to be delivered; the Maximum Power-transfer Theorem states that load-impedance must exactly match the internal-impedance of a generator. When the driving-point impedance of an aerial exactly matches the characteristic impedance of the feeder then, at the other end of that feeder, the transmitter is presented with a resistive load that is normally 50-ohms.
When the aerial/feeder matching fails then, at the other end of the feeder the transmitter load becomes a complex impedance but, in itself, this is not a problem. The tuning controls of the P.A. cope with this variation but, in doing so, they either increase the voltage developed across the active device (either valve or transistor) or they increase the demand for current. Problems are few where the P.A. uses a valve but with transistor P.A.'s it is the Bank balance which comes under pressure. An A.T.U., once properly adjusted, protects the P.A. but it serves no other useful purpose; the one thing an ATU cannot do when housed in the Shack is to tune the aerial (long wires excepted).
When transmitting , a mismatch between aerial and feeder causes the appearance of a standing-wave on the feeder; this increases feeder losses but, at the power levels permitted by the Amateur Licence, it does little harm. A badly-adjusted A.T.U. may increase the standing-wave because it causes a re-reflection but it cannot reduce the basic reflection at the aerial. Thus an A.T.U. may reduce the bandwidth of an aerial but it cannot improve it unless it is hauled up the mast and connected between the aerial and the feeder.
In reception the story is different; a mismatch between aerial and feeder means that the aerial fails to deliver the maximum possible power to the feeder and a loss of overall sensitivity results. Now the "load" on the feeder is the input-impedance of the receiver and, because the A.T.U. produces (hopefully) a perfect match between feeder and receiver, a standing-wave does not appear on the feeder. However it can be advantageous to deliberately mismatch the receiver input-impedance and so cause a standing-wave to appear with the receiver terminals at an antinode (a node is a point of no-displacement — a minimum). Because of the conflicting requirements however this technique demands separate aerials for transmitting and receiving.
The radiation pattern of a compound aerial such as a yagi is designed and adjusted with the device matched to its feeder. With a mismatch there must be a phase-displacement between driving voltage and driving current which must be reflected in the aerial's performance. Not only is an A.T.U. unable to correct this but it might even make the situation worse. The wise will limit yagi-aerial building to the three basic elements and so preserve some knowledge of which way their signal is going. Use an A.T.U. by all means (I do) but not as a panacea for lazy engineering.
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