Ground Mounted Vertical Antennas For The HF Amateur

Ground Mounted Vertical Antennas For The HF Amateur Bands A presentation for The Newport County Radio Club February 8 2010 Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Some Different Types of Vertical Antennas Ground Plane Vertical Dipole Ground Mounted In this presentation we will talk about the Ground Mounted HF Vertical HF Propagation and Antennas © J King June 2009

Ground Mounted Vertical Antennas True Facts, Old Wives’ Tales, and Internet Folklore Which of the following statements are True? • Verticals radiate poorly in all directions (55. 1%) • All verticals are low angle radiators • Verticals are always better for DX than dipole antennas • Ground mounted verticals require very little space • Short verticals don’t work very well • Good, high performance verticals are expensive • Vendors are always truthful in their advertisements • TANSTAAFL ( There ain’t no such thing as a free lunch ) Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Why? Q: Why would we want to use a ground mounted vertical antenna? A: To radiate a low angle signal on the lower HF bands Q: Why not just use a horizontal wire dipole antenna? A: Horizontal antennas are effective low angle radiators only when they are mounted at least one half wavelength above the ground. This becomes increasingly difficult at the lower frequencies: Half Wavelength Heights 40 m 70 feet 80 m 140 feet 160 m 280 feet Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Why do we want low angles of antenna radiation? HF Propagation and Antennas © J King June 2009

Ground Mounted Vertical Antennas The Ionosphere Isn’t As High As We Sometimes Imagine HF Propagation and Antennas © J King June 2009

Ground Mounted Vertical Antennas Here’s an actual picture from space. Courtesy NASA Long distance communication requires very low angles HF Propagation and Antennas © J King June 2009

Ground Mounted Vertical Antennas 3λ Elevation Patterns for Various Vertical Radiator Lengths 1λ 3/4 λ 1/2 λ 5/8 λ 1/4 λ HF Propagation and Antennas © J King June 2009

Ground Mounted Vertical Antennas Before we delve further into Verticals, let’s take a look at the Dipole antenna: Transmitter Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Half-wave Dipole Antenna λ/ 2 Radiation Resistance (Rrad) = 72 Ω Feedpoint Impedance = 72 +j 0 Ω at the resonant frequency Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Understanding Radiation Resistance This 72 Ohms is easy to visualize… 72 Ω Transmitter Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Understanding Radiation Resistance …but this? 72 Ω Transmitter Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Visualizing Current Flow in a Dipole Antenna Transmitter Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Visualizing Current Flow in a Dipole Antenna Space Displacement currents (Aether) Transmitter Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Feeding The Half Wave Dipole Antenna λ/ 2 72 Ω 50 Ω Transmission Line VSWR with 50 Ω transmission line = 72 / 50 = 1. 44 Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Coax-fed Half-wave Dipole Antenna SWR and Reflected Power ½ wave Dipole, no matching 1. 44 Reflected power = 3. 2 % (0. 14 d. B) Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Ground Mounted Vertical Monopole Antenna Transmitter Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Visualizing Current Flow in a Ground Mounted Vertical Monopole Antenna Transmitter Zero Ohm Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Quarter-wave Ground Mounted Vertical Monopole Antenna ¼λ Rrad = 36 Ω Feedpoint Impedance = 36 +j 0 Ω Zero Ohm Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Coax-fed Quarter-wave Ground Mounted Vertical Antenna VSWR with 50 Ω transmission line = 50 / 36 = 1. 39 50 Ω Transmission Line 36 Ω Zero Ohm Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Coax-fed Quarter-wave Vertical Antenna SWR and Reflected Power ¼ wave vertical, no matching 1. 39 Reflected power = 2. 7 % (0. 12 d. B) Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Real ( Non – Zero Ohm ) Earth “Unstratified glacial drift deposited directly by the ice and consisting of clay, sand, gravel, and boulders…” Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Effect of Real Earth on Current Flow Transmitter Non-Zero Ohm Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Equivalent Circuit – Vertical Antenna Over Real Earth Rrad Antenna Radiation Resistance Transmitter Ground Resistance Rg Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Effect of Ground Resistance on Feedpoint Impedance Rrad Feed Point Impedance Rrad + Rg Rg Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Effect of Ground Resistance on Feedpoint Impedance As an example, let’s plug in 14 Ohms of ground resistance: 36 Ω Feed Point Impedance 36 Ω + 14 Ω = 50 Ω VSWR with 50 Ω transmission line = 50 / 50 = 1. 00 ! Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Effect of Ground Resistance on Antenna Efficiency 72 Watts 36 Ω Transmitter 100 Watts 14 Ω 28 Watts VSWR = 1. 00 Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Effect of Ground Resistance on Antenna Efficiency 72 Watts Radiated 36 Ω Transmitter 100 Watts 14 Ω 28 Watts Warm The Ground VSWR = 1. 00 Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Radiation Efficiency = = Power Radiated Into Space Power Entering The Feedpoint 72 x x 100 % 100 = 72 % Only the power dissipated in Rrad is radiated Power dissipated in Rg is wasted Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Additional Losses Rrad Conductor resistance Rant Rg Matching network resistance Rmatch Only the power dissipated in Rrad is radiated – the rest is lost as heat. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Radiation Efficiency = Rrad + Rg + Rant + Rmatch x 100 % We need to minimize these for best efficiency Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Minimizing Loss Resistances • Minimizing Conductor Resistance (Rant) • Use good conductors for the vertical element and all interconnections • Minimizing Matching Network Resistance (Rmatch) • Don’t use a matching network • Use high Q components in the matching network • Minimizing Ground Resistance (Rg) • Buy oceanfront property • Install radial wires on or in the ground This is the toughest nut to crack Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas RF Ground Systems Ordered by Effectiveness • Infinitely large, perfectly conductive surface • Salt water • 120 radial wires ( on or just below the ground ) • Between 20 and 120 radial wires • 20 radial wires • Several radial wires • One or more ground rods • Fresh water Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas RF Ground Systems The quality of the RF ground beneath a ground mounted vertical antenna will make or break the antenna’s performance. Unless a vertical antenna can be mounted our over salt water, a wire radial system will be needed to minimize ground resistance Rg. Since RF grounds are so important to antenna performance, lets spend a few minutes looking at ground radial systems. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Wire Radial Ground Systems Currents returning through low resistance wire All current seeks to return here Many radials Partial radial field shown Currents flow radially back to the base of the antenna Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Wire Radial Ground Systems All current seeks to return here Few radials Partial radial field shown Current returning through lossy earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Ground Radial Systems • Ground radials are not ‘tuned’ (Quarter-wave, etc. ) • Radials should be on, or just under the soil surface • The maximum effective radial length is about 0. 4 wavelength • Radials can be insulated or bare copper • Virtually any wire gauge is acceptable • Radials should be soldered at the common point • It is not possible to accurately predict the RF resistance of any given radial system. To make matters more interesting, it is very difficult, if not impossible, to precisely measure the RF ground resistance. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas What if the…. (pick one or more): ( Big House, Driveway, Pool, Dog House, Greenhouse, Outhouse, Wife’s Garden ) is in the way? Obstruction Run copper strap, tubing, or wire around the perimeter of the obstruction. Solder radials to the strap and extend the radials beyond the obstruction. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas My vertical is at the edge of my property. Extending radials onto the neighbor’s property is not an option. Can I put down radials in a 180 degree semi-circle? Will it change my antenna’s radiation pattern? First the good news: An asymmetrical radial field won’t alter the horizontal or the vertical radiation patterns of the antenna very much. (Rudy Serverns, N 6 LF, has done some excellent real world studies on this subject. See http: //www. antennasbyn 6 lf. com/ ) And the not-so-good news: All those displacement currents landing in the neighbor’s yard must return to the base of your antenna through your neighbor’s lossy earth. Rg will be higher than it would be with a 360 degree radial field. You can double up on the number of radials on your side of the fence to get ‘your’ Rg as low as possible. It is probably worthwhile to put even very short radials in the restricted direction if you can. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Ground Radial Systems Theoretical Ground Resistance for Radial Systems Over Very Good Ground ‘Your mileage may vary’ – and it probably won’t be better. Expect actual resistance values to be higher over average or poor soil, especially the values on the left side of the table. Table From “ON 4 UN’s Low-Band Dxing”, Devoldere Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas The FCC Standard Ground For AM BC Stations “A standard ground system consists of 120 copper radials, each 1/4 wavelength in length, buried in the earth surrounding the central tower (at 3 degree intervals). These radials look like the spokes of a bicycle wheel, but they are not connected at the outer ends. Each radial is electrically connected to the other radials at the bottom of the base insulator beneath the vertical tower section. ” Q: Is this the ultimate ground radial system for a vertical antenna? A: No. A system with more, or longer radials (or a salt water ground) would be superior to the FCC’s standard ground. The FCC’s standard ground system is sufficient to produce predictable antenna performance over widely differing types of soil. The FCC’s intent is to insure that the station’s coverage area is blanketed with a signal that will provide good reception. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas AM BC radial system – Example 1 Photo: AM Ground Systems Company Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas AM BC radial system – Example 2 Photo: AM Ground Systems Company Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas AM BC radial system – Example 3 Where’s the tower…. ? Photo: AM Ground Systems Company Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Photo: AM Ground Systems Company Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Amateur Ground Radial Systems It’s not hard to do it like the pros Home Depot has everything you need And you’ll save money! Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Amateur Ground Radial Systems Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Rules For Radials • There are few hard and fast absolute rules • More radials are better than fewer radials • Longer radials are better than shorter radials • One cannot have too many radials For a given length of bulk radial wire, there exists an optimum configuration of ‘n’ radials of ‘x’ length. Example: Given a 1, 000 foot spool of wire, twenty 50 foot radials would be a better use of wire than four 250 foot radials. References are included on the course CD. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas ARRL Survey Mar 14, 2008 How many radials do you have connected to your ground-mounted HF vertical antenna? My vertical isn't ground mounted I don't own a vertical antenna (532) (1149) Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas What About Elevated Radials? Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Elevated Radials • A vertical antenna with elevated radials is a Ground Plane (GP) Antenna • GP Antennas are efficient radiators when mounted high above the ground • How high is high? • That depends – on: • Frequency • Number of radials • Who you ask • A system of many (100+) radials can be efficient relatively close to the ground • A system with few radials ( 4 ) achieves maximum efficiency at 0. 25 – 0. 50 λ above the ground Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Elevated Radials – Height Above Ground These currents cannot ‘see’ the ground. All current returns through the radials. These currents can ‘see’ the ground. Some current returns through the radials, and some returns through the lossy earth. These radials are too high to couple current into the earth. These radials are close enough to the ground to couple some of their RF current into the lossy earth. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Elevated Radials • Size does matter • Elevated radials are tuned ( one-quarter wavelength ) • Large currents flow in few radials – requiring large gauge conductors • High voltages are present on the radials • Radials close to the ground are influenced by soil environmental variations • Moisture content • Frost depth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Elevated Radials • Does all of this mean that elevated radials don’t work at HF? • No. Elevated radials ‘work’ – but TANSTAAFL applies Credible and generally respected amateurs (ON 4 UN, W 8 JI, and others) have conducted real world tests comparing carefully constructed tuned elevated radial systems to their benchmark conventional radial systems. These real-world tests have generally shown that elevated radial systems (at modest heights) are less effective than mid-to-large sized conventional radial systems. An elevated radial system is a viable alternative to a conventional system if a conventional radial system cannot be installed, and is certainly better than no radial system. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Multi-band vertical antennas are available from many vendors. Some are good, and some not so good. As in everything else, TANSTAAFL applies. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Multi-band Vertical Antenna Transmitter ¼ λ 10 m ¼ λ 15 m ¼ λ 20 m Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Multi-band Trap Vertical Antenna Parallel Resonant Circuits 15 m 10 m Transmitter ¼ λ 10 m ¼ λ 15 m ¼ λ 20 m Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Multi-band Fan Vertical Antenna Transmitter Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas ‘Magic’ Multi-band Verticals Then… and now. THUNDERCLAP 43 Foot Vertical 160 -10 meter vertical multi-band! Attractive! Durable! Optimized! Slow taper! Massive fiberglass insulator! Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas ‘Magic’ Multi-band Verticals Q: Which of these Magic antennas is truly a multi-band antenna? This one? Neither? THUNDERCLAP 43 Foot Vertical Or both? Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas ‘Magic’ Multi-band Verticals A 1: Neither. Both of these ‘miracle’ antennas are simply lengths of aluminum tubing with a dubious ‘matching’ device at the base, to which one connects 50 ohm coaxial cable. Neither provides a good match to the coaxial cable. Neither antenna is resonant on any ham band. Both are fine examples of creative marketing. A 2: Both. Either of these antennas can be excellent multi-band antennas - if we replace the manufacturer’s ‘matching’ device with a real matching network. In a few minutes we will design a proper matching network for the Thunderclap vertical antenna. But first some background about vertical antenna matching in general… Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Antenna Matching A quarter wavelength vertical antenna presents a 36 Ohm resistive feedpoint impedance. This is a reasonably close match to 50 Ohms. We can connect 50 Ohm coaxial transmission line directly to the base of a quarter wave vertical antenna and accept the slight mismatch without any serious penalty. Vertical antennas of lengths of other than a quarter wavelength present feedpoint impedances other than 36 Ohms resistive. Some impedances may be very low or very high, and most will have a significant reactive component. These antennas require a matching network between the vertical element and the coaxial transmission line. The purpose of the matching network is to make the antenna appear as a 50 Ohm resistive load to the coaxial transmission line. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Antenna Matching Ground mounted vertical antennas are resonant at all multiples of a quarter wavelength: ¼ λ, ½ λ, ¾ λ, 1 λ, etc. At these resonant lengths, antennas present a purely resistive component – Rrad – at the feedpoint. There is no capacitive reactance nor inductive reactance at resonance. (More accurately, the capacitive reactance (XC) and inductive reactance (XL) precisely cancel each other out at resonance. ) As the length of the vertical element changes, a reactive component appears in series with Rrad. This reactance is capacitive (-j) for lengths between zero and one quarter wavelength, and inductive (+j) between one quarter and one half wavelength. Beyond one half wavelength, this cycle repeats. Rrad varies too, as we change the length of a vertical antenna. The 36 Ohm value of Rrad that we used previously is the value of radiation resistance for a ¼ λ vertical. At any other length, Rrad will be some different value. There is nothing particularly special about the quarter wave length with regard to vertical antenna performance. A quarter wave is, however, a very convenient length because we can directly connect coaxial cable to the feedpoint of a quarter wave vertical antenna. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Thinking in Degrees 1λ 0° 90° 180° 225° ¼λ ½λ 5/8 λ 270° 360° ¾λ 1λ Antenna engineers prefer to express antenna length in degrees, rather than wavelength because antenna math involves trigonometric relationships. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Radiation Resistance vs. Length 500 450 400 350 300 250 200 150 100 36 50 50 1/4 λ 1/2 λ 1λ 3/2 λ 2λ Courtesy: Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Reactance vs. Antenna Length Most of our verticals will follow this curve (1 inch diameter on 80 m) Inductive (+j) Capacitive (-j) 20 degrees is a really fat vertical! (15 feet on 80 m) Graph From “ON 4 UN’s Low-Band Dxing”, Devoldere Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Antenna Matching OK, enough of theory and graphs and funny symbols. Let’s tackle a real world vertical antenna matching exercise. We will use the 43 -foot Thunderclap vertical in our exercise, and to make things interesting we will choose 160 meters as our design frequency. But “Wait…” , you might ask, “…isn’t 43 feet awfully short for a 160 m vertical? ” Well, yes…. And no. A quarter wavelength (90 degrees) on 160 meters is 40 meters, about 130 feet. The 43 foot (13. 1 meter) Thunderclap works out to be about 1/12 wavelength or 30 degrees on 160 m. How effective could this antenna be on 160 m? We should take a brief detour here, and take a closer look at short vertical radiators. Hold on to your hat – some of this stuff is quite amazing. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Short Vertical Antennas Jerry Sevick, W 2 FMI (sk), startled many in the Amateur community with his 1973 QST article “The W 2 FMI Ground-Mounted Short Vertical. ” Dr. Sevick made the seemingly outrageous claim that the performance of a very short vertical – a one or two foot vertical on 40 meters – would be within 0. 07 S-units of a full size quarter wavelength vertical, both transmitting and receiving. Sevick made it quite clear in his article that he wasn’t plowing any new ground. The references he cited in his article were works that had been published more than a quarter-century earlier. In fact, antenna engineers had been successfully designing and building very short, yet effective VLF antennas a half century before 1973. Somehow, that word hadn’t gotten out in the Amateur community – or at least it wasn’t widely known. Most hams today remember Jerry Sevick as “the balun guy. ” His contributions to amateur antenna art are at least as important as his work on transmission line transformers (baluns. ) His QST antenna articles are very worthwhile reading for anyone interested in vertical antennas. Ground Mounted Vertical Antennas © J King Feb 2010

HF Propagation and Antennas Field Strength vs. Antenna Length From Radio Antenna Engineering, Laport, 1952 Mc. Graw-Hill HF Propagation and Antennas © J King June 2009

HF Propagation and Antennas Field Strength vs. Antenna Length 40 m 80 m 160 m 3. 5 ft 7 ft 14 ft 43 Foot Thunderclap on 160 m 33 ft 67 ft 130 ft From Radio Antenna Engineering, Laport, 1952 Mc. Graw-Hill The difference between 19. 5 u. V and 18. 7 u. V is 0. 36 d. B or 0. 06 S-unit HF Propagation and Antennas © J King June 2009

Ground Mounted Vertical Antennas Short Vertical Antennas Q. OK, what’s the catch here? If a 43 foot (30 degree) vertical, or a 14 foot (10 degree) vertical can radiate nearly the same signal as a 130 foot (90 degree) vertical, why would anyone ever put up a large antenna? A. It’s that darn free lunch thing again. While it is true that even an extremely short antenna will radiate nearly as much energy as a much larger antenna, a very short antenna’s radiation resistance Rrad will be some very small value – perhaps as low as 1 Ohm. A low value of Rrad is not, in itself, a problem. We can design an efficient matching network to transform a low value of Rrad to 50 Ohms without too much difficulty. But, recall the formula for overall Radiation Efficiency: Radiation Efficiency = Rrad + Rg + Rant + Rmatch x 100 % Can you spot the problem here? If Rrad is a small number (say 1, for example) then the other terms (Rg + Rant + Rmatch) in the denominator can swamp Rrad and dominate the efficiency equation. A short vertical requires very low losses from Rg, Rant, Rmatch if it is to achieve reasonably good efficiency. This is difficult to do, especially in making Rg small over real earth. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Antenna Matching Exercise Back to our 43 foot Thunderclap matching exercise. . . Let us assume that out Thunderclap is mounted over a very good radial ground system which has an RF resistance of 8 Ohms. In order to design a matching network we need to know the Thunderclap’s feedpoint impedance at the design frequency. There are several ways we can find this value: • Direct measurement (Most reliable) • Computer simulation (Can be remarkably reliable) • Graphical references (Can get in the ballpark) • Longhand math (Good luck) Using one of these methods, we find the feedpoint impedance at 1. 820 MHz to be: 10. 4 – j 635 Ohms Note that this is the Feedpoint Impedance, which is the vector sum of the Radiation Resistance Rrad, the Ground Resistance Rg, and the antenna’s reactance. The antenna’s impedance alone is 2. 4 – j 635 Ω The ground impedance alone is 8 +j 0 Ω Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Antenna Matching Exercise Thunderclap vertical, as it comes wired from the manufacturer, has a 4 -to-1 step down ‘un-un’ transformer between the feedpoint and the coax connector. Factory configuration 4 -to-1 Step Down Transformer Clearly, this is not what we’re looking for in a matching network on 160 m. We wish to match the feedpoint impedance to 50 Ohms, but the Thunder ‘matching’ unit actually moves the feedpoint impedance the wrong way! We will need to bypass the Thunder network and insert our own matching network to properly match the 10. 4 – j 635 Ω feedpoint impedance to 50 Ω. 10. 4 – j 635 Ω Coax Connector Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Antenna Matching Exercise Using our favorite impedance matching network design method, we design an L-Network to transform 10. 4 – j 635 Ω to 50 + j 0 Ω. 57 u. H 10. 4 – j 635 Ω To L-network 3400 p. F 50 + j 0 The L-network can then be placed in a weatherproof enclosure at the base of the antenna in place of the Thunder ‘matching’ transformer. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Antenna Matching Exercise Now let’s calculate the Radiation Efficiency of our homebrew-matched Thunderclap: Radiation Efficiency = Rrad x 100 % Rrad + Rg + Rant + Rmatch = 2. 4 + 8. 0 = 23 % The radiation efficiency of our antenna is 23%, or 6. 4 d. B (1. 06 S-units) less than a theoretical antenna of 100% efficiency. A full sized 130 foot vertical over the same 8 Ohm ground radial system would have a radiation efficiency of 82%. The difference between our modified 43 foot Thunderclap and a full size 130 foot vertical is 5. 6 d. B, or 0. 93 S-units. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Capacitive Top Loading The Closest Thing Yet to a Free Lunch Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Capacitive Top Loading Transmitter Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Visualizing Current Flow with Capacitive Top Loading Transmitter Earth Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Capacitive Top Loading: • Increases Radiation Resistance Rrad • Decreases antenna series reactance • Increases antenna bandwidth Radiation Efficiency increases because: • Rrad becomes larger • There is less reactance to cancel, requiring less inductance in the matching network (lower loss from Rmatch) Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Capacitive Top Loading Large Top Hat No Top Hat A very large top hat can quadruple Rrad. From Radio Antenna Engineering, Laport, 1952 Mc. Graw-Hill Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Capacitive Top Loading Example Let’s put a capacitance hat on our 160 m 43 foot Thunderclap and run the numbers again. We will keep the antenna over the same 8 Ohm ground radial system. We will make our top hat from eight five-foot long, ½ inch diameter spokes, with a wire running around the outer periphery. With the top hat, the feedpoint impedance at 1. 820 MHz now becomes: 12. 2 – j 375 Ohms The antenna’s impedance alone is now 4. 2 – j 375 Ω (was 2. 4 – j 635) The ground impedance alone remains 8 +j 0 Ω The L and C values in the L-network now become: 35 u. H ( Was 57 u. H and 3400 p. F ) 3100 p. F Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Capacitive Top Loading Now re-calculate the Radiation Efficiency of our Top Loaded Thunderclap: Radiation Efficiency = Rrad x 100 % Rrad + Rg + Rant + Rmatch = 4. 2 (Was 2. 4) 4. 2 + 8. 0 (Was 2. 4 + 8. 0) = 34 % (Was 23 %) The radiation efficiency of our antenna is 34%, or 4. 6 d. B (0. 77 S-units) less than a theoretical antenna of 100% efficiency. A full sized 130 foot vertical over the same 8 Ohm ground radial system still has a radiation efficiency of 82%. The difference between our 43 foot Top Hat Thunderclap and a full size 130 foot vertical is 3. 8 d. B, or 0. 63 S-units. (Was 5. 6 d. B, or 0. 93 S-units) Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Multi-banding The Thunderclap In no case does the Thunder ‘un-un’ ‘matching’ device help in matching the antenna – throw it away. TANSTAAFL – A matching network is required for each band. Some bands (160, 80) may require two networks (or a component tweak) to achieve end-to-end band coverage. 160 a/b 80 a/b 40 Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Multi-banding The Thunderclap Auto-tuner block diagram (MFJ 998) Auto. Tuner Coax to shack An automatic tuning at the base of the antenna. Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Some Small Amateur Antenna Tuning Units Photos: Phil Salas, AD 5 X Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas A Poster Child Amateur Antenna Tuning Unit Output Vacuum Relays Vacuum Variable Capacitors Edge Wound Inductors Input Vacuum Relays Tuner Envy! Photos: Marvin Gorden, KC 9 VF Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas KC 9 VF Vertical Photo: Marvin Gorden, KC 9 VF Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas The Inverted L Antenna 100 Feet Radiation Both wires radiate 40 Feet Match Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas The Tee Antenna 100 Feet Fields cancel No radiation 40 Feet Radiation Match Feedpoint Impedance = 13. 9 – j 38 Ω Rrad = 5. 9 Ω Rg = 8. 0Ω Radiation Efficiency = 42% Ground Mounted Vertical Antennas © J King Feb 2010

Ground Mounted Vertical Antennas Questions, Discussion, and Homebrew Brainstorming Ground Mounted Vertical Antennas © J King Feb 2010