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What is a groundplane antenna? By PHIL WATSON The term “groundplane antenna” often means different things to different people. There are two quite distinct antenna designs under this heading, a myriad of variations in between and lots of confusion as a result. T HE GROUNDPLANE antenna is probably the best known and most commonly used of all transmit­ting antennas, both in commercial and amateur roles. It is omnidirectional, simple to construct, uses low-cost materials and is equally suitable for base or mobile use. In its basic form, it consists of a quarter-wavelength vertical radiator, mounted above four quarter-wavelength horizon­tal radials spaced 90° apart. These horizontal radials form the so-called “groundplane”. This type of antenna is fed via coaxial 66  Silicon Chip cable, the inner conductor going to the radiator and the outer braid to the groundplane assembly. This is the configuration with which most amateurs will be familiar and it sounds simple enough. But confusion begins imme­diately we tackle the task of matching the impedance of this antenna to the impedance of the transmitter. Nowadays, by conven­ tion, transmitters are designed to work into a 52Ω load and to be connected to the antenna via a 52Ω coaxial cable. In practice, this means that the antenna should provide a 52Ω load. In reality, very few antennas provide such a load naturally and the ground­ plane is no exception. As a result, we have to modify the antenna, or the coupling to it, to present the transmitter with the correct load. There are many well established ways of doing this but first, we need to know the natural impedance of the antenna, the mismatch that this creates, and the best way to correct it. So what is the natural impedance of the groundplane antenna? Put this question to most amateurs and nine times out of ten they will nominate 36Ω, a figure that’s frequently quoted in the textbooks. However, as more than one amateur has learned to his dis­may, any attempt to develop a matching system based on this figure is doomed to failure. So is this a case where theory and practice don’t agree? This is where things become confusing. Groundplane development In order to better understand the problem, let’s first take a look at the groundplane’s history and clarify some of the published data. The groundplane antenna evolved from the basic half-wave, centre-fed antenna; ie, a half-wavelength long radiator, broken at the centre to form a feed point. A half-wave centre-fed anten­na has a natural radiation resistance of 72Ω, may be polarised horizontally or vertically, and is a very efficient antenna in its own right. This type of antenna is most popular in the horizontal mode, particularly at the lower frequencies. It isn’t used as much in the vertical mode because it would be impractically long at low frequencies, while the centre-feed requirement is an awkward arrangement in some applications. The original groundplane antenna was designed to overcome these limitations. The radiator was reduced to a quarter-wave vertical element and this was mounted above a large conducting surface. In theory, the quarter-wave element is reflected by the conducting surface, thus providing the other half of the antenna which would thus be equivalent to a half-wave centre-fed system. Theory also suggests that the reflecting surface should be infinitely large and have zero resistance. In practice, the Earth itself serves as the reflector and although it isn’t perfect, it can be made very effective. Various tricks are often employed to enhance its performance, such as selecting a moist area of ground area and burying wires in the ground, radiating outwards from the vertical element. The theoretical radiation resistance of this type of antenna is 36Ω (ie, half 72Ω) but, in practice, this varies according to the efficiency of the groundplane. Typical examples are the antennas used by radio stations in the broadcast band. A single mast acts as the radiator and, in its simplest form, is a quarter wavelength long. However, the length may vary, with some installations embracing the five-eighth wavelength concept or some other means to control the radiation angle. Often, the mast is located above moist or swampy ground, into which many radials are buried. OK, so that’s the background to what might be called the “original” or “earth” groundplane; names deliberately chosen to avoid confusion as we progress. It is popular with many amateurs, particularly for the higher HF bands – up to 30MHz – where the physical size of the radiator is more manageable. However, it does have a disadvantage. Although fine for use out in the country, in the traditional 40-acre paddock with few nearby obstructions, it is less attractive in suburban backyards which are often surrounded by buildings on all sides. And as we go higher in frequency and the radiator becomes shorter, these obstructions become more and more detrimental. The elevated groundplane It’s here that we come to another version of the ground­plane antenna. Known as the “elevated groundplane”, this is the version that’s most familiar to amateurs working at VHF. Its development is usually credited to Dr George H. Brown and J. Epstein of RCA and took place around 1938, when interest in frequencies above 30MHz was increasing rapidly. As mentioned earlier, it consists of a quarter-wave verti­ cal radiator and four quarter-wave horizontal radials, emanating from the base of But at least there was agreement on one point; the elevated groundplane has a lower impedance than the original groundplane and this was recognised by its creators back in 1938. They meas­ured two values: 25Ω for one version and 21Ω for another. Figures like this continued to be quoted for many years, with some writers having a bet each way by quoting 20-30Ω. What appears to be one of the first references to a realistic value is in the “RSGB Amateur Radio Handbook”, Third Edition (page 365), where the value is quoted as being “less than 20Ω.” Later, in the “RSGB Radio Communication Handbook”, Third Edition (page 12.81) is what appears to be the first mathematical explanation. In simplified form, this states that it is the theoretical value of a dipole feedpoint (73Ω) divided by four, or 18.25Ω. It adds that measured values are usually a little higher. A later (6th) edition of this handbook expands on this theme. It quotes the dipole feedpoint impedance as the more usual 72Ω, thus making the calculated value 18Ω, and provides Any attempt to develop a matching system based on an impedance of 36 ohms for an elevated groundplane antenna is doomed to failure. the radiator. The radials form an artificial groundplane which is no longer earthbound, allowing the com­ plete antenna system to be mounted high above surrounding obsta­cles. And so the scene was set for confusion, with two somewhat different antenna configurations using the same name. Granted, one evolved from the other and for the most part, their be­ haviour is similar, even when it comes to the angle of radiation. But one characteristic of the two antennas is significantly different – the feedpoint impedance. So what is the feedpoint impedance of an elevated groundplane antenna? This is a figure that has been difficult to accurately pin down. Indeed, one might take the cynical view and say that it depended on the last reference consulted. a more detailed explanation as to why this value may vary somewhat in practice. So that’s the basic background to the elevated groundplane antenna and, in particular, its feedpoint impedance. And, if it appears that this point has been unduly laboured, it was for a very good reason – to put to rest the confusion over feedpoint impedance that’s occurred over the years. This confusion has arisen because many well-known publica­tions and textbooks have failed to recognise and make clear this all-important distinction between the two antennas. And at least one textbook has positively stated that the (elevated) ground­plane, clearly portrayed diagrammatically, has an impedance of JUNE 1999  67 30-35Ω. Not only that, but it goes on to describe a matching stub, based on this figure, which is supposed to match it to a 75Ω coaxial cable – this some 30 years after the inventors, Brown and Epstein, had suggested a value as low as 20Ω. Practical considerations But the situation is really quite clear. The original or “earth” groundplane has a theoretical feedpoint impedance of 36Ω and a value close to this figure can be achieved given a favour­able situation and an elaborate setup. Otherwise, the value may vary considerably. On the other hand, the elevated groundplane has a theoreti­cal figure of 18Ω and this value or one very close to it can also be achieved in practice. Between 18Ω and 20Ω is a frequently quoted range but the writer’s own experience suggests that calcu­lations based on 18Ω work out to be extremely close. Having said that, it is necessary to but may call for more attention at HF. At 14MHz (20 metres), for example, the required clearance would be 10 metres. Matching problems For now, let’s settle for the true elevated version and accept an impedance value of 18Ω. Unfortunately, this is not exactly a convenient figure when it comes to matching the 52Ω impedance of the transmitter and the associated coax cable. Indeed, it represents a mismatch of nearly three to one (2.88:1). And that brings us to the practical side – how do we match the two? Broadly speaking, there are two possible approaches: (1) interpose a matching transformer (typically a quarter-wavelength of a suitable value coax), or (2) modify the antenna design itself so that it presents the desired impedance. The author has tried both approaches, with near perfect results in both cases. However, this article will con- By juggling the element dia­meters, we can continuously vary the feed impedance over a wide range. In short, we can design an antenna to have exactly the impedance we require. point out that there can be intermediate conditions between these two configurations. A typical example is the mobile version – a vertical quarter-wave radiator above a vehicle body as the groundplane. There are so many variables here that the impedance is anybody’s guess. It satisfies neither the elevated version nor the earth version. So how long is a piece of string? If in doubt there is only one way to find out; measure it and see. But that’s another story. Another variable factor is the distance between the elevat­ed ground­ plane and the true earth, and/or other conducting sur­faces. This should be at least 0.5 wavelengths, or greater if possible. The most likely effect of nearby conducting surfaces is to raise the impedance towards the 36Ω. Maintaining good separa­tion is not a difficult requirement to satisfy at VHF 68  Silicon Chip centrate on the latter approach, mainly because it is physically simpler but also because it has some advantages in its own right. In simple terms, the method is a variation of the folded dipole concept, except that it uses a folded monopole. This is in no sense an original concept. It has been known and used in both amateur and commercial circles for many years. However, it has never attracted much publicity. As is well known, a folded dipole has an impedance that’s four times that of a simple dipole – ie, 288Ω. This figure is usually rounded to 300Ω. The same applies to a folded monopole, which has a feedpoint impedance of 4 x 18Ω, or 72Ω. Admit­tedly, this is still not a perfect match to a 52Ω system but it is a good deal better than that of a simple monopole. In fact the error is now only 1.4:1. To digress briefly, this approach was used extensively during the early days of VHF mobile radio systems, mainly for base antennas. The transmitters of the day were designed for a 75Ω load, using 75Ω cable. The basic folded monopole presented an impedance of 72Ω; as near perfect a match as one could wish for. This approach to a 72Ω load require­ ment is suggested in the “RSGB Radio Communication Handbook”, 3rd edition, p12.82 (Fig.12.123(d)) and further confirms the 18Ω basic value. The 52Ω standard is not quite so easily accommodated but we have another trick up our sleeve. In its basic form, the folded radiator uses the same diameter conductors for both the active and passive elements. And in this form the spacing between the elements is not critical. But when we use different diameter conductors for the two elements, the picture changes. The spacing now becomes a factor in determining the feed impedance and by also juggling the element dia­ meters, we can continuously vary the feed impedance over a wide range. In short, we can design an antenna to have exactly the feed impedance we require. A formula and a graph, which can be used to calculate the design of a folded dipole, have appeared in several publications, including the “ARRL Antenna Book”, 14th Edition (p2-29) and this is equally applicable to the folded monopole concept. The formula is as follows: r = [1 + log(2S/d1)/log(2S/d2)]2 where S = spacing between elements d1 = driven element diameter d2 = passive element diameter r = impedance ratio As can be seen, in this configuration the formula solves the impedance ratio for any nominated combination of element diameters and spacing. Unfortunately, this is not the most con­venient way of going about things because, given the element diameters and the required impedance ratio, it is necessary to make a series of trial and error calculations to find the correct spacing. In practice, we would prefer to directly calculate the element spacing to give the required ratio, after first nominat­ing the element diameters we wish to use. These diameters will in ¼ λ Fig.1: basic concept of an elevated groundplane antenna. It consists of a quarter-wave verti­cal radiator plus four quarter-wave horizontal radials, which form an artificial groundplane, emanating from its base. turn depend on the material to hand or on what can be obtained. Unfortunately, transposing this equation so that we can directly calculate the spacing (S) is not straightforward. This problem was solved by sticking to the trial and error approach but letting a spreadsheet do all the calculations. This method was used to produce a list of ratios from given element diameters, with the spacing increasing in 1mm steps. Although this approach might seem a little clumsy, it works very well and was used for the practical design described below. Note that this calculation gives the space between the element centres. This means that, in some cases, the physical spacing between the two elements will be quite small when their diameters are taken into consideration. In fact, it may even be impossible to space them correctly, since the theoretical figure would require the two elements to overlap. The answer here, of course, is to recalculate the ratios using elements with different diameters. Putting all this theory into practice resulted in the fol­lowing dimensions for an antenna designed for 146MHz and measur­ing 470mm. Using a 2.89 (ie, 52 ÷ 18) multiplication factor and taking advantage of available materials, a prototype was con­structed using ELECTRONIC COMPONENTS & ACCESSORIES • RESELLER FOR MAJOR KIT RETAILERS • • PROTOTYPING EQUIPMENT • FULL ON-SITE SERVICE AND REPAIR FACILITIES • LARGE RANGE OF ELECTRONIC DISPOSALS (COME IN AND BROWSE) CB RADIO SALES AND ACCESSORIES Ph (03) 9723 3860 Fax (03) 9725 9443 Come In & See Our New Store M W OR A EL D IL C ER O M E ¼ λ a 5/16-inch diameter driven element (made of brass tube), a 1/8-inch brass rod passive element, and a spacing of 28mm between the outside dia­meters. And the result? Although the prototype was rather hurriedly constructed, it came up with an SWR ranging from 1.05:1 to 1.1:1 across the 2-metre band. So the theory and practice can be made to agree very closely. And had it been considered worthwhile, the spacing could have been juggled a fraction to come even closer to optimum. And that brings us to the other advantages of this arrange­ment, hinted at earlier. First, the folded element is inherently broadband, so rather than suffering any trade-offs with this arrangement, we actually score a bonus. Secondly, it is at earth potential in the DC sense, a valu­able feature where there is a risk of a lightning discharge. In this case, the discharge is directed directly to earth, rather than via the equipment. The actual construction details are best left to the indi­vidual and will vary with available materials and workshop facil­ities. Truscott’s ELECTRONIC WORLD Pty Ltd ACN 069 935 397 27 The Mall, South Croydon, Vic 3136 email: truscott<at>acepia.net.au www.electronicworld.aus.as References (1). Harold C. Vance, Sr. K2FF. “The Ground Plane Antenna: Its History and Development.” Ham Radio, January 1977, pages 26-28 (2). Amateur Radio Techniques, 6th Edition. Pat Hawker, G3VA. Pages 242-243. Published by R.S.G.B. (3). RSGB Amateur Radio Handbook. 3rd Edition. Pages 364-365. (4). RSGB Radio Communication Handbook. 3rd Edition Page 12-81 (18.25Ω) (5). RSGB-Radio Communication Handbook. 4th Edition. September 1968. Page 13-69 (20Ω or less) (6). RSGB-Radio Communication Handbook. 6th Edition. (7). Radio Handbook, 17th edition, 1967, edited by William I. Orr, W6SAI. Published by Editors & Engineers. Pages 359 & 407 Acknowledgements Many fellow amateurs contributed to this article. There are too many to mention individually but the following deserve spe­cial mention: W. A. (Blue) Easterling, VK4BBL (ex VK2­ ABL); I. Pogson, VK2AZN; A Walker, VK2ZEW; C. Wallis, VK6CSW (ex VK2DQE); J. Yalden (ex VK2YGY).SC P.C.B. Makers ! • • • • • • • • • If you need: P.C.B. High Speed Drill P.C.B. Guillotine P.C.B. Material – Negative or Positive acting Light Box – Single or Double Sided – Large or Small Etch Tank – Bubble or Circulating – Large or Small U.V. 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