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This section covers antennas with special 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 or 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.
Not on the exam, but three parallel reflectors, wires a little longer than the antenna can improve the upward radiation, with a chicken-wire mesh 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 an old 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.
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.
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.
These have a second benefit, not explored by the examiner, of being more omnidirectional than a low dipole.
While a resonant inverted-vee, not requiring a tuner, is the best option where room allows, things like the G5RV may be installed this way.
There are several loop designs.
A fullwave loop, be it a square, rectangle, delta, circle, or what I need to get motivated to complete on 10 metres, a trapezoid, is radiates broadside to the loop, with a null to the side.
The electrically small loop, less that one third (⅓) of a wavelength in circumference is sensitive to a signal in its plane, meaning edge-on, and has a null broadside to it.
The third option is the large horizontal loop, several times the wavelength of the lowest frequency used. These typically require four large trees, or other supports.
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 horizontally polarised. These work well on harmonically related bands.
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 for a period. 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 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.
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, and this TV on the lowest channels, ending it is no longer used, and 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. Or a stupid stock image site calling it a cellular site here.
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.
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 feedpont, as well as opposite it. See: PAR Ominangles.
Big Wheel designs are larger, consisting of 3 petals, each projecting a a quarterwave from the centre, and having a half-wavelength contributing to the circumference of the wheel. (At 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 on: Widipedia (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 images and List 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.
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 using from a battery powered screwdriver to drive a contact up and down over a section of the coil which has has the insulation removed. This changes the inductance.
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.
Off the exam, centre loaded antennas, featuring a fairly large coil can be used, the coil referred to as a "bug-catcher". If mounded 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 larger vertical may be attached to the mount when stationary.
These are actual questions from the General exam pool.
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 left-hand circularly polarized antenna
D. A right-hand circularly polarized antenna
This is the low dipole, answer A.
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.
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.
What is the primary purpose of antenna traps?
A. To permit 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.
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.
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.
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 increases, as described in A.
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.
What is the primary use of a Beverage antenna?
A. Directional receiving for 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 low HF (and MF) signals, answer A.
In which direction or directions does an electrically small loop (less than 1/3 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.
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.
What is the common name of a dipole with a single central support?
A. Inverted V
B. Inverted L
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.
Each of the others are real, useful, antennas. The sloper tends to be a an unbalanced or single wire antenna, rather than a dipole, although these do exist. The Lazy H has two driven dipoles, and has significant gain if the spacing is large (¾ wavelength) but needs 4 or 6 supports.
If a taut central rope is used, two supports allow a yagi style antenna to be constructed from several wires, each in an inverted V format.
What is the combined vertical and horizontal polarization pattern of a multi-wavelength, horizontal loop antenna?
A. A figure-eight, similar to a dipole
B. Four major loops with deep nulls
C. Virtually omnidirectional with a lower peak vertical radiation angle than a dipole
D. Radiation maximum is straight up
The big loop is omnidirectional, with a low radiation angle, answer C.
On to the final page: Safety
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, April 2022.
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