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Antennas are a vital part of an Amateur station, and a station with a reasonable rig, and a good antenna will typically perform as well as a station with a fantastic rig, but an ordinary antenna. For many uses height is a good thing. That said, any antenna is better than none. Having the option to switch between, say, a vertical and a NVIS dipole can be beneficial. Some tuners offer push-button antenna selection.
One simpler antenna is the random wire. These are great as receiving antennas, but if used for for transmitting then there is the risk that high RF levels will be present in the shack, resulting in painful RF burns. Especially for transmission, these are usually used with an antenna tuner, typically going to a screw terminal or a banana socket.
One way to mitigate the "RF in the shack" problem is to use a coaxial cable to a remote tuner placed outside, or even just to a terminal box or un-un on the fascia board of your house.
Many are in the form of an "Inverted L", but for a question on the next page, they require 2 supports, and are NOT a dipole!
Quarter-wave (and ⅝λ) antennas mounted on poles need some form of ground-plane. In some cases, these are horizontal, but a better option is to lower these to about 45 degrees. This increases the feedpoint impedance to 50 ohms.
Several factors affect the impedance of a dipole. If we move the feedpoint away from the centre, the impedance increases. Lowering a dipole decreases the impedance. That said, I have cut a lightweight dipole for 40 metres, taken it on a trip, and strung it low between two trees, and worked from Brisbane to Sydney with a good match.
As in the Technician exam, we also need to determine the length of resonant dipoles and verticals. A wavelength in metres is 300 (or 299.7) divided by the frequency in megahertz. Overall a dipole is half this (each leg being a quarter), and likewise a quarter-wave is a quarter. In each case a factor of 0.95 is applied, due to velocity factor and "end effect".
Once we get the answer we need in meters, we need to divide the answer by 0.3048, the length of a foot in metres.
It is easy to get to the answer by halving or quartering the band name, then multiplying by 3 (or 3.3) for feet.
If we were asked about 7.106 MHz we would know this is in the 40 metre band, so a halfwave is about 20 metres long, and a quarterwave vertical about 10 metres. Thus the answers are about 3.3 times this, 66 or 33 feet. The distractors will be significantly different to this, meaning an estimate is fine. λ = 299.7 / 7.106 = 42.1756 metres. A halfwave is 21.0878 metres, and a quarterwave 10.5439 m. Apply the 0.95 factor, and get 20.0334 m and 10.0167 m for the actual lengths. We have to divide these by 0.3048 to get 65.726 feet and 32.863 ft respectively.
In free space, a dipole radiates in a figure 8 broadside to the wire. However, dropping the antenna below half a wavelength results in a near omnidirectional radiation pattern. Omni means all, as in omnibus, a vehicle available to all.
There are many loop configurations, including loops square on plan, needing 4 poles or trees. However, the paper discusses loops which are suspended from the top or top corners, and typically fed at the bottom, consisting of a wavelength of wire. These can be square, diamond, hexagonal, round, or a delta, among other shapes. If the feed is at the bottom the antenna is horizontally polarised; fed at the side, vertical. Feeding a delta Δ with a coax at the top may give a mixed polarity, suitable for working stations of either polarisation. A square loop will have a quarter-wave per side, and a delta a third, assuming an equilateral triangle.
One thing I have been thinking of making is a trapezoidal loop for 10 metres, as this will occupy less space than a full delta, and should allow a base less than a third of a wavelength, as the dowel I can find is a little short. The top will be a metre or so of brazing rod.
Loops have some gain compared to a dipole, and at harmonics, gain increases.
There are also small loops, for HF, but these involve high voltages and currents, and require high voltage tuning capacitors. People use vacuum capacitors, but many of these are set-and forget devices, so constantly adjusting them will cause the diaphragm to fail.
Take a simple dipole, folded dipole, delta loop, or square loop, and add additional straight elements or loops, and you get an antenna which more strongly directs the signal in the desired direction. Usually these are placed along a boom, made from either metal, or varnished timber.
A quad is a beam antenna consisting of square loops, with each side of the driven element about a quarter wave long. Given the reflector and directors are also square loops, the whole thing is roughly cubical, hence the alternative name "cubic quad".
The element behind the driven element is the reflector, and it is about 5% larger than the driven element. Thus the reflector on a quad is a little over a quarter wavelength. There are Yagi style antennas without reflectors.
The first director (D1) is typically around 5% shorter than the driven element. In longer antennas the addition directors may be a similar length to D1, or D2, D3, D4 might be a little shorter, but a common size, then a step down to D5, D6, D7, shorter again, but common size. A longer boom, and more elements means more gain.
The spacing is often 0.2 of a wavelength, but both spacing and element size can be optimised for different specifications using antenna design software. There are also many websites with yagi designs for different bands, and different band segments.
One specification is front-to-back ratio. Suppose there are two repeaters some distance from our station, in opposite directions, but on the same frequency, and neither have CTCSS. We need an antenna with a high front to back ratio, so we don't trigger the more distant repeater while using the nearer. This figure is the the ratio of the power sent forward, and that going backwards (in the exact opposite direction to the main lobe, to quote the question), and it is normally in dB.
A Yagi with thicker elements has a greater bandwidth. The broadcasting site in Cooma, NSW has retired yagis for TV broadcasting which have elements at least 75 mm thick.
Not on this exam, there are also corner reflector antennas, and parabolic dish antennas, with significant gain.
These are actual questions from the General exam pool.
What is one disadvantage of a directly fed random-wire HF antenna?
A. It must be longer than 1 wavelength
B. You may experience RF burns when touching metal objects in your station
C. It produces only vertically polarized radiation
D. It is more effective on the lower HF bands than on the higher bands
This is referring to an antenna where the wire goes from the back of the radio, or usually the antenna tuner, out a window, and to a tree, etc, or it is the vertical element of an inverted-L antenna (more like a Gamma - Γ, but that is used elsewhere). These are great for short-wave listening across multiple bands, but when transmitting, especially at high power levels, these can put high RF voltages into the shack, and perhaps result in your getting zapped by touching metal objects, answer B.
Which of the following is a common way to adjust the feed point impedance of a quarter wave ground plane vertical antenna to be approximately 50 ohms?
A. Slope the radials upward
B. Slope the radials downward
C. Lengthen the radials
D. Shorten the radials
The radials are located below the vertical antenna, and having them sloping downwards (roughly 45 degrees) gives an impedance of about 50 ohms, answer B.
Which of the following best describes the radiation pattern of a quarter-wave, ground-plane vertical antenna?
A. Bi-directional in azimuth
D. Omnidirectional in azimuth
These radiate in all horizontal directions (to all points of the compass), meaning omnidirectional in azimuth, answer D.
What is the radiation pattern of a dipole antenna in free space in the plane containing the conductor?
A. It is a figure-eight at right angles to the antenna
B. It is a figure-eight off both ends of the antenna
C. It is a circle (equal radiation in all directions)
D. It has a pair of lobes on one side of the antenna and a single lobe on the other side
A 2D representation of this is a figure-8 shape, at right angles to the antenna, answer A.
Examining a 3D representation would tell you that this is two sections of a dough-nut, as radiation is, say east and west, but also up and down, assuming a horizontal dipole. At harmonics the antenna has multiple lobes, but the last answer is silly.
How does antenna height affect the horizontal (azimuthal) radiation pattern of a horizontal dipole HF antenna?
A. If the antenna is too high, the pattern becomes unpredictable
B. Antenna height has no effect on the pattern
C. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional
D. If the antenna is less than 1/2 wavelength high, radiation off the ends of the wire is eliminated
A low HF dipole has a pattern which is almost omnidirectional, meaning if your antenna runs north to south, you can still work a station to the north or south. Depending on the band, such an antenna can provide great local or regional coverage via NVIS, answer C.
Where should the radial wires of a ground-mounted vertical antenna system be placed?
A. As high as possible above the ground
B. Parallel to the antenna element
C. On the surface of the Earth or buried a few inches below the ground
D. At the center of the antenna
This describes an antenna most often used on the "low bands", 160, 80, and maybe 40 metres; as well as 630 metres and 2200 metres; plus the MW / MF / AM broadcast band. In these cases a large mat of radials on, or just below, ground level is used, answer C.
A length of ⅝ wavelength is ideal, and is used even if the mast is only a quarterwave, also called "90 degrees" in the trade.
How does the feed point impedance of a 1/2 wave dipole antenna change as the antenna is lowered below 1/4 wave above ground?
A. It steadily increases
B. It steadily decreases
C. It peaks at about 1/8 wavelength above ground
D. It is unaffected by the height above ground
As a dipole become very low, the feedpoint impedance drops, answer B.
Ground conductivity is probably a factor in this too.
How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends?
A. It steadily increases
B. It steadily decreases
C. It peaks at about 1/8 wavelength from the end
D. It is unaffected by the location of the feed point
Various "OCF", or off-centre fed antennas are popular with Amateurs, with some "Windom" antennas falling into this class, although the "Carolina Windom" uses a single-wire feed. Anyway, for a coax fed OCF antenna, the impedance increases as the feedpoint moves away from the centre, answer A.
If it helps, remember that the end-fed halfwave, still termed a dipole due to the length, has a very high impedance, requiring a transformer at the feedpoint.
Which of the following is an advantage of a horizontally polarized as compared to a vertically polarized HF antenna?
A. Lower ground reflection losses
B. Lower feed point impedance
C. Shorter Radials
D. Lower radiation resistance
These have lower ground reflection losses, answer A.
What is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz?
A. 8 feet
B. 16 feet
C. 24 feet
D. 33 feet
20 metre band, halve for a half-wave, get 10 metres, and multiply by 3, and the nearest number of archaic units is 33, answer D. But you are paying me for a mathematical answer, so L = 299.7 / 14.250 x 0.5 x 0.95 / 0.3048 = 9.99 / 0.3048 = 32.77559 feet.
What is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz?
A. 42 feet
B. 84 feet
C. 132 feet
D. 263 feet
80 metre band, so halve, get 40 metres, multiply by 3, and get 120 feet, near enough to 132, answer C. L = 299.7 / 3.550 x 0.5 x 0.95 / 0.3048 = 40.1007 / 0.3048 = 131.56399 feet.
What is the approximate length for a 1/4 wave vertical antenna cut for 28.5 MHz?
A. 8 feet
B. 11 feet
C. 16 feet
D. 21 feet
10 metre band, quarter it for 2.5 metres, multiply by 3, and 7.5 old-fashioned units is near enough to A. L = 299.7 / 28.5 x 0.25 x 0.95 / 0.3048 = 2.4975 / 0.3048 = 8.1938 feet.
Grab a drink, let the non-directional antenna stuff sink in, then settle in for a bit of a long haul on directional antennas.
Which of the following would increase the bandwidth of a Yagi antenna?
A. Larger diameter elements
B. Closer element spacing
C. Loading coils in series with the element
D. Tapered-diameter elements
Increasing the diameter of the elements, but taking this into account in the calculating software, generates a yagi with greater bandwidth than one with thin elements, answer A.
Software allows optimisation for different parameters.
What is the approximate length of the driven element of a Yagi antenna?
A. 1/4 wavelength
B. 1/2 wavelength
C. 3/4 wavelength
D. 1 wavelength
This refers to the over-all length of the element, even if it is cut in the centre, this being a half-wave long, answer B.
Note that some feed systems manage to use a single, undivided rod or tube, and also that if it was referring to each half of a cut centre element, then it would have used the plural "elements".
How do the lengths of a three-element Yagi reflector and director compare to that of the driven element?
A. The reflector is longer, and the director is shorter
B. The reflector is shorter, and the director is longer
C. They are all the same length
D. Relative length depends on the frequency of operation
The reflector is longest, and the director the shortest element, answer A.
How does antenna gain stated in dBi compare to gain stated in dBd for the same antenna?
A. dBi gain figures are 2.15 dB lower than dBd gain figures
B. dBi gain figures are 2.15 dB higher than dBd gain figures
C. dBi gain figures are the same as the square root of dBd gain figures multiplied by 2.15
D. dBi gain figures are the reciprocal of dBd gain figures + 2.15 dB
The salesman's figure, the dBi one, is inflated by 2.15 dB, compared to the engineer's dBd figure, answer B.
How does increasing boom length and adding directors affect a Yagi antenna?
A. Gain increases
B. Beamwidth increases
C. Front to back ratio decreases
D. Front to side ratio decreases
More gain! A longer yagi, with more elements (directors) means more gain, answer A.
What configuration of the loops of a two-element quad antenna must be used for the antenna to operate as a beam antenna, assuming one of the elements is used as a reflector?
A. The driven element must be fed with a balun transformer
B. There must be an open circuit in the driven element at the point opposite the feed point
C. The reflector element must be approximately 5 percent shorter than the driven element
D. The reflector element must be approximately 5 percent longer than the driven element
The reflector must be about 5% longer that the driven element, answer D.
What does "front-to-back ratio" mean in reference to a Yagi antenna?
A. The number of directors versus the number of reflectors
B. The relative position of the driven element with respect to the reflectors and directors
C. The power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction
D. The ratio of forward gain to dipole gain
A Yagi has forward gain, but some signal radiates backwards. The front-to-back ratio is the ratio of this forward power to the power going backwards, answer C.
Again, this is a factor for which an antenna can be optimised using software.
What is meant by the "main lobe" of a directive antenna?
A. The magnitude of the maximum vertical angle of radiation
B. The point of maximum current in a radiating antenna element
C. The maximum voltage standing wave point on a radiating element
D. The direction of maximum radiated field strength from the antenna
The main lobe is the lobe in which the greatest amount of energy is contained, answer D.
How does the gain of two 3-element horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single 3-element Yagi?
A. Approximately 1.5 dB higher
B. Approximately 3 dB higher
C. Approximately 6 dB higher
D. Approximately 9 dB higher
Two yagis means double the gain, or 3dB higher, answer B.
Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?
A. The physical length of the boom
B. The number of elements on the boom
C. The spacing of each element along the boom
D. All these choices are correct
All these factors can be adjusted the optimise the performance of the antenna, answer D.
Which HF antenna would be the best to use for minimizing interference?
A. A quarter-wave vertical antenna
B. An isotropic antenna
C. A directional antenna
D. An omnidirectional antenna
This requires a directional antenna, which would be orientated for minimum interference, rather than maximum signal, answer C.
Remember, as isotropic antenna is a mathematical tool, but a physical impossibility.
Which of the following is an advantage of using a gamma match with a Yagi antenna?
A. It does not require that the elements be insulated from the boom
B. It does not require any inductors or capacitors
C. It is useful for matching multiband antennas
D. All of these choices are correct
A potential weakness in Yagi antennas is the plastic mounting used insulate the elements from the boom, especially of the driven element is cut at the centre. A gamma match, named for upper-case Γ, where a thin rod or stiff wire runs parallel to the driven element and is connected some distance from the centre, allows the elements to be riveted, bolted, or welded directly to the boom, answer A.
Approximately how long is each side of the driven element of a quad antenna?
A. 1/4 wavelength
B. 1/2 wavelength
C. 3/4 wavelength
D. 1 wavelength
As with many loops, the total is a full wavelength, so each side is λ/4, answer A.
How does the forward gain of a two-element quad antenna compare to the forward gain of a three-element Yagi antenna?
A. About the same
B. About 2/3 as much
C. About 1.5 times as much
D. About twice as much
These antennas have similar gain, answer A.
A retired question indicates that the same is true of a two-element delta-loop beam.
Even a loop by itself (round, square, triangular (delta), or whatever) has greater bi-directional gain than a dipole.
What is meant by the terms dBi and dBd when referring to antenna gain?
A. dBi refers to an isotropic antenna, dBd refers to a dipole antenna
B. dBi refers to an ionospheric reflecting antenna, dBd refers to a dissipative antenna
C. dBi refers to an inverted-vee antenna, dBd refers to a downward reflecting antenna
D. dBi refers to an isometric antenna, dBd refers to a discone antenna
"dBi" is relative to a theoretical isotropic antenna, "dBd" is relative to a dipole, answer A.
What is a beta or hairpin match?
A. It is a shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching
B. It is a ¼ wavelength section of 75 ohm coax in series with the feed point of a Yagi to provide impedance matching
C. It is a series capacitor selected to cancel the inductive reactance of a folded dipole antenna
D. It is a section of 300 ohm twinlead used to match a folded dipole antenna
This is typically a U-shaped stub of aluminium wire, placed across the feedpoint for impedance matching, answer A.
An efficient and rather easy vertical antenna VHF is the "Flowerpot, by John VK2ZOI (SK). There are several options for 2m, 2m & 70cm, and 6 metres. Wire cutters, a METRIC pocket tape, a power drill, file, and a hacksaw or similar are all that is needed, along with a some RG-58, a length of plastic conduit, an end-cap, some fishing line (or dental floss?), heatshrink tubing, and a connector. See: VK2ZOI.COM
On to: Special Antennas
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, April 2022.
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