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In this section we will take a look at the types of wire useful for antenna elements, the different types of transmission lines, and grounding. The wire used in an antenna project can make or break the results, especially the reliability of the project over time. Antennas, especially outdoor antennas, are exposed to a lot of stress and environmental factors that tend to deteriorate both the performance and life expectancy.
TYPES OF WIRE
There are basically two types of wire used for the radiating element and other parts of antennas: solid ( Ill. 1A) and stranded ( Ill. 1B). The solid type of wire is made of a single thick strand of copper or copper alloy.
Pure copper is rarely used because of both cost and strength problems, while aluminum is almost never used because it does not take solder without special, high-cost, industrial equipment. Solid wire is extruded from a single piece of copper. This is the type of wire used most often in household electrical wiring. In its smaller sizes, it's also the type of wire most commonly used as hook-up wire in electronic construction projects.
The stranded form of wire is made of two or more (sometimes many more) lengths of solid wire twisted together ( Ill. 1B). The number of strands used to make the wire sets the flexibility of the wire. Coarse stranded wire has fewer strands, and is less flexible than wire of the same overall diameter that's made up of many more strands of finer wire.
ILL. 1 (a), (b)
Antennas should normally be built with stranded wire. The reason is that the stranded wire stands up better to the constant flexing an antenna experiences, especially in windy areas. All wire will begin to stretch because of both gravity and wind. Solid wire will stretch rather rapidly, and eventually come to a point where it's too thin at the strain point to hold its own weight - and will come tumbling down. Stranded wire, on the other hand, withstands the normal stresses and strains of antenna installations much better, and will stay up a lot longer. Stranded wire antennas may come tumbling down as well, but in general it takes a lot more time and effort for the stranded wire antenna to break, compared to the solid wire type.
Both stranded and solid wire comes in certain standard sizes. The specifications for each size wire were set basically for their ability to carry certain standard loads of direct current or the 50/60 Hz alternating current (AC) used in power systems. Tbl. 1 gives some of the standard wire sizes in the range normally used by antenna constructors. In most cases, it's a good idea to stick to sizes around AWG 14 (SWG 16) for antenna work. The larger sizes are expensive, and difficult to work (most antenna hardware assumes AWG 14 or SWG 16, unless otherwise marked). The smaller sizes, on the other hand, are nice to use but often provide poor reliability - they break easily. The trade off is to use wires, of whatever standard size, that are in the 1-2.2 mm diameter region.
TBL. 1 AWG ( USA) | Diameter (mil , mm) | Nearest SWG (UK)
8 10 12 14 16 18 20 22
128.5 101.9 80.8 64.1 50.8 40.3 32.0 25.3
3.264 2.588 2.053 1.628 1.291 1.024
10 12 14 16 18 19 21 24
Pure copper wire is not terribly good for radio antenna work because it's too soft to withstand the rigors of being outdoors, or supporting its own weight plus the weight of the fixtures and transmission lines used. As a result, one needs to find a better wire. Some people will use copper wires of harder alloys, but even that type of wire is not all that good. Others buy 'hard drawn' copper wire, and while it's better than straight electrical wiring copper wire it's not the best suited to antenna construction. The best wire for radio antennas is copper-clad steel wire, perhaps the most famous brand of which is Copperweld. This type of wire is made of a solid steel core coated with a copper layer. It looks like ordinary copper wire on the outside. The steel provides strength and thereby increases the reliability of the antenna installation. But steel is not a terribly good electrical conductor, so the manufacturer places a layer of copper over the outside of the wire.
The standard wire sold with the better antenna kits and by the better sources is copper-clad steel, stranded wire of AWG 14 (SWG 16) size. Each strand is copper-clad steel, and the strands are wound together to form the antenna wire.
The copper-clad wire is better for the purposes of strength, but there is a workability problem with it: copper-clad steel-core wire knots easily. If you get a knot or kink (even a minor bend) in it, that feature is always present from then on. Even if you attempt to straighten it out, it will not come anywhere near the original state. Although ordinary copper wire also possesses this attribute, it's not nearly as severe as with copper-clad steel-core wire, and can often be worked into a negligible size when it does occur. The trick is to make sure that the wire does not knot or kink during the installation process (and that can be a monumental task!).
One fair question regarding copper-clad steel-core wire concerns its resistivity. After all, any basic electrical text tells us that the cross-sectional area of the wire sets the current-carrying capacity. That is true for direct current (DC), and power line frequency AC, but only partially true for higher-frequency AC. Radio signals cause AC currents to be set up in the antenna wire, and because their frequency is so much higher than power line AC the currents flow only near the surface. The operating principle here is called the skin effect.
The skin effect refers to the fact that AC tends to flow on the surface of a conductor. While DC flows in the entire cross-section of the conductor, AC flows in a narrow band near the surface (Ill. 2). Current density falls off exponentially from the surface of the conductor toward the center (inset to Ill. 2). At the critical depth (_), also called the depth of penetration, the current density is 1/", or 1/2.718 = 0.368, of the surface current density.
The value of _ is a function of operating frequency, the permeability (_)of the conductor, and the conductivity (s).
WIRE SIZE AND ANTENNA LENGTH
The size of the wire used to make an antenna affects its performance. For amateur radio operators, one concern is the current-carrying capacity. For normal amateur radio RF power levels, the AWG 14 wire size works well, although those tempted to use smaller diameters with the higher legally permitted powers are forewarned: it's not such a good idea.
Both short-wave listeners and amateur radio operators need to be aware of another problem with the wire: the length to diameter ratio, or as it's sometimes specified, the signal-wavelength-to-wire-diameter ratio, determines the velocity factor (VF) of the conductor. The VF is defined as the velocity of the wave in the medium (i.e. wire) compared with the velocity in free space (which is c, the speed of light).
Ill. 3 shows the velocity factor as a function of the L=D ratio. Wire antennas tend to have very high L=D ratios, especially at high frequencies.
For example, a half wavelength at 10 MHz is 15m (or 15 000 mm), while AWG 14 wire is about 1.63mm diameter, resulting in L=D = 15 000= 1:63 = 9200. The VF of this L=D ratio is around 0.97-0.98. At very high/ ultrahigh frequencies (VHF/UHF), on the other hand, the L=D ratio declines. At 150MHz, the AWG 14 wire half-wavelength antenna has a physical length of 1500m, making L=D = 1500=1:63 = 920. The VF drops to about 0.95-0.96. At 450 MHz, the L=D ratio drops to 200, and the VF runs to 0.94 or so.
So what? Why is the VF important in antenna design? Most of the antennas in this guide are designed to be resonant, which means that the element lengths are related to wavelength: quarter wavelength (lambda/4), half wavelength (lambda /2), full wavelength (lambda), and so forth. These lengths are required electrical lengths, and are only found in free space with perfect connectors. In antennas and transmission lines, the VF shortens the physical length required to achieve a specified electrical length. To find the physical length, multiply the required electrical length by the velocity factor. For example, a half wavelength in free space is defined as 150/F_MHz, while in a wire it's 150 _ VF = F_MHz. The result is that antenna elements must be shortened a small amount compared to the free space length. If you cut some antennas to a physical length that's comparable to the wavelength of the desired resonant frequency, then you will find that the actual resonant frequency is a tad low-meaning that the antenna is a little bit too long.
This effect is almost negligible on the 80m band, but becomes more substantial at 10m and downright annoying at 2 m.
The wire between the antenna and either a transmitter or receiver is called the transmission line. Although it's tempting to think of the transmission line as a mere wire, it's actually equivalent to a complex inductor-capacitor network. The details of the transmission line network are a bit beyond the scope of this guide, but can be found in antenna engineering textbooks for those who are interested. The main aspect of the transmission line network that you need to understand is that the line possesses a property called the characteristic impedance or surge impedance, which is symbolized by Z_0. The rigorous definition of Z_0 can get a bit involved, but it breaks down to one thing for most practical situations: Z_0 is the value of load impedance that will result in the maximum power transfer between the antenna and the transmission line, or between the receiver/rig and transmission line. The act of making these impedances equal to each other (a very desirable thing, as you will find out) is called impedance matching, and is covered in Section 11.
Transmission lines come in a bewildering variety of species, but there are only a few basic types: parallel open lead, twin-lead, and coaxial cable are the main types that we need to consider. Ill. 4 shows all of these varieties.
Parallel open-wire line
Parallel line is shown in Ill. 4A. It consists of a pair of wires, run parallel to each other, and separated by a constant space. Insulating spacers (made of ceramic, nylon, or some other material) are used in practical parallel open line to keep the distance between the conductors constant.
The characteristic impedance of open line is determined by the diameter (d) of the conductors and the spacing (s) between them. Typical values run from 300 to 1000 ohms. Although you can calculate the spacings needed (and the wire diameters are given in some of my other guides), there are some general guidelines in Tbl. 2. The conductor type is along the vertical axis on the left, and the characteristic impedances are along the horizontal axis at the top. The entries in each cell are the spacings (s)in centimeters. As you can see from the table, only some of the calculated values are useful (who wants to make a 129 cm wide parallel line?).
Parallel open-wire transmission line can be either built or bought. For some applications, such as tuned feeders, it might be the best selection. But for many balanced antennas that use parallel line one might want to consider using twin lead instead.
TBL. 2 Characteristic impedance, Z_0 (ohms) Conductor 300 400 500 600 800
The twin-lead form of transmission line is shown in Figures 3.4B and 3.4C.
This type of line is like parallel line, except that the conductors are buried in an insulating material that also spans the space between the conductors.
Spacings are shortened a little bit compared to the open-wire variety because the dielectric constant of the insulating material reduces the VF.
The most common form is 300 ohm television antenna twin-lead, of the sort shown in Ill. 4B. This line is about 1 cm wide, and has a characteristic impedance of 300 ohms. Television twin-lead is easily available from a wide variety of sources, including shops catering for television and video customers. It can be used for transmitting at powers up to about 150W, but even that level is questionable if the line is of a cheaper variety.
Receive-only installations have no such limitation.
Television 300 ohm twin-lead may be solid as shown, or may have oblong or rectangular holes cut into the insulation separating the conductors. This tactic is used to reduce the losses inherent in the transmission line. In other cases, the center insulating material will be rounded and hollow. This approach to design also reduces the losses, and is especially effective at UHF.
The type of twin-lead shown in Ill. 4C is 450 ohm line. This type of line is specialist, and must be sought at shops catering for amateur and commercial radio operators. It is about twice as wide as the 300 ohm variety, and almost always comes with rectangular or oblong loss-reducing holes cut into the insulation. The 450 ohm twin-lead is considerably heavier than 300 ohm line, and uses larger conductors. As a result, it can handle higher power levels.
Twin-lead is wonderful stuff, but can cause problems in some installations. The problems are especially severe at VHF/UHF, but nonetheless exist at HF as well. For example, care must be given to where the line is run: it should not be in close proximity to metal structures such as aluminum house siding, rain gutter downspouts, and so forth. It is also susceptible to picking up local electrical fields, which means a possibility of interference problems. One solution is to use coaxial cable transmission line.
This type of cable is shown in Ill. 4D. The name 'coaxial' comes from the fact that the two conductors are cylindrical (the inner one being a wire) and share the same axis. The inner conductor and outer conductor are separated by the inner insulator. The outer conductor is usually a braided wire or foil shield, and is covered with an outer insulating material.
The characteristic impedances of commercial coaxial cables are of the order of 36-120 ohms, with 52 and 75 ohms being the most common. The specific impedance value is a function of the conductor sizes and the spacing between them. There are several standard types of coaxial cable, and these are available in many subtypes (Tbl. 3) that use similar numbering.
The terms 'thick' and 'thin' would have been replaced with actual dimensions if this guide were being written only a few years ago, but because of the wide array of different types now available it was decided to use these designations instead. One reason is that there are some RG-8/U specialist cables used by amateur radio and commercial antenna installers that are smaller than regular RG-8/U but larger than RG-58/U or RG-59/U.
One of the differences between the different forms of coaxial cable is the insulating material used for the center insulator. Several types are used: polyethylene, polyfoam and Teflon. A principal effect of these materials is to change the VF of the line.
Velocity factor The VF (denoted by V in some textbooks) of transmission line is calculated in exactly the same way as the VF of other conductors: it's the ratio of the velocity of the signal in the transmission line over the velocity of the signal in free space. The free space velocity is the speed of light (c), or about 300 000 000 m/s. The VFs of several popular transmission lines are shown in Tbl. 4.
The effect of the VF is to reduce the physical length of the coaxial cable that's cut for a particular frequency. And this foreshortening can be quite drastic. For example, if polyethylene cable is cut for an electrical half wave length at, say, 5 MHz, the actual length L =ð150 _ 0:66Þ=5 = 19:8m, while the wavelength of the signal in free space is 1.52 times longer, or 30m.
TBL. 3 Characteristic impedance, Z_0 (ohms)
CONNECTING THE TRANSMISSION LINE TO THE RIG OR RECEIVER
TBL. 4 Type of line :
Parallel open-wire 300 ohm twin lead 300 ohm twin lead (with holes in insulation) 450 ohm twin lead Coaxial cable Polyethylene Polyfoam Teflon
The method used to connect the transmission line to a receiver or transmitter depends on the type of output/input connector that's provided for the antenna, and the type of transmission line. In the discussion to follow, the illustrations show a receiver, but apply equally well to transmitters. The only difference is that parallel output on transmitters tends to be large ceramic feedthrough insulators, while on receivers, parallel line inputs tend to be small screw terminals on a plastic, Bakelite or other insulating carrier.
Ill. 5 shows the basic coaxial connector on a receiver. Although an SO-239 'UHF' connector is shown, and may well be the standard, you may also find BNC, Type-N, or RCA 'phono' plug connectors used on some rigs (I've even seen phono plug connectors used on 100W transmitters, but that's not the recommended practice!). In this type of installation, the coaxial cable from the antenna (or any antenna tuning unit that might be present) is fitted with a connector that matches the chassis connector on the receiver.
The threads of the male PL-259 connector are tightened onto the female SO 239 connector. Or, in the case of BNC connectors, they are pressed on and then rotated a quarter of a turn (it is a bayonet fitting), and phono plugs are just pushed into the socket.
If the receiver or transmitter is fitted with a ground connection, then it should be used. In short order we will discuss what a 'good ground' means in this context, but for the time being rest assured that you will need a heavy wire to the 'good ground' (whatever it's ). If your receiver or transmitter lacks a ground terminal, then wonder why and consider that next time you select a new model.
Although most modern receivers are fitted with coaxial connectors for the antennas, there are a few models (including many that are old but still useful) which are fitted with a balanced input scheme such as that shown in Ill. 6. In the case of Ill. 6A, there are three screw terminals: A1, A2, and G (or the variants 'ANT1', 'ANT2', and 'GND'). The A1 and A2 terminals form a balanced input for a parallel transmission line, while G is the chassis ground. When an unbalanced antenna transmission line, such as a single-wire downlead (Ill. 6B) is used, then the line goes to either A1 or A2, while the other is connected to G (in the case shown, A1 receives the antenna downlead, while A2 and G are connected together and then connected to the earth ground). Some models are equipped with only two terminals ( Ill. 6C), and in that case the selection is easy: connect the downlead to A1 (or just 'A' or 'ANT'), and the ground lead to
'G' or 'GND.' When parallel line, twin lead, or any other form of balanced line is used ( Ill. 6D), then connect 'G' or 'GND' to ground, and the two conductors of the transmission line to A1 (or ANT1) and A2 (or ANT2).
CONNECTING WIRES TOGETHER
Wire antennas often require that two or more wires be spliced together (for example, two antenna elements or an antenna element and a downlead).
There are right ways and wrong ways to accomplish this task. Let us look at some of the right ways.
All of the correct methods (only a few of which are shown here) have two things in common: they are (a) electrically sound, and (b) mechanically sound. Electrical soundness is created by making a good electrical connection, with the wires bound tightly together, and protected from the elements.
The electrical soundness of the joint is made better and longer lasting by soldering the connection.
Ill. 7 shows how to make the connection mechanically sound.
Overlap the two wires being spliced by a few centimeters ( Ill. 7A), and then wrap each one onto the other ( Ill. 7B), forming seven or more tight turns ( Ill. 7C). Once the splice is made, then solder both knots to form a better electrical connection.
One mistake made by novice antenna builders is to assume that the purpose of soldering is to provide mechanical strength. This is false.
Solder only improves and keeps good the electrical connection. Solder for radio work is a 50/50 or 60/40 lead/tin mixture with a resin core (NEVER acid core!). It is soft and has no inherent mechanical strength. Always depend on the splice to provide mechanical strength, and NEVER use solder for strength.
CONNECTING WIRES TO ANTENNAS AND SUPPORTS
The methods for connecting wires and antenna elements are basically the same as above, but with differences to account for the types of hardware being used in the antenna construction. Ill. 8 shows how the ends of the wire antenna are supported. An end insulator is used. These insulators may be of glass, ceramic, or a synthetic material (nylon is common), but all will have a body and two holes. The wire from the antenna element is passed through one hole, and then looped back onto itself to form seven or more turns. As in the previous case, the splice may be plated with solder to prevent corrosion from interfering with the electrical integrity of the connection. The other end of the insulator is fitted to a rope that's run to a tree, edge of the building, or a mast of some sort. The rope should be treated similarly to the wire in that it should be wrapped over on itself several times before being knotted.
ILL. 8 ILL. 9
If you use an antenna that has a single-wire downlead, then you should connect it to the antenna in the manner of Ill. 9. The downlead is almost always insulated wire, even when the antenna is not insulated. The insulation of the downlead prevents it being shorted out somewhere along its run. The scheme in Ill. 9A shows how most people connect the downlead (and, indeed, how I have connected them quite often). This is not the best practice, although it will work most of the time. The problem is that the connection is stressed by the downlead, and may break. More than once I have been off-the-air due to a spliced downlead breaking right at the junction with the antenna wire. A better solution is shown in Ill. 9B. In this installation, the downlead wire is passed through the hole in the insulator, and then wrapped back on itself three or four times before being spliced to the antenna wire in the usual manner. This arrangement provides some strain relief to the downlead, and that (usually) translates into a longer life expectancy.
ILL. 10 , ILL. 11
Some antennas, such as the center-fed half-wavelength dipole (a very popular wire antenna), use a center insulator to separate the two halves of the antenna. Coaxial cable is connected such that the inner conductor is spliced to one antenna element, and the shield (outer conductor) is connected to the other antenna element. Good practice is to splice the shield to the element closest to the house, if this is a factor. The method shown in Ill. 10A will work, but it's also a bit foolish (I know, I know, I have used it and even recommended it in the past). The problem is that the center conductor is quite weak, and may be even smaller than single-wire down leads. As a result, it has no strength. To make matters worse, coaxial cable weighs more length-for-length than single-conductor downleads (or even some twin lead). The connection of Ill. 10A will work, but only for a while - it will surely come down prematurely. A better scheme is shown in Ill. 10B. In this case, the coaxial cable is wrapped once around the center insulator, and secured with a length of twine or string. The electrical connections are made in exactly the same manner as shown in Ill. 10A.
Perhaps the best solution is to use a manufactured center insulator (Ill. 11). These devices come in a wide variety of sizes and shapes, so this illustration represents a lot of different devices. What they all share in common, however, are means for strain relieving the antenna element wires and coaxial cable. Some devices may be straight insulators, while others may contain BALUN transformers inside (of which, more shortly). Some models include special strain relief features for the coaxial cable, or they may have a screw-eye on top to secure the antenna center to a mast or other support.
The electrical connections between the center insulator and the wire antenna elements are done in the usual manner (see above), but the ends of the wires are usually attached to eyelet connectors, which are crimped and then soldered. It has been my experience that these connectors are often corroded (as seen by the dull appearance) when they arrive, so brighten them with sandpaper or steel-wool before attempting to make the connections permanent.
Lightning is messy stuff, and can create havoc with your radio - and might set your house on fire. But there are some things you can do to protect yourself. But first, let us take on one myth right away: contrary to popular belief, antennas do NOT inherently attract lightning. Lightning will usually strike the highest object around, and if that happens to be an antenna, then the antenna will take a hit. And that hit could set the house on fire, and the radio will be damaged or even destroyed. The lightning would have come anyway, and if it takes out the antenna rather than your house, then it acted like a lightning rod and protected your property.
If lightning actually strikes your antenna, then it will probably wipe out your transmitter or receiver no matter what you do. But a direct hit is not needed for fatal damage to occur: even lightning overhead or striking nearby will induce a large enough voltage into your antenna to wreak havoc inside the equipment. And it may be silent damage because it's not so spectacular as the problems seen with direct hits.
The key to protecting your equipment is to use a lightning arrestor (Ill. 12). These devices come in several types, each suitable for coaxial line, parallel line, or single-wire downleads. Each will have at least one connection for the antenna wire, and a ground connection. It is placed in the line between the antenna lead and the receiver or transmitter, and should be mounted outside the house. There is very little that can be done for a direct hit, but I have heard tales of near hits only causing a small amount of rig or receiver damage because an effective lightning arrestor was in place.
Also, keep in mind that some local laws or insurance regulations make the lightning arrestor mandatory. If you fail to use one, you may be liable for a fine from the local government, or find your homeowner's insurance not valid after a lightning episode. Lightning arrestors are cheap, and so make very reasonably priced insurance.
Both impedance transformation and conversion between unbalanced and balanced lines can be accomplished with a BALUN transformer (the name comes from BALanced-UNbalanced). Two sorts of transformers are used (Ill. 13). The 1:1 BALUN transformer converts the balanced load to an unbalanced form (so it can be fed with coaxial cable). The impedance of the load must be the same as the source. The version shown in Ill. 13B is a 4:1 BALUN. This transformer is used to provide not only a balanced-to-unbalanced conversion, but also a 4:1 impedance transformation. For example, a half-wavelength folded dipole antenna is balanced with respect to ground, and has a feedpoint impedance near 300 ohms. A 4:1 BALUN transformer converts the 300 ohm load to 75 ohms, and makes it appear an unbalanced rather than balanced load.
BALUN transformers can be easily built using toroidal coil forms or ferrite rods. The 1:1 BALUN is wound in the trifliar style, i.e. the three wires are wound side by side so that all three coils interleave with one another. The 4:1 BALUN is wound in the bifilar manner. If you don't wish to make your own BALUN transformers, then you will find that there are many commercially available BALUN transformers on the market.
VHF/UHF scanner users will find few BALUNs specially for their needs, but there are plenty of devices available. The television antenna transformers are not usually called 'BALUNs,' but when you find a transformer that converts 75 ohm coaxial cable to 300 ohm twin-lead, then it's (rest assured) a BALUN, and will work over the entire VHF/UHF range unless marked otherwise. BALUN transformers are bidirectional devices, so can be used either way around.
Antenna safety is an absolute must. First, let me make one thing absolutely, positively, clear in your mind:
DO NOT EVER TOSS AN ANTENNA WIRE OVER THE AC POWER LINES!
If you plan to do this trick, then have your family prepare for some solemn praying, sad music, and slow marching, for your funeral is imminent. This action kills radio hobbyists every year. There is no safe way to do it, so don't . Even if both the antenna wire and the power line are insulated, the forces of erecting the antenna will slice through both insulations! When that happens, the wires will come into contact with each other and with you - with disastrous consequences: human bodies make lousy fuses.
The next matter is like unto the first: don't erect your antenna where it can fall onto the power lines, or other people's property, either during erection or as a result of a structural failure. Falling onto the electrical lines can make the antenna deadly, and falling onto your neighbor's property is just plain rude (and might get you sued).
Finally, don't install an antenna without assistance. I know they are lightweight, and all that, but they have a remarkable 'sail area,' and even very weak winds will increase their apparent weight. I've suffered injury myself (wrenched back) by working alone, and a friend of mine broke a pelvis and a leg falling off a roof while trying to install a television antenna.
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|EMC Testing | Environmental
Updated: Thursday, 2014-11-20 23:39 PST