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If you own a large property, a rhombic antenna is an option for an HF with a good gain, and a low angle of radiation, in a particular direction. They are more usually used for military communications; at the BBC's monitoring station, where several were used until recently to cover different regions; and in the past, for things like radio-telephone links between Australia and Pacific islands; or California and Shanghai. Each segment is around 1 to 2 wavelengths long, and they must be (at least) a half-wave above ground. Even at UHF, they are several metres long.
In some cases, for tasks such as HF (SW) broadcasting, they have been replaced by antennas such as curtain arrays. They do however have the benefit of relative simplicity, and broad bandwidth. You just need to stand four tall poles in a diamond pattern, and run wires between them.
An important feature is that there is often a terminating resistor at the far end of the antenna from the feedpoint. This make the antenna unidirectional (transmitting in the direction away from the feedpoint), rather than bidirectional. An alternative to the resistor, which reduces the efficiency of the antenna, is a resonant stub of balanced feedline.
You can see drawings in the Wikipedia article, and follow links to the patents: Rhombic antenna
If you drive past a MW broadcasting site, in many cases you will see just one tower, but in some cases, especially in the US, there are two, or more, antennas in a line, or some other arrangement. The reason these arrays are used for MW in the US is that they have a very large number of MW broadcasters, re-using the same frequency, or on nearby frequencies, in close proximity. The pattern is determined by the spacing and feed method (in-phase or out-of-phase), and helps to "protect" the other stations on the same frequency.
One option is a half-wave spacing, which gives a figure-8 pattern. If they are fed in-phase, then the pattern is broadside to the line of the antennas. If fed out-of-phase, the pattern is along the line of the array.
The other option explored is to site the two antennas a quarter wavelength apart, and feed the antennas 90 degrees out of phase. This generates a cardioid, or heart-shaped, pattern.
US broadcast station systems can have something like 9 masts, to generate complex patterns, in some cases switched in at night, along with a power reduction.
For HF these may fit a suburban yard unlike some of the models above, and are listed by the examiner under "practical" antennas.
These are a half-wave end-fed antenna, consisting of a single half-wave wire, or an odd multiple of a half-wave, such as 3λ/2. These are termed "end-fed dipoles". It is important to note that special feed arrangements are necessary, due to the very high impedance of such end-fed antennas. The name Zepp is from German "Zeppelin" airships. These used a resonant dangling wire antenna, a half-wavelength long, or 3λ/2, 5λ/2, etc.
If you can read German, you can read the patent, dated 1909, here, and see a overview of this (with brief translation), and other interesting antenna patents here.
An interesting tie-in is that the Zeppelins were built in Fredrichshafen, and it is possible to visit the Zeppelin Museum while visiting one of the largest Ham Radio events in the world, held in this southern German city. The company is still making airships, now helium filled.
What is even stranger is that people either can't separate the concept of a hydrogen filled craft from that of one powered by hydrogen safely stored in tanks, or they are being deliberatly thick.
There are several variations on the OCF or OCFD theme, with the version discussed by the examiner consisting of a dipole fed one-third the way along, using a 4:1 balun. These antennas allow operation of a range of HF bands.
A variation used by a friend is the Carolina Windom. Their version consists of a continuous horizontal element, connected to a single drop wire, grounded via the winding of a transformer, with coax feeding the other side of the transformer.
In the Technician section we discussed the folded dipole, for VHF and UHF, made from stainless steel or aluminium tubing. For HF a folded dipole can be made from parallel wires, totalling a full wavelength. These have an impedance of 300 ohms; and thus some form of balun is often used.
Construction is somewhat similar to ladder-line, with some form of spacer between the lines.
Note that many companies sell a variation with a resistor opposite the feed-point, as some sort of "miracle" wideband antenna, touted as needing no tuner. These are however not efficient, as energy is absorbed by the resistor. Perhaps their function is to transfer funds from the pockets of the gullible to those of the vendor, rather than to efficiently radiate signals?
This is a dipole antenna, designed by, and named for, Louis Varney G5RV. Also termed a doublet, the classic version is 31.1 metres, or 102 feet long. It is fed with ladder line or TV ribbon for 8 - 10 metres, depending on the impedance of the line. It is then connected to 50 ohm coax, either directly, or ideally via a 1:1 balun. The antenna may work on 14 MHz without a tuner, but can be used on several other HF bands, but certainly not all, using a tuner.
There are a range of modified versions, including the "half G5RV", the ZS6BKW, and the W0BTU; plus a version for 17 metres.
The article Wikipedia: G5RV, and linked pages, should provide enough information to build one, should you wish.
A better alternative, and one suggested by G5RV himself, is to run open-wire line (typically 420 - 450 ohms) all the way to a balanced tuner in the shack.
One method of using a single dipole on two or more bands is to use "traps", these being a resonant parallel circuit. If we want an 80 and 160 metre dipole we would use traps resonant on a frequency such as 3650 kHz. The traps are placed around 20 metres from the centre feedpoint. The trap prevents signals from the transmitter on 80 metres reaching the remainder of the antenna, and thus the transmitter sees a resonant antenna while transmitting on this band; and while transmitting on 160 metres, it sees the full antenna.
Traps are traditionally made from either a large diameter (50-125 mm) coil, or a toroidal inductor; and a fairly high voltage ceramic, silver mica, or other capacitor. The alternative is a using coils of coax. Capacitors are often several tens to 100 picofarads.
The downside is that if we have a 160 metre transmitter which is generating harmonics, especially on 80 metres, then this antenna will radiate them well, as the harmonic sees a good antenna, whereas on a standard dipole, it would see a high impedance.
Q stands for Quality Factor. High Q can be our friend when we want a good filter to resolve a weak Morse signal in a noisy band, but is our enemy when we want to use a single antenna to cover a whole low-HF band with good SWR. High Q reduces the bandwidth of the antenna.
Read more on Q in the Wikipedia article: Q factor
For various reasons we may not be able to use a quarter-wave antenna on HF, so some form of shortened arrangement is needed. Helicoil winding up a rod is one option. Another is to use a single loading coil. While "base loading" is an option, centre loading gives the best performance. These were popular for CB whips, on 27 MHz, where the 2.5 to 3 metre length for a full-sized antenna was often inconvenient. For amateur bands, often larger "bug-catcher" sized coils may be used.
One disadvantage of antennas shortened by the use of coils is narrower bandwidth than a full-length antenna. It is also important that these coils have low resistance, for good efficiency.
Part of the function of these coils is to cancel the capacitive reactance of a short antenna.
Note that the centre loading coil is typically a close-would coil of enamelled wire, rather than the open wound coils on Australian UHF CB, or old mobile 'phone whips; which act as phasing sections separating resonant sections. These antennas are sometimes termed "co-linear", as the elements are in a line.
Loading can also be used to build shortened dipoles, and similar antennas. The Buddipole is an example, often configured as a dipole, and mounted on a stand for field use.
One parameter in selecting or configuring an antenna is take-off angle. To "work DX", meaning make contacts with distant stations, we want a low angle of radiation, so that the signal reaches the ionosphere at the greatest distance, meaning the signal travel the greatest distance.
A vertical, especially a 5/8 wavelength generates a signal with low angle of radiation, especially if it is over a conducting surface, such as salt-water. Poorly conducting rocky ground typically results in a higher angle.
For horizontal antennas, the higher the antenna, the lower the angle of radiation. Factors, such as being on a slope, affect the angle. On a slope, the take-off angle on the down-hill side is lower than over flat ground.
Covered in other papers, in some cases we do want a high angle of radiation, such as NVIS.
These are actual questions from the published Extra exam pool.
What is the radiation pattern of two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase?
C. A figure-8 broadside to the axis of the array
D. A figure-8 oriented along the axis of the array
Given that 180 degrees is another way of saying half a wavelength, at a point a half wavelength from the antenna is at 180 degrees to what it is at the antenna, f we add a second antenna at this point, fed out of phase, we reinforce the signal along the axis of the two antennas. This reduces the signal broadside to the array. We get a figure-8 pattern oriented along the axis of the array, answer D.
What is the radiation pattern of two 1/4 wavelength vertical antennas spaced 1/4 wavelength apart and fed 90 degrees out of phase?
B. A figure-8 end-fire along the axis of the array
C. A figure-8 broadside to the axis of the array
This results in a cardioid pattern, answer A.
This is essentially a circular pattern with a null in one direction, somwhat reminiscent of the cleft in the heart symbol, ♡.
What is the radiation pattern of two 1/4 wavelength vertical antennas spaced a 1/2 wavelength apart and fed in phase?
C. A Figure-8 broadside to the axis of the array
D. A Figure-8 end-fire along the axis of the array
While the two signals would cancel each other along the line of the antennas, if we were a kilometre or so away from them, at 90 degrees, and could see the currents in the antennas, we would see them moving up and down in phase. We would also experience stronger signals than we would from a single antenna. The radiation pattern is figure-8, broadside to the antenna array, answer C.
What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased?
A. The lobes become more perpendicular to the wire
B. The lobes align more in the direction of the wire
C. The vertical angle increases
D. The front-to-back ratio decreases
Lobes on a 1/4 wave wire are at 90 degrees to the wire. At 3/4 wavelength antenna are at roughly 45 degrees to the wire, and they get closer to it as the wire becomes longer, answer B.
Which of the following is a type of OCFD antenna?
A. A dipole fed approximately 1/3 the way from one end with a 4:1 balun to provide multiband operation
B. A remotely tunable dipole antenna using orthogonally controlled frequency diversity
C. A folded dipole center-fed with 300-ohm transmission line
D. A multiband dipole antenna using one-way circular polarization for frequency diversity
The is the off-centre fed dipole, so as you may expect, they are cut as the 1/3 point. The selling point is the ability to work on several bands, answer A.
Many standard Remote Ham Radio stations use these, due to their multi-band ability.
What is the effect of adding a terminating resistor to a rhombic antenna?
A. It reflects the standing waves on the antenna elements back to the transmitter
B. It changes the radiation pattern from bidirectional to unidirectional
C. It changes the radiation pattern from horizontal to vertical polarization
D. It decreases the ground loss
It makes the antenna radiate in a single direction, answer B.
There were several at the OTC transmitting station at Doonside in Sydney, aimed at different Pacific islands. It would be undesirable to have high powered signals also fired backwards across rural NSW and beyond, potentially causing interference to other users, so they would have been terminated, to make them unidirectional.
What is the approximate feed point impedance at the center of a two-wire folded dipole antenna?
A. 300 ohms
B. 72 ohms
C. 50 ohms
D. 450 ohms
This is 300 ohms, higher than for a standard dipole, answer A.
As an aid-to-memory: To make an FM broadcast band receiving antenna, cut a length of 300 ohm TV ribbon to a half-wavelength, and short each end together. Then snip one of the two wires at the half-way point, and terminate these to a length of ribbon.
What is a folded dipole antenna?
A. A dipole one-quarter wavelength long
B. A type of ground-plane antenna
C. A half-wave dipole with an additional parallel wire connecting its two ends
D. A dipole configured to provide forward gain
This can be considered a dipole with an extra wire linking the two ends, as described in the FM antenna above, answer C.
Which of the following describes a G5RV antenna?
A. A multi-band dipole antenna fed with coax and a balun through a selected length of open wire transmission line
B. A multi-band trap antenna
C. A phased array antenna consisting of multiple loops
D. A wide band dipole using shorted coaxial cable for the radiating elements and fed with a 4:1 balun
This is a dipole usable on multiple bands. Its distinguishing feature is a "drop" of balanced feedline, such as TV ribbon, joined to 50 ohm coax, ideally through a balun, as in answer A.
Which of the following describes a Zepp antenna?
A. A dipole constructed from zip cord
B. An end fed dipole antenna
C. An omni-directional antenna commonly used for satellite communications
D. A vertical array capable of quickly changing the direction of maximum radiation by changing phasing lines
This is an end fed dipole antenna, answer B.
How is the far-field elevation pattern of a vertically polarized antenna affected by being mounted over seawater versus soil?
A. The low-angle radiation decreases
B. Additional higher vertical angle lobes will appear
C. Fewer vertical angle lobes will be present
D. The low-angle radiation increases
A benefit for DX-pedition stations activation remote islands, the low-angle radiation increases, answer D.
Which of the following describes an Extended Double Zepp antenna?
A. A wideband vertical antenna constructed from precisely tapered aluminum tubing
B. A portable antenna erected using two push support poles
C. A center fed 1.25 wavelength antenna (two 5/8 wave elements in phase)
D. An end fed folded dipole antenna
This is a large centre-fed antenna, 1.25 wavelength long. Each arm consists of a 5/8 wave element, answer C.
How does the radiation pattern of a horizontally polarized 3-element beam antenna vary with its height above ground?
A. The main lobe takeoff angle increases with increasing height
B. The main lobe takeoff angle decreases with increasing height
C. The horizontal beam width increases with height
D. The horizontal beam width decreases with height
The horizontal beamwidth decreases, answer B.
This potentially makes the antenna better for DX.
How does the performance of a horizontally polarized antenna mounted on the side of a hill compare with the same antenna mounted on flat ground?
A. The main lobe takeoff angle increases in the downhill direction
B. The main lobe takeoff angle decreases in the downhill direction
C. The horizontal beam width decreases in the downhill direction
D. The horizontal beam width increases in the uphill direction
The take-off angle reduces in the down-hill direction, answer B.
How much does the gain of an ideal parabolic dish antenna change when the operating frequency is doubled?
A. 2 dB
B. 3 dB
C. 4 dB
D. 6 dB
The dish might go from being 10 wavelengths wide to 20, and from 15 to 30 wavelengths high, so the dish covers four times the number of wavelengths squared. As we increase something by 4 times, a 6 dB increase applies, so it is answer D.
How can linearly polarized Yagi antennas be used to produce circular polarization?
A. Stack two Yagis fed 90 degrees out of phase to form an array with the respective elements in parallel planes
B. Stack two Yagis fed in phase to form an array with the respective elements in parallel planes
C. Arrange two Yagis perpendicular to each other with the driven elements at the same point on the boom fed 90 degrees out of phase
D. Arrange two Yagis collinear to each other with the driven elements fed 180 degrees out of phase
Two yagis perpendicular to each other, with the driven elements at the same point on the boom fed 90 degrees out of phase, answer C.
A few other similar arrangements are possible.
Where should a high Q loading coil be placed to minimize losses in a shortened vertical antenna?
A. Near the center of the vertical radiator
B. As low as possible on the vertical radiator
C. As close to the transmitter as possible
D. At a voltage node
It should be at or near the centre of the vertical element, answer A.
Why should an HF mobile antenna loading coil have a high ratio of reactance to resistance?
A. To swamp out harmonics
B. To lower the radiation angle
C. To minimize losses
D. To minimize the Q
Heavy, low resistance material in the coil minimises losses, answer C.
What usually occurs if a Yagi antenna is designed solely for maximum forward gain?
A. The front-to-back ratio increases
B. The front-to-back ratio decreases
C. The frequency response is widened over the whole frequency band
D. The SWR is reduced
The F-to-B ratio often decreases, answer B
What happens to the SWR bandwidth when one or more loading coils are used to resonate an electrically short antenna?
A. It is increased
B. It is decreased
C. It is unchanged if the loading coil is located at the feed point
D. It is unchanged if the loading coil is located at a voltage maximum point
The Q of the coil means that bandwidth is reduced, answer B
The upside is that efficiency at the frequency to which it is tuned to may be better, due to the coil.
What is an advantage of using top loading in a shortened HF vertical antenna?
A. Lower Q
B. Greater structural strength
C. Higher losses
D. Improved radiation efficiency
Top loading increases radiation efficiency, answer D.
My understanding is that the capacitance between the top loading "hat" and the ground increases current in teh antenna element, and thus the energy radiated.
What happens as the Q of an antenna increases?
A. SWR bandwidth increases
B. SWR bandwidth decreases
C. Gain is reduced
D. More common-mode current is present on the feed line
High Q reduces bandwidth, answer B.
What is the function of a loading coil used as part of an HF mobile antenna?
A. To increase the SWR bandwidth
B. To lower the losses
C. To lower the Q
D. To cancel capacitive reactance
Inductance tends to cancel capacitive reactance, answer D.
What happens to feed-point impedance at the base of a fixed length HF mobile antenna when operated below its resonant frequency?
A. The radiation resistance decreases and the capacitive reactance decreases
B. The radiation resistance decreases and the capacitive reactance increases
C. The radiation resistance increases and the capacitive reactance decreases
D. The radiation resistance increases and the capacitive reactance increases
As frequency reduces, the radiation resistance decreases, and the capacitive reactance increases, answer B.
Which of the following conductors would be best for minimizing losses in a station's RF ground system?
A. A resistive wire, such as spark plug wire
B. A wide flat copper strap
C. Stranded wire
D. Solid wire
Low loss implies low resistance, and this implies copper, over resistive wire; and for RF, due to skin effect we want the maximum surface area, so wide copper strap is best, answer B.
Which of the following would provide the best RF ground for your station?
A. A 50 ohm resistor connected to ground
B. An electrically short connection to a metal water pipe
C. An electrically short connection to 3 or 4 interconnected ground rods driven into the Earth
D. An electrically short connection to 3 or 4 interconnected ground rods via a series RF choke
The best option is a short connection to several ground rods, answer C.These should be spaced 1.8 to 2 metres apart. More rods, and more strings of them, are better.
On to: Antennas 3 - Transmission Lines
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, September 2022.
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