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This section covers antennas with somewhat more specific purposes, although for many Amateurs an NVIS is a basic antenna, used for club net operation, and emergency support work.
NVIS is Near Vertical Incidence Sky-wave, and this uses antennas strung around a tenth to a quarter of a wavelength above ground. These low dipoles, or other low antennas, direct signals upwards (at a Near Vertical angle), towards the ionosphere, making it a skywave. NVIS works well on 80 metres, 60 metres, and 40 metres. The range is typically several hundred kilometres. This does not mean such an antenna will not pick up and radiate signals longer distances, but it is not ideal for this job.
The examiner puts the ideal height at 1/10 and 1/4 wavelength above ground. I have operated with a simple 40 metre dipole, about 2 metres (1/20 wavelength) above ground using low trees at coastal Manly in Brisbane, and confused Sydney people who though the good signal indicated I was in the Sydney version of Manly.
People involved in remote area land search and rescue may carry small HF radios, and deploy wire antennas across brush, etc, if there are no trees, in order to check in with the control station periodically. Licensed frequencies around 5 MHz (60 metres) often work well for this.
Not on the exam, but three parallel reflectors, wires a little longer than the antenna run along the ground can improve the upward radiation, with a chicken-wire mesh also doing the same thing. I believe that this also reduces the ground-wave component, as close-in stations can have the NVIS and groundwave components cancel each other.
NVIS can also be achieved with HF whip antennas pulled back or forward over the vehicle; and military vehicles feature antennas consisting of a metal tube or strip spaced 15 to 20 cm above the vehicle roof, and if you have something like a Land Rover, and you don't mind drilling holes in the roof, you could replicate these.
One method of building a multi-band antenna is to use traps. A trap consists of a coil of stiff wire, and a capacitor. The trap is placed at the point the antenna would end if it was for only the highest frequency. The inductance prevents significant amounts of the signal reaching the remainder of the antenna. Low frequency signals pass through the trap, and can use the entire length of the wire or element.
While I personally have never used one, they are far from uncommon, especially on permanently installed dipoles.
Named for the inventor, a Mr Harold H. Beverage, these antennas are typically from half a wavelength, to several wavelengths long, and receive signals from the end of the antenna. The wire is around 2.5 metres from the ground. The far end is grounded via a resistor of 400 to 800 ohms (450 is common). They are used for receiving only, as they are lossy, but pick up less noise than the alternatives. If used with a transceiver, you need either a smart antenna tuner, a relay box, or a radio with a receiver antenna port, which selects this antenna for receiving, and a vertical or sloper antenna for transmitting.
The longest were 14 kilometres long, used in the north-eastern US to receive long-wave signals from Europe. An array of 5 km antennas, hundreds of metres wide, was used in Europe for reception in the first radio based transatlantic telephone service.
It appears that the limited conductivity of the soil below them is important. To receive signals from behind the antenna, a two wire system with a transformer at the far end, which then uses the antenna as its own feedline can be set up. The DX newsletter from HCJB suggested a buried engine block for the ground, something I suppose tends to available in the dump area of a rural property large enough to run one of these antennas. The typical construction method is that of a telegraph line - a wire on insulators supported by timber poles.
Typical use would be on the 180, 630, and perhaps 2200 metre bands, and for long range MF/AM broadcast band reception. Ditto the tropical broadcast band at 2.3 MHz (120 metres).
When a dipole is supported by a single mast, pole, or tree branch, it forms an upside-down V shape. The ends can be tied to a fence, star-picket, etc. The included angle must not be less than 90 degrees, that is, less than a 45 degree drop from horizontal on each side, if possible. The lengths are the same as a dipole. Inverted-vee is another spelling.
These have a second benefit, not explored by the examiner, of being more omnidirectional than a low dipole.
While a resonant inverted-V which does not require a tuner is the best option where room allows, things like the G5RV can be installed this way.
It is possible to connect more than one pair of legs to a feed-point, generally for different bands. Each pair should run as close to straight as possible.
This can be used as a field operations antenna, in a rental property, or as a permanent installation. During or after a disaster a flagpole may well work well, and they often survive high intensity wind events. A star-picket with a tube or light pipe zip-tied, gaffer-taped or wired to it also works. Application is across the HF bands, although there is no reason they won't work on higher bands.
Their efficiency is good.
If a strong central rope is used, two supports allow a yagi style antenna to be constructed from several wires, each in an inverted V format.
There are several loop designs, each quite different in their behaviour.
A fullwave loop, be it a square, rectangle, delta, circle, or what I need to get motivated to complete on 10 metres, a trapezoid, radiates broadside to the loop, with a null to the side. Read about my Delta project.
The electrically small loop, often less than 1/10 of a wavelength (and certainly less than one third (⅓)) in circumference is sensitive to a signal in its plane, meaning edge-on, and has a null broadside to it. They are often made from things like thin Heliax, such as FSJ1-50; or copper tubing. (The examiner has reduced the size in the question, as shown above).
These can generate very high voltages and/or significant currents, even at low input powers.
The subject of a now deleted question, the third option is the large horizontal loop, using wire totalling several times the wavelength of the lowest frequency used. These typically require four large trees, or other supports. Borrow a TARDIS, and go back to plant these in a square around your house, about 15 years ago.
These are omnidirectional, with a radiation angle lower that a dipole. This makes them better for DX than the dipole, as the signal travels further before it hits the ionosphere, and therefore returns to earth further away. The signal is both vertically and horizontally polarised. These work well on harmonically related bands. While companies sell various kits, you can just buy wire and insulators, along with a dipole centre or 4:1 balun.
Also sold as a LPDA, these are directional antennas with limited gain, but often very wide bandwidth. Like a Yagi, the have a central boom, and an array of elements reducing significantly in size along the boom. A pair of coupling straps zig-zag along the boom, one connecting the left half of the back element to the next on the right, and the following one on the left, and so on, while the other strap connects the opposing elements. An alternative uses two booms, with each half element connected to alternative booms.
They are used widely by the military, due to being a directional antenna which operates on a wide range of channels. USS Blue Ridge had an HF one mounted on the fore-deck from its 1970 commissioning, there when it visited Sydney in 1988, but removed by the time it supported INTERFET in East Timor, in early 2000, where it was photographed with large dome-covered satellite antennas instead.
There are TV versions, typically covering the upper VHF to UHF spectrum used for DVB-T; and some which cover maybe 700 to 2100 MHz, used for rural 'phone and mobile (cellular) data connections. Note that despite having many elements, these only have the gain of a two or three element Yagi. If you do not need the bandwidth, a long Yagi will give better gain. As for the idea of mounting two of these antennas, each at 45 degrees, for the customer end of a cellular system in rural areas, assuming beef production, there is a lot of this idea on the ground already...
The term "log periodic" refers to the logarithmic relationship between the length of the elements, and their spacing, at least in some versions of the antenna.
From maybe 700 MHz up, these can be made from Printed Circuit Board, as can a range of other designs.
Increasing the thickness of the elements of a Yagi antenna increases its bandwidth.
A pretty hardcore example is this antenna system, designed for Channel 0 TV in Cooma, Australia. A PAL TV channel was 7 MHz wide, in this case from 45 to 52 MHz, meaning the bandwidth is 15.555% of the lowest frequency. With PAL analogue TV closing, and with it the use of the lowest channels, it is no longer used. I think it would make a great antenna for a 6 metre beacon. And yes, Ch 0 TV did prevent operation in lower parts of 6 metres in many areas.
Take a look here: Nanny Goat Hill (their spammy site weirds-out if I direct-link the image). Or a stupid stock image site calling it a cellular site here (includes farcebook trackers).
Wandering off topic, opening a site in Internet Explorer on Windows Server 2019 or 2022 (both free downloads with re-armable 180 day free use periods) lists each server a page uses. I test these pages in it, as it reproduces the <TT> text I use for questions correctly when I paste them into OpenOffice to check spelling.
Most often used while operating SSB or CW on VHF or UHF while mobile, the halo consists of a near loop of tubing which is mounted horizontally, with a small gap opposite the mast and feed point. It is used as the operator desires an omnidirectional signal, that is, one covering every azimuth. It is used in preference to a vertical antenna, as other stations using these modes use horizontal antennas. They are usually fed by a gamma match, and are a half wavelength in circumference.
The other stations communicating with the halo user are typically located in the same plane as the antenna.
VHF+ contest Rover stations may use these for all operations, or only while in transit between operating points, usually in different gridsquares, switching to a directional antenna once on the hilltop, etc.
There are also square versions, again using a half-wave of tubing, and also with a gap.
Available from 50 to 432 MHz, PAR Electronics "Omniangles" are roughly triangular, which is claimed to make the pattern more omnidirectional. Again, half a wavelength of conductor is used, although these are fed like a conventional dipole, as there is a gap as the feedpoint, as well as opposite it. See: PAR Ominangles. I've had a 2 metre one on my Christmas list, unsuccessfully, for quiet a few years.
Big Wheel designs are larger, consisting of 3 petals, each projecting a quarter-wave from the centre, and having a half-wavelength contributing to the circumference of the wheel. (As 3 is less than π, there must be a small gap between each petal). Thus they are about 1.5 wavelengths in circumference. You can view an image Wikipedia (Danish) - Big Wheel, and follow links to the references, mostly in English, should you wish. It is also shown on: Wikipedia (French) - 2 metre band.
All can be stacked for improved gain. There is no reason they cannot be used at home, although a larger yagi is better if gain is the goal, with the downside being that a rotator will likely be necessary.
These antennas also have application in propagation beacons. The European 8 metre band is perhaps the lower limit for a practical Halo, or 6 m for mobile use. For home use, a big wheel on 6 metres is probably the limit; and 2, or maybe 4 metres if mobile. At the other end of the spectrum, as it were, a 13 cm band (2.450 GHz) Big Wheel can be printed on a 40 mm diameter PCB.
The OZ7IGY beacons use Bigwheels and Halos: Main page, with small images and details of beacons.
There are range of helically wound antennas for HF. These can be single band, or tuned using a "wander lead" to connect to a tap for each band. Some designs use half a wavelength of wire, and a small vertical section at the top.
The alternative is a whip with a loading coil in the base. One option is to tune this loading coil using a motor and gearbox from a battery powered screwdriver to drive a contact up and down over a section of the coil which has had the insulation removed. This changes the inductance. I believe others use relay switched inductors and capacitors, as many auto-tuners do.
The whip is typically a 102" (2590.8 mm) or 108" (2743.2 mm) quarter-wave CB antenna. The shorter one in intended to use used with a spring for CB, but for ham use, one can be used with either. The spring needs to contain a braid linking its two ends, which not all do.
These are all inefficient, especially on the lower HF bands, and some "screwdrivers" are fragile. If being able to make contact in an emergency is important, make sure you have a means to connect a wire antenna to the radio. (If no trees are available, laying an antenna on brush or even dry soil will work).
Off the exam, centre loaded antennas, featuring a fairly large coil can be used, the coil referred to as a "bug-catcher". If mounted at the rear of the vehicle, heavy fishing mono-filament line or some similar insulating material may be used to guy it against wind loads. A version with the coil inside a plastic tube was popular during the 1970s CB era (noting this is different to the phasing coils in Australian UHF CB antennas).
In some cases having the whip arc over the vehicle with a bungy cord or similar arrangement attached towards the end both keeps the height sensible, and assists in NVIS propagation.
As well as Amateur use, similar antennas are popular with militaries, non-state actors, remote area emergency services, civil defence, remote area tourism and transport companies, off-road tourists, and presumably mining companies in such areas.
A larger vertical can be attached to the mount when stationary, ideally along with counterpoise wire(s) draped on the ground. 5.6 metre telescoping antennas are available out of China, with an M10 (10 mm) stud on the bottom. The slightly shorter, but potentially stronger, MFJ version has a ⅜", 24 TPI stud. They can also be mounted on other supports. Note that Manfroto's ⅜" stud is 16 TPI UNC.
In Australia 4.6 metres is the maximum height generally permitted to the top of an antenna, measured from the road surface. To operate on 40 metres (7 MHz), an antenna using a fairly thick fishing rod blank with 10 metres (a quarter wavelength) of wire wound over it will form an antenna with good efficiency. This can be mounted using a 50 mm square section, welded to another piece inserted into a trailer hitch receiver, rising about 50 cm above the receiver. A reasonably heavy insulated, non-radiating feed wire can run up the protected car side of it, to the rod.
You would however need to select petrol stations by awning height, and may need to avoid level crossings with electrical overhead, plus low bridges, shopping centres, tunnels, etc. Victoria has an online map if low bridges, etc. The Australian Standard for tram overhead is 5.64 metres (clearly converted from 18' 6"). Some junk food locations use randomly placed bars around open carparks, probably to discourage heavy vehicles.
In some cases if you have a 144 MHz radio which is transmitting a strong harmonic on 288 MHz the 2 metre quarter-wave antenna will be a half-wave at this higher frequency, and thus have a high impedance, poor SWR, and likely poor radiation characteristics for the harmonic.
If an antenna is designed to radiate will on 2 metres and 70 cm, the third harmonic of a signal at 146 MHz, being at 438 MHz, will radiate well. The same applies between HF bands, so the 7.2 MHz harmonics of a 3.6 MHz signal will radiate well from a trapped dipole covering both bands. If we radiate 1500 watts at 80 metres, and suppression is only 30 dB we will radiate 1.5 watts of signal, more than enough to cause interference to other users. Radiate a second harmonic on 2 metres, at 288 - 296 MHz, and you may make the military grumpy!
These are actual questions from the General exam pool.
G9D01
Which of the following antenna types will be most effective as a Near Vertical Incidence Skywave (NVIS) antenna for short-skip communications on 40 meters during the day?
A. A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground
B. A vertical antenna placed between 1/4 and 1/2 wavelength above the ground
C. A horizontal dipole placed at approximately 1/2 wavelength above the ground
D. A vertical dipole placed at approximately 1/2 wavelength above the ground
This is the low dipole, answer A.
G9D02
What is the feed point impedance of an end-fed half-wave antenna?
A. Very low
B. Approximately 50 ohms
C. Approximately 300 ohms
D. Very high
These have very high impedance, requiring a transformer with a large ratio, answer D.
G9D03
In which direction is the maximum radiation from a portable VHF/UHF "halo" antenna?
A. Broadside to the plane of the halo
B. Opposite the feed point
C. Omnidirectional in the plane of the halo
D. Toward the halo's supporting mast
These are typically mounted so the element is in the horizontal plane, and they radiate a signal which is roughly omnidirectional in this plane, answer C.
G9D04
What is the primary function of antenna traps?
A. To enable multiband operation
B. To notch spurious frequencies
C. To provide balanced feed point impedance
D. To prevent out-of-band operation
Traps are a form of low-pass filter, allowing a wire or beam antennas to operating on more than one band, answer A.
G9D05
What is an advantage of vertical stacking of horizontally polarized Yagi antennas?
A. It allows quick selection of vertical or horizontal polarization
B. It allows simultaneous vertical and horizontal polarization
C. It narrows the main lobe in azimuth
D. It narrows the main lobe in elevation
This narrows the angle of the signal in elevation, directing more energy towards the horizon, answer D.
G9D06
Which of the following is an advantage of a log-periodic antenna?
A. Wide bandwidth
B. Higher gain per element than a Yagi antenna
C. Harmonic suppression
D. Polarization diversity
These have a wide bandwidth, answer A.
G9D07
Which of the following describes a log-periodic antenna?
A. Element length and spacing vary logarithmically along the boom
B. Impedance varies periodically as a function of frequency
C. Gain varies logarithmically as a function of frequency
D. SWR varies periodically as a function of boom length
The length and spacing varies, as described in A.
G9D08
How does a "screwdriver" mobile antenna adjust its feed point impedance?
A. By varying its body capacitance
B. By varying the base loading inductance
C. By extending and retracting the whip
D. By deploying a capacitance hat
The motor arrangement varies the induction of the base loading, answer B.
G9D09
What is the primary use of a Beverage antenna?
A. Directional receiving for MF and low HF bands
B. Directional transmitting for low HF bands
C. Portable direction finding at higher HF frequencies
D. Portable direction finding at lower HF frequencies
Beverage antennas are used for directional reception of MF and low HF signals, answer A.
G9D10
In which direction or directions does an electrically small loop (less than 1/10 wavelength in circumference) have nulls in its radiation pattern?
A. In the plane of the loop
B. Broadside to the loop
C. Broadside and in the plane of the loop
D. Electrically small loops are omnidirectional
Unlike larger loops, the null is broadside to the loop, answer B.
G9D11
Which of the following is a disadvantage of multiband antennas?
A. They present low impedance on all design frequencies
B. They must be used with an antenna tuner
C. They must be fed with open wire line
D. They have poor harmonic rejection
If they operate in multiple bands, and we are using a transmitter on one of the lower frequencies, and it is generating harmonics, these will be radiated by the antenna, answer D.
G9D12
What is the common name of a dipole with a single central support?
A. Inverted V
B. Inverted L
C. Sloper
D. Lazy H
The inverted V can be a very convenient antenna to set up, needing only a singe mast, at the centre, answer A.
For many this might be the mast used to support a VHF and UHF antennas, or a single tree, chimney, etc may also be used.
Each of the others are real, useful, antennas. The sloper can be an unbalanced or single wire antenna, or it can be a dipole, but nore in this case the support is NOT central. The Lazy H has two driven dipoles, and has significant gain if the spacing is large (¾ wavelength) but these need 4 or 6 supports.
G9D13 - Question deleted
Circularly polarised antennas are real, but removed as a distractor above. Polarisation can be right-hand (clockwise) or left-hand (anti-clockwise). CP signals are most often used from the VHF-FM broadcast band, where arrays of crossed and intermeshed dipoles are used (such as this, this, and this), and up into the microwave bands. For FM broadcasting they appear to reduce drop-outs and "picket-fencing", where a vehicle's antenna passes between strong and weak points in the radiated signal, and the received audio varies rapidly, somewhat like a car's engine noise reflecting from a fence with perforations. I have noticed that the signal from a low powered FM broadcasting site 1 km from my house varies as I walk towards it with a handheld on its frequency, cycling every 3 metres. It uses a simple vertical antenna, rather than a CP one.
GPS signals are RHCP.
In ham radio high gain CP antennas are often used in Moon-bounce (EME) and with satellites. They take the form of a helix, or crossed Yagis, where phased elements may be used. An array of such antennas gives greater gain.
I am not aware of CP antennas used at HF, and an antenna for use on lower HF bands would require huge amounts of curved metal tubing or similar material, and a very large insulated support structure. Thus a simple low dipole is a much more sensible option for NVIS.
Boats and ships may use vertical antennas on HF, as well as wires strung between masts, etc. Typically used with an ATU, on larger vessels verticals can be tall enough to be efficient. Either a metal hull, or an earthing plate, makes the water / ocean a vast ground-plane.
While a yacht's backstay may make an excellent antenna, an alternative antenna must be available in the case of dismasting, or even a roll-over.
Shakespeare have two 23 ft (7 metre) verticals, the 390 shipped in 2 parts; 393 in 3. These are fed from an Antenna Tuning Unit using 15 kV wire such as GTO-15, standing for Gas Tube Only, meaning for neon and similar tubes. This appears to come in plain insulated, and insulated with a sheath similar to coax sheath. I'm not sure about the "Only" bit, maybe it is a discouragement against use as spark plug lead, as making these from such wire can cause them to radiate RF noise covering at least MF to VHF. Alternatives are compressed carbon within the wire, or a spiral of fine stainless wire.
On to the final, and most important, page: Safety
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, March 2026.
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