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I can't find a proper etymology of "Beam Antenna", but it appears to relate to a beam of light, that the signal is directed in a particular direction. The best know is the "Yagi–Uda", invented by the Japanese inventor Shintaro Uda, and patented by Hidetsugu Yagi. These consist of multiple elements along a boom. Typically, the second from rear-most element is the driven element, the rear-most, and longest is the is the reflector, and the forward elements are the directors, shorter than the driven element, and sometimes stepping down in side marginally along the antenna.
While it is possible to optimise these antennas using software, for factors such as wider bandwidth, better side or rear signal rejection, and to some extent, forward gain, if someone is claiming more than a dB or so above the industry standard for an antenna of a certain number of elements, or boom length, they are, um, exaggerating.
Note that there are both 2 element Yagis, with no directors, especially on HF, where they are large; and designs without a reflector.
|An MFJ-1763 3 element portable Yagi. The elements can be removed, and should fit into a suitcase when disassembled. The driven element is in the centre, the short directors at the top, the reflector at the bottom. It uses an SO-239 style socket.|
|This is the antenna configured for vertical use, suitable for FM voice and packet. Note that for vertical use it should not be set up with the pole going close to the driven element in the same plane, unless it is fibreglass.|
|This time it is shown set up for CW, SSB, and weak-signal digital modes. When set up for horizontal signals, the pole can pass through it vertically, at 90° to the elements, although the end mounting point could also be used.|
Another directional antenna is the Quad, or cubic quad, which also uses a boom, but instead of straight elements, they use a elements folded into a cage-like arrangement. More info here.
One version was designed at HCJB in Quito, Ecuador for broadcasting at high altitudes, as the thin air 3km up meant that huge corona discharges occurred off the ends of the Yagi elements, both burning them, and making an terrifying noise. The closed nature of the loops prevented arcing and discharge, even during high humidity at night. While small quad can be made with stiff elements, the alternative is to make crosses in + or × format, and wind diamonds or squares of flexible wire around them.
The Log-Periodic (LPDA) looks somewhat like a Yagi, but with all elements connected to the feedline; diminishing significantly in length going to the front. They are broadband, and directional, but have limited gain. The directional antennas in the question; the quad, Yagi, and dish are however NOT log-periodics. There are HF and VHF-UHF versions, the former popular with the military.
Dishes are capable of very high gain. The signal is directed at the dish, just as light directed into a parabolic reflector in a car headlamp or "PAR" stage lamp is directed forward. There are several kinds, the prime focus dish, where the feed is placed in the centre of a round dish, the downside being that the feed blocks part of the dish area.
A variation has a small convex reflector placed at the focal point, with the feed passing through the skin of the dish, called the Cassegrain feed. While you normally see these at NASA Deep Space Network sites in Canberra (at Tidbinbilla, run by CSIRO), Goldstone, and Madrid; there is one on the transmitter site hill in Oberon. One benefit of this is that instead of pointing at the warm, noisy earth, they are pointing at the cool, quiet sky. Voyager also uses one. A related feed is the Gregorian, with a concave reflector.
Modern pay-tv and many other dishes are "offset" dishes, using an oval dish, with the feed out of the path from the dish to the target. In Ham radio, dishes are used to direct signals in the various microwave bands to other stations, typically in hilltop to hilltop operations, including during contests. They can also be used in EME, and some satellite work. For the high microwave bands, a 25cm dish can have significant gain, but accuracy of the surface must be very good. For 70 cm, you need to build a 6 metre diameter dish to get real gain. This can be made from tensioned timber batons, and covered in fine chicken mesh.
Some clever folks at the Centre for Appropriate Technology in the Northern Territory have worked out that a dish a somewhat over a meter in diameter, pointed towards a distant mobile 'phone base can be used to make a community member or tourist's 'phone work where the signal would be too weak, by placing it on a small shelf at the feed-point, where the signal from the base is concentrated, and from which the signal from the handset is directed to the base station. It is normally used in hands-free mode.
|These two dishes concentrate energy in a particular direction, and also receive signals with considerable gain. You can see the electronics to generate and receive the signals on the back, with heatsinking on the enclosure. Uses include connecting mobile telephony and mobile data bases to the network, and broadcasting links, as well as linking telephone exchanges, and carrying corporate data networks.|
The smaller units are for higher frequencies, and maybe shorter distances, but may be "patch" antennas.
|A UHF train antenna, most likely recovered from a Pacific National locomotive, given the blue. These are bolted to the roof, with the coaxial connected to the N-connector. It is just 68 mm high, and even should it contact fallen wires, the energy would be shorted to the body.|
Beyond the exam, while a directional antenna can be set up pointing at a favourite distant repeater, or a friend's QTH; they can also be rotated, either by the "Armstrong Method", meaning manually; or more often, via a motorised rotator. A dial on the controller indicates the direction the antenna is pointing. For HF antennas programmes are available to calculate the direction the beam needs to pointed, and can even command the rotator controller to point in te required direction. Satellite tracking programs can control Elevation-Azimuth (El-Az) rotators.
As mentioned, a vertical antenna generates an electric field perpendicular to the earth, in line with the element. Vertical monopole antennas are practical for many amateur bands, HF Yagis are usually horizontal, including perhaps for practical reasons. VHF and UHF yagis can be vertical, the practise for FM and packet.
A dipole is an antenna with two equal wires, which if strung between two trees, poles, etc, are horizontally polarised. A dipole can be made from two stiff elements. A dipole can also be vertical, either a pair of stiff elements offset from a pole, or wires between a high branch, and a rope to the ground.
Hand-held radios, especially for 10m, 6m, 4m, 2m, and sometimes on 70cm are supplied with antennas which are less than a quarter-wavelength. They normally consist of a coil of wire with a rubber sheath, giving them the nickname "rubber duck", or "rubber ducky". The new question calls these the "integral" antenna, even if that implies they are non-removable. These are less efficient than quarter-wave, half-wave, or five-eighth antennas. Antennas of various lengths can be purchased from radio radio stores, and online. A telescopic half-wave has the benefit that they are "ground-plane independent". One thing you can try to improve gain is a "tiger-tail", a ⅝ length of insulated wire clipped over the BNC or SMA connector, and hanging down.
Using any hand-held device inside a vehicle with its rubber duck or similar antenna, signal strength will be significantly lower than if using it outside the vehicle. That said, if close to a repeater, or talking between two cars near each other, it will still work. Loss is typically higher at VHF than UHF. A ⅝ wave antenna has more gain than a quarter-wave one, but is physically longer. A steel whip has lower wind-loading than a braid-covered fibreglass rod, so far less likely to blow over while driving at freeway speeds*. An example is the Hustler MX-2.
*Australia is the only country with an open-road limit, 130 km/h in the Northern Territory, higher than the highest motorway or free-way limit, 110 in the various states. The states even include the revenue raised in their budget estimates, and so, more and more people die in fatigue and distraction related accidents in these states. "Insanity is doing the same thing over and over again, but expecting different results", not Einstein, but a character in a Rita Mae Brown novel.
As well as connecting your hand-held to a vehicle mounted antenna, you can use one mounted on your house. In either case, I suggest you use an speaker-microphone to save moving the radio constantly while using its internal microphone, repeatedly flexing the coax, causing the connection to eventually fail.
As a refresher, the wavelength in metres can be found by dividing 300 by the frequency in MHz. Some vertical antennas are a quarter this length, with better ones five-eighth; dipoles total a half-wavelength. In most cases a factor of 0.95 is applied to these lengths. To get millimetres or centimetres, multiply by 1000 or 100. Divide this by 25.4 or 2.54 to get the answer in ye olde inches.
By the way, say that as "the old", as "y" was substituted for the similar looking thorn, or þorn, in modern form Þ & þ. The e on "olde" is silent, and rare in real Old English. This is NOT the second person plural pronoun, "ye", replaced by you.
One problem with operating on HF from a vehicle is that even a quarter-wave antenna will be of impractical length. This can be solved using "loading" of various kinds. You may have seen HF CB antennas with a tube of about 100 mm length half way up the antenna, known as "centre loading". Another is base loading, where the coil is at the base (a little different to a coil used at the base of a five-eighth vertical, but this means that a ⅝ for 2 metres works well as a slightly short quarter-wave on 6 metres). The last coil option is "linear" loading, where a length of wire, usually a half wavelength, is wound over a fibreglass rod, getting tighter at the top, often with a short straight segment at the very top. Loading can also be used on dipoles. Not on the exam, capacitive top loading, consisting of a disc is probably less used on vehicles than broadcasting masts.
The pigtail coils on taller 70 cm, UHF CB, and cellular antennas have a different function, they are phasing several half-wave or other elements together. In stationary applications this can be a sideways U shaped element.
Adding a spring to a large HF antenna is a good idea, as it absorbs shocks both from hitting branches, and from bumps in the road. Some are below the feed-point, others above. These need to contain an internal braid strap to directly connect the base and antenna.
You will sometimes read something like λ/4, meaning quarter wavelength; or ⅝λ.
Below are a couple of example of popular cables, relevant to the exam.
|RG-58 is a 5 mm diameter cable, popular in Amateur Radio. OK for short runs at 100 watts or so, and interconnecting gear; and is popular for mobile use. It is 50 ohms, with RG-59 the 75 ohm version, this having a thinner inner conductor. Yes, this version is brown. It is shown with a crimp PL-259 style connector; the tip can instead be crimped, rather than soldered. The RG-58C/U version has a multi-strand core. LMR-200 (0.2") is similar. RG-223 is a dual shield version, with two braids.|
|The next on the exam is the RG-8, a 10 mm cable. Belden is a major coax maker, and also number this 9913. This can handle high power, and often has a solid core. It has significantly lower loss than RG-58. This goes far beyond the reduction in DC resistance, to factors such as the lower capacitance between the inner and shield. RG-213 is a similar sized cable, with a stranded core; as is LMR-400 (0.4"); while RG-224 is dual shield. Note that RG-8X is similar to RG-58. As above, this shown with a PL-259 connector, with a soldered inner connection.|
|This shows the thick pin on the PL-259 style connector. This means these so called "UHF" connectors have a low characteristic impedance, which means that at UHF part of the signal passing through it will be reflected back to the source. The name was created when anything above 30 MHz was the exotic world of UHF, and what we call VHF was used for things like experimental radar. Note that low impedance in this case does NOT mean low loss.|
Some air-cored cable is the made from copper pipe, and is rigid. Instead of normal connectors, it uses a "bullet" piece to join the inners, and flanges on the outer; and rather than bending, elbow joints are used. Another version is made from corrugated copper tubing, and these can be gently curved when installed. In both cases PTFE (Teflon) spacers are used to separate the inner conductor from the shield. Appropriate precautions are required to keep moisture out, and in some cases dry air or nitrogen is fed into the cable to force moisture out, and if flow is monitored, to indicate a leak. I expect, especially for black sheathed cable, it heats up, pushing air out; and when it cools, moist evening air is sucked back in by the vacuum created. Sizes range from around 10 cm to over 20. There are 50 ohm and 75 ohm versions.
Larger sizes of Heliax, made by Andrew are air-dielectric, while smaller types use a low-density plastic foam.
This is FSJ1-50A, terminated on a TNC plug. The 1 indicates that is it 1/4", which appears to be the inside diameter of the shield; the 50 the impedance.|
There is a -75 version, 75 ohms. You can see the ripples in the sheath, following the corrugated tube below. As you can see, there is a glue-lined heat-shrink fitted.
Somewhat ironically, while we have been saying fatter is better for higher frequencies, once we are into the microwave, there are limits on the size of the cable, as it starts to act as waveguide. Each size of hardline I saw on one site has a highest channel usable TV listed. Thus at microwave, a thin hard-line is often used with a solid teflon dielectric. As even short lengths have significant loss, at high microwave frequencies the electronics are mounted on the back of the usually small dish.
Beyond the exam, but useful if you get into building repeaters, or other duplex systems, where there is simultaneous transmission and reception via one cable, is that braid over foil cable should NOT be used, as micro-diodes form between the aluminium foil and the tinned braid, and these can cause inter-modulation products (a form of interference). The same is true where you might have several repeaters feeding a single antenna (including a mix of Amateur and other services). Either Heliax, or dual-braid cables (RG-214 or RG-223) can be used.
There are a few types of SWR meter. The classic style involves setting the transmitter to low power AM, typically around 10 watts. CW can also be used, or FM. SSB is NOT suitable, as the signal varies widely with voice modulation. Set the power level switch, if needed, and the FWD - REF to FWD (forward), transmit (identify, adding "Testing"), and adjust knob so the meter goes to the SET mark at the end of the scale. Un-key, flip the switch to REF, and key again. You can then read the SWR off the scale.
Another type is the "cross-needle" meter with why meter mechanisms on one meter face. One is deviated by the forward power, the other by the reverse power. In addition to the regular arcs with power, a series of lines with SWR numbers on them. The line at which the needles cross is the SWR reading.
These do not require power, although some have a power input socket for a lamp to back-light the meter
There are also various digital meters, some with a remote sensing element, and these do require power.
Yep, actual exam questions, from the published NCVEC Technician pool.
What is a beam antenna?
A. An antenna built from aluminum I-beams
B. An omnidirectional antenna invented by Clarence Beam
C. An antenna that concentrates signals in one direction
D. An antenna that reverses the phase of received signals
A beam "beams" signals in one direction, answer C.
Which of the following describes a type of antenna loading?
A. Inserting an inductor in the radiating portion of the antenna to make it electrically longer
B. Inserting a resistor in the radiating portion of the antenna to make it resonant
C. Installing a spring in the base of a mobile vertical antenna to make it more flexible
D. Strengthening the radiating elements of a beam antenna to better resist wind damage
Loading is inserting a coil into an antenna to make it appear electrically longer, answer A.
Which of the following describes a simple dipole oriented parallel to the Earth's surface?
A. A ground wave antenna
B. A horizontally polarized antenna
C. A rhombic antenna
D. A vertically polarized antenna
An antenna with the wired horizontal is horizontally polarised, answer B.
What is a disadvantage of the “rubber duck” antenna supplied with most handheld radio transceivers when compared to a full-sized quarter-wave antenna?
A. It does not transmit or receive as effectively as a full-sized antenna
B. It transmits a circularly polarized signal
C. If the rubber end cap is lost it will unravel very quickly
D. All of these choices are correct
These antennas are less efficient than a full sized antenna, answer A.
How would you change a dipole antenna to make it resonant on a higher frequency?
A. Lengthen it
B. Insert coils in series with radiating wires
C. Shorten it
D. Add capacitive loading to the ends of the radiating wires
To raise the resonant frequency you must shorten the elements, answer C. The other actions would lower the resonant frequency.
What type of antennas are the quad, Yagi, and dish?
A. Non-resonant antennas
B. Log periodic antennas
C. Directional antennas
D. Isotropic antennas
These are directional antennas, answer C.
What is a disadvantage of using a handheld VHF transceiver, with its integral antenna, inside a vehicle?
A. Signals might not propagate well due to the shielding effect of the vehicle
B. It might cause the transceiver to overheat
C. The SWR might decrease, decreasing the signal strength
D. All of these choices are correct
The vehicle partially shields the signal, so it is weaker than if the radio is used outside the car, or if it connected to a magnetically mounted (or other) antenna on the roof, assuming it is steel, not aluminium, answer A.
What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
A quarter of 2 meters is about 0.5 metres, but in old units this is about 19 inches, answer C.
What is the approximate length, in inches, of a 6 meter 1/2-wavelength wire dipole antenna?
Off the top of my head, 3 metres is between 9 and 10 feet, or 108 to 120 inches, and the only one in this range is C, 112". (Calculating 112" out, this takes into account the 0.95 factor, for an antenna built for 50.1 MHz, the boundary between the CW and SSB segments, and fine for either mode).
In which direction does a half-wave dipole antenna radiate the strongest signal?
A. Equally in all directions
B. Off the ends of the antenna
C. Broadside to the antenna
D. In the direction of the feed line
Dipoles radiate broadside to the antenna, answer C.
What is the gain of an antenna?
A. The additional power that is added to the transmitter power
B. The additional power that is lost in the antenna when transmitting on a higher frequency
C. The increase in signal strength in a specified direction when compared to a reference antenna
D. The increase in impedance on receive or transmit compared to a reference antenna
Put a reflector behind a lamp, and the brightness in one direction is increased at the detriment of other directions; and likewise, a directional antenna does this, the increase is relative to a reference antenna, such as a dipole or isotropic radiator, answer C.
What is an advantage of using a properly mounted 5/8 wavelength antenna for VHF or UHF mobile service?
A. It has a lower radiation angle and more gain than a 1/4 wavelength antenna
B. It has very high angle radiation for better communicating through a repeater
C. It eliminates distortion caused by reflected signals
D. It has 10 times the power gain of a 1/4 wavelength design
It has a lower angle of radiation, so coverage is often better, answer A.
T9A13 & T9A14 removed.
Why is it important to have low SWR when using coaxial cable feed line?
A. To reduce television interference
B. To reduce signal loss
C. To prolong antenna life
D. All of these choices are correct
A good match, indicated by a low SWR allows efficient transfer of power, and thus a lower signal loss, answer B.
What is the impedance of most coaxial cables used in amateur radio installations?
A. 8 ohms
B. 50 ohms
C. 600 ohms
D. 12 ohms
The usual coax for ham radio and other two-way systems is 50 ohms, answer B. No other answers here are in any way sensible.
Why is coaxial cable the most common feed line selected for amateur radio antenna systems? A. It is easy to use and requires few special installation considerations B. It has less loss than any other type of feed line C. It can handle more power than any other type of feed line D. It is less expensive than any other type of feed line
Unlike unshielded balanced cables such as ladder-line, coax cable can be zip-tied or taped to metal masts and towers, or bundled together, or passed through holes in walls quite easily, so answer A.
What is the major function of an antenna tuner (antenna coupler)?
A. It matches the antenna system impedance to the transceiver's output impedance
B. It helps a receiver automatically tune in weak stations
C. It allows an antenna to be used on both transmit and receive
D. It automatically selects the proper antenna for the frequency band being used
An antenna tuner presents the transceiver with a 50 ohm impedance, matching the antenna's impedance to this, answer A. This has been re-written to say "major function", as there are certainly tuners which can select antennas automatically, the SGC MAC-200 being an example; others manually.
In general, what happens as the frequency of a signal passing through coaxial cable is increased?
A. The characteristic impedance decreases
B. The loss decreases
C. The characteristic impedance increases
D. The loss increases
As the frequency increases, loss increases, answer D.
Which of the following connectors is most suitable for frequencies above 400 MHz?
A. A UHF (PL-259/SO-239) connector
B. A Type N connector
C. An RS-213 connector
D. A DB-25 connector
The Type N connector is a constant impedance connector, so they are more suitable for UHF frequencies, unlike the PL-259 & SO-239 pairing, called UHF when UHF was 30 MHz+, so answer B. The Yaesu FT-847 has an N on the 70 cm port, and European FT-857D has one for 2m & 70 cm. RS-213 makes no sense, presumably conflating RG-213, often used with N connectors in systems where reasonably low loss is needed, such as VHF & UHF bands; with RS-323C, a serial data protocol used between PCs and dial-up modems, TNCs, and some transceivers, pre-dating USB. DB-25 is a 25 pin connector in a housing longer than the DE-15 High Density one used for VGA on PCs, used for serial data up to a few hundred kilobits, among other things, NOT for radio signals; although there is a highly modified version which can mix data pins, power pins and lower-powered coaxial RF connections. While the US version of the FT-857D has a SO-239 on the 2m and 70cm port, this is not ideal.
Which of the following is true of PL-259 type coax connectors?
A. They are preferred for microwave operation
B. They are water tight
C. They are commonly used at HF frequencies
D. They are a bayonet type connector
PL-259 is really only good at HF, where they are common, answer C. They are certainly not suitable for microwaves, and is a threaded connector. All connectors need additional water-proofing if used outdoors.
Why should coax connectors exposed to the weather be sealed against water intrusion?
A. To prevent an increase in feed line loss
B. To prevent interference to telephones
C. To keep the jacket from becoming loose
D. All of these choices are correct
Moisture getting into coax cable affects the dielectric material, increasing loss, answer A. It also causes corrosion in the shield.
What might cause erratic changes in SWR readings?
A. The transmitter is being modulated
B. A loose connection in an antenna or a feed line
C. The transmitter is being over-modulated
D. Interference from other stations is distorting your signal
Erratic SWR can be caused by a loose connection, answer B.
What is the electrical difference between RG-58 and RG-8 coaxial cable?
A. There is no significant difference between the two types
B. RG-58 cable has two shields
C. RG-8 cable has less loss at a given frequency
D. RG-58 cable can handle higher power levels
This question now requires knowledge that RG-8 is significantly larger in diameter than RG-58; and from this realise that it has lower loss, answer C. RG-58 has a single shield, unlike some other cables.
Which of the following types of feed line has the lowest loss at VHF and UHF?
A. 50-ohm flexible coax
B. Multi-conductor unbalanced cable
C. Air-insulated hard line
D. 75-ohm flexible coax
Made from concentric copper tubing, and having a large diameter, air dielectric hard-line is most likely to have the lowest loss, answer C.
On to: Operating 3
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Written by Julian Sortland, VK2YJS & AG6LE, February 2018.
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