Misplaced Pages

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79-441: The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a shorted quarter-wave parallel transmission line stub . For a transmitting antenna to operate efficiently, absorbing all the power provided by its feedline, the antenna must be impedance matched to the line; it must have a resistance equal to the feedline's characteristic impedance . A half-wave antenna fed at one end has

158-435: A 'circular aerial' because of the shape) and the halo antenna . Higher-gain omnidirectional antennas can also be built. "Higher gain" in this case means that the antenna radiates less energy at higher and lower elevation angles and more in the horizontal directions. High-gain omnidirectional antennas are generally realized using collinear dipole arrays . These consist of multiple half-wave dipoles mounted collinearly (in

237-791: A characteristic impedance of 76.7 Ω. When more common dielectrics are considered, the lowest insertion loss impedance drops down to a value between 52 and 64 Ω. Maximum power handling is achieved at 30 Ω. The approximate impedance required to match a centre-fed dipole antenna in free space (i.e., a dipole without ground reflections) is 73 Ω, so 75 Ω coax was commonly used for connecting shortwave antennas to receivers. These typically involve such low levels of RF power that power-handling and high-voltage breakdown characteristics are unimportant when compared to attenuation. Likewise with CATV , although many broadcast TV installations and CATV headends use 300 Ω folded dipole antennas to receive off-the-air signals, 75 Ω coax makes

316-423: A common ground at the house. See ground loop . External fields create a voltage across the inductance of the outside of the outer conductor between sender and receiver. The effect is less when there are several parallel cables, as this reduces the inductance and, therefore, the voltage. Because the outer conductor carries the reference potential for the signal on the inner conductor, the receiving circuit measures

395-416: A convenient 4:1 balun transformer for these as well as possessing low attenuation. The arithmetic mean between 30 Ω and 77 Ω is 53.5 Ω; the geometric mean is 48 Ω. The selection of 50 Ω as a compromise between power-handling capability and attenuation is in general cited as the reason for the number. 50 Ω also works out tolerably well because it corresponds approximately to

474-421: A current node at its feedpoint, giving it a very high input impedance of around 1 000–4 000 ohms . This is much higher than the characteristic impedance of transmission lines, so it requires an impedance matching circuit between the antenna and the feedline. A shorted quarter-wave stub, a transmission line one quarter of the wavelength long with its conductors shorted together at one end, has

553-409: A function of frequency, voltage handling capability, and shield quality. Coaxial cable design choices affect physical size, frequency performance, attenuation, power handling capabilities, flexibility, strength, and cost. The inner conductor might be solid or stranded; stranded is more flexible. To get better high-frequency performance, the inner conductor may be silver-plated. Copper-plated steel wire

632-493: A good choice both for carrying weak signals that cannot tolerate interference from the environment, and for stronger electrical signals that must not be allowed to radiate or couple into adjacent structures or circuits. Larger diameter cables and cables with multiple shields have less leakage. Common applications of coaxial cable include video and CATV distribution, RF and microwave transmission, and computer and instrumentation data connections. The characteristic impedance of

711-571: A line), fed in phase. The coaxial collinear (COCO) antenna uses transposed coaxial sections to produce in-phase half-wavelength radiators. A Franklin Array uses short U-shaped half-wavelength sections whose radiation cancels in the far-field to bring each half-wavelength dipole section into equal phase. Another type is the Omnidirectional Microstrip Antenna (OMA). Omnidirectional radiation patterns are produced by

790-444: A non-circular conductor to avoid current hot-spots. While many cables have a solid dielectric, many others have a foam dielectric that contains as much air or other gas as possible to reduce the losses by allowing the use of a larger diameter center conductor. Foam coax will have about 15% less attenuation but some types of foam dielectric can absorb moisture—especially at its many surfaces—in humid environments, significantly increasing

869-423: A similar high impedance node at its open end, making a good match to the antenna. The input impedance seen at a point along the stub varies continuously, decreasing monotonically from this high value to zero at the shorted end. So any value of input impedance can be obtained by connecting the feedline to the proper point along the stub. One arm of the stub is extended a half wavelength to make the antenna. By attaching

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948-527: A similar way to how a folded dipole is related to a dipole . The Slim Jim is one of many ways to form a J-Pole. Introduced by Fred Judd (G2BCX) in 1978, the name was derived from its slim construction and the J ;type matching stub ( J I ntegrated M atching). The Slim Jim variation of the J-pole antenna has characteristics and performance similar to a simple or folded half-wave antenna and identical to

1027-485: A solid copper, stranded copper or copper-plated steel wire) surrounded by an insulating layer and all enclosed by a shield, typically one to four layers of woven metallic braid and metallic tape. The cable is protected by an outer insulating jacket. Normally, the outside of the shield is kept at ground potential and a signal carrying voltage is applied to the center conductor. When using differential signaling , coaxial cable provides an advantage of equal push-pull currents on

1106-423: A solid metal tube. Those cables cannot be bent sharply, as the shield will kink, causing losses in the cable. When a foil shield is used a small wire conductor incorporated into the foil makes soldering the shield termination easier. For high-power radio-frequency transmission up to about 1 GHz, coaxial cable with a solid copper outer conductor is available in sizes of 0.25 inch upward. The outer conductor

1185-550: A type of waveguide . Power is transmitted through the radial electric field and the circumferential magnetic field in the TEM mode. This is the dominant mode from zero frequency (DC) to an upper limit determined by the electrical dimensions of the cable. Coaxial connectors are designed to maintain a coaxial form across the connection and have the same impedance as the attached cable. Connectors are usually plated with high-conductivity metals such as silver or tarnish-resistant gold. Due to

1264-616: A weaker signal at the end of the cable and radio frequency interference to nearby devices. Severe leakage usually results from improperly installed connectors or faults in the cable shield. For example, in the United States, signal leakage from cable television systems is regulated by the FCC, since cable signals use the same frequencies as aeronautical and radionavigation bands. CATV operators may also choose to monitor their networks for leakage to prevent ingress. Outside signals entering

1343-522: Is a good approximation at radio frequencies however for frequencies below 100 kHz (such as audio ) it becomes important to use the complete telegrapher's equation : Applying this formula to typical 75 ohm coax we find the measured impedance across the audio spectrum will range from ~150 ohms to ~5K ohms, much higher than nominal. The velocity of propagation also slows considerably. Thus we can expect coax cable impedances to be consistent at RF frequencies but variable across audio frequencies. This effect

1422-423: Is also used as an insulator, and exclusively in plenum-rated cables. Some coaxial lines use air (or some other gas) and have spacers to keep the inner conductor from touching the shield. Many conventional coaxial cables use braided copper wire forming the shield. This allows the cable to be flexible, but it also means there are gaps in the shield layer, and the inner dimension of the shield varies slightly because

1501-568: Is corrugated like a bellows to permit flexibility and the inner conductor is held in position by a plastic spiral to approximate an air dielectric. One brand name for such cable is Heliax . Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the spacing between the center conductor and shield. The dielectric losses increase in this order: Ideal dielectric (no loss), vacuum, air, polytetrafluoroethylene (PTFE), polyethylene foam, and solid polyethylene. An inhomogeneous dielectric needs to be compensated by

1580-415: Is obtained. Being a half-wave antenna, it provides a small gain of just under 1 dB over a quarter-wave ground-plane antenna. Primarily a dipole, the J-pole antenna exhibits a mostly omnidirectional pattern in the horizontal (H) plane with an average free-space gain near 2.2 dBi (0.1 dBd) . Measurements and simulation confirm the quarter-wave stub modifies the circular H-plane pattern shape increasing

1659-418: Is often used as an inner conductor for cable used in the cable TV industry. The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or air with spacers supporting the inner wire. The properties of the dielectric insulator determine some of the electrical properties of the cable. A common choice is a solid polyethylene (PE) insulator, used in lower-loss cables. Solid Teflon (PTFE)

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1738-553: Is referenced in IEC 61917. A continuous current, even if small, along the imperfect shield of a coaxial cable can cause visible or audible interference. In CATV systems distributing analog signals the potential difference between the coaxial network and the electrical grounding system of a house can cause a visible "hum bar" in the picture. This appears as a wide horizontal distortion bar in the picture that scrolls slowly upward. Such differences in potential can be reduced by proper bonding to

1817-400: Is roughly inversely proportional to the cutoff frequency . A propagating surface-wave mode that only involves the central conductor also exists, but is effectively suppressed in coaxial cable of conventional geometry and common impedance. Electric field lines for this TM mode have a longitudinal component and require line lengths of a half-wavelength or longer. Coaxial cable may be viewed as

1896-439: Is still a seam running the length of the cable. Foil becomes increasingly rigid with increasing thickness, so a thin foil layer is often surrounded by a layer of braided metal, which offers greater flexibility for a given cross-section. Signal leakage can be severe if there is poor contact at the interface to connectors at either end of the cable or if there is a break in the shield. To greatly reduce signal leakage into or out of

1975-449: Is supported by a spiral strand of polyethylene, so that an air space exists between most of the conductor and the inside of the jacket. The lower dielectric constant of air allows for a greater inner diameter at the same impedance and a greater outer diameter at the same cutoff frequency, lowering ohmic losses . Inner conductors are sometimes silver-plated to smooth the surface and reduce losses due to skin effect . A rough surface extends

2054-432: Is used as a transmission line for radio frequency signals. Its applications include feedlines connecting radio transmitters and receivers to their antennas, computer network (e.g., Ethernet ) connections, digital audio ( S/PDIF ), and distribution of cable television signals. One advantage of coaxial over other types of radio transmission line is that in an ideal coaxial cable the electromagnetic field carrying

2133-418: Is used for straight-line feeds to commercial radio broadcast towers. More economical cables must make compromises between shield efficacy, flexibility, and cost, such as the corrugated surface of flexible hardline, flexible braid, or foil shields. Since shields cannot be perfect conductors, current flowing on the inside of the shield produces an electromagnetic field on the outer surface of the shield. Consider

2212-401: The monopole antenna , consisting of a vertical rod conductor mounted over a conducting ground plane , and vertical dipole antenna , consisting of two collinear vertical rods. The quarter-wave monopole and half-wave dipole both have vertical radiation patterns consisting of a single broad lobe with maximum radiation in horizontal directions, so they are popular. The quarter-wave monopole,

2291-536: The skin effect , the RF signal is only carried by the plating at higher frequencies and does not penetrate to the connector body. Silver however tarnishes quickly and the silver sulfide that is produced is poorly conductive, degrading connector performance, making silver a poor choice for this application. Coaxial cable is a particular kind of transmission line , so the circuit models developed for general transmission lines are appropriate. See Telegrapher's equation . In

2370-418: The skin effect . The magnitude of an alternating current in a conductor decays exponentially with distance beneath the surface, with the depth of penetration being proportional to the square root of the resistivity. This means that, in a shield of finite thickness, some small amount of current will still be flowing on the opposite surface of the conductor. With a perfect conductor (i.e., zero resistivity), all of

2449-618: The H-plane of the Collinear J antenna is from 4.6 to 5.2 dBi (2.4 dBd to 3.1 dBd). The graph compares the E-plane gain of the above three variations to the conventional J antenna. The conventional J antenna and SlimJIM variation are nearly identical in gain and pattern. The Super-J reveals the benefit of properly phasing and orienting a second radiator above the first. The Collinear J shows slightly higher performance over

J-pole antenna - Misplaced Pages Continue

2528-470: The RG-series designations were so common for generations that they are still used, although critical users should be aware that since the handbook is withdrawn there is no standard to guarantee the electrical and physical characteristics of a cable described as "RG-# type". The RG designators are mostly used to identify compatible connectors that fit the inner conductor, dielectric, and jacket dimensions of

2607-454: The Super-J. The basic J antenna resonates on the third harmonic of its lowest design frequency. Operating a ⁠ 3 / 2 ⁠ wavelengths this way produces an antenna pattern unfavorable for terrestrial operation. To address the pattern change a variety of techniques exist to allegedly constrain a J antenna operating at or near the third harmonic so only one half-wave is active in

2686-468: The antenna's feedline to the proper point along the transmission line, the stub will transform this impedance down to match the lower feedline impedance, allowing the antenna to be fed power efficiently. During construction the proper attachment point for the feed-line is found by sliding the connection of the feedline back and forth along the stub while monitoring the SWR until an impedance match (minimum SWR)

2765-420: The antenna's operation. Later research confirms the tendency of the mast or grounding wire to draw current from the antenna potentially spoiling the antenna pattern. A common approach extends the conductor below the bottom of the J-pole resulting in additional and undesirable RF currents flowing over every part of the mounting structure. This modifies the far field antenna pattern typically, but not always, raising

2844-457: The area inside the cable. Coaxial lines can therefore be bent and moderately twisted without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them, so long as provisions are made to ensure differential signalling push-pull currents in the cable. In radio-frequency applications up to a few gigahertz , the wave propagates primarily in the transverse electric magnetic (TEM) mode , which means that

2923-399: The assumption of a sin ⁡ ( b θ ) / b θ {\displaystyle \sin(b\theta )/{b\theta }} pattern shape is: Coax cable Coaxial cable , or coax (pronounced / ˈ k oʊ . æ k s / ), is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield , with

3002-405: The axis ( elevation angle ), declining to zero on the axis. When graphed in three dimensions (see graph) this radiation pattern is often described as doughnut-shaped . This is different from an isotropic antenna , which radiates equal power in all directions, having a spherical radiation pattern. Omnidirectional antennas oriented vertically are widely used for nondirectional antennas on

3081-466: The braid cannot be flat. Sometimes the braid is silver-plated. For better shield performance, some cables have a double-layer shield. The shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers, such as "quad-shield", which uses four alternating layers of foil and braid. Other shield designs sacrifice flexibility for better performance; some shields are

3160-535: The cable ( Z 0 ) is determined by the dielectric constant of the inner insulator and the radii of the inner and outer conductors. In radio frequency systems, where the cable length is comparable to the wavelength of the signals transmitted, a uniform cable characteristic impedance is important to minimize loss. The source and load impedances are chosen to match the impedance of the cable to ensure maximum power transfer and minimum standing wave ratio . Other important properties of coaxial cable include attenuation as

3239-401: The cable can cause unwanted noise and picture ghosting. Excessive noise can overwhelm the signal, making it useless. In-channel ingress can be digitally removed by ingress cancellation . An ideal shield would be a perfect conductor with no holes, gaps, or bumps connected to a perfect ground. However, a smooth solid highly conductive shield would be heavy, inflexible, and expensive. Such coax

J-pole antenna - Misplaced Pages Continue

3318-505: The cable from water infiltration through minor cuts in the jacket. For internal chassis connections the insulating jacket may be omitted. Twin-lead transmission lines have the property that the electromagnetic wave propagating down the line extends into the space surrounding the parallel wires. These lines have low loss, but also have undesirable characteristics. They cannot be bent, tightly twisted, or otherwise shaped without changing their characteristic impedance , causing reflection of

3397-536: The cable, by a factor of 1000, or even 10,000, superscreened cables are often used in critical applications, such as for neutron flux counters in nuclear reactors . Superscreened cables for nuclear use are defined in IEC 96-4-1, 1990, however as there have been long gaps in the construction of nuclear power stations in Europe, many existing installations are using superscreened cables to the UK standard AESS(TRG) 71181 which

3476-478: The coax itself, affecting the radiation pattern of the antenna. With sufficient power, this could be a hazard to people near the cable. A properly placed and properly sized balun can prevent common-mode radiation in coax. An isolating transformer or blocking capacitor can be used to couple a coaxial cable to equipment, where it is desirable to pass radio-frequency signals but to block direct current or low-frequency power. The characteristic impedance formula above

3555-425: The coaxial cable is terminated in a pure resistance equal to its impedance. Signal leakage is the passage of electromagnetic fields through the shield of a cable and occurs in both directions. Ingress is the passage of an outside signal into the cable and can result in noise and disruption of the desired signal. Egress is the passage of signal intended to remain within the cable into the outside world and can result in

3634-516: The conventional J and connects the two with a phase stub to ensure both vertical half-wave sections radiate in current phase. The phasing stub between the two half-wave sections is often of the Franklin style . The Super-J antenna compresses the vertical beamwidth and has more gain than the conventional J-pole design. Both radiating sections have insufficient separation to realize the maximum benefits of collinear arrays, resulting in slightly less than

3713-455: The conventional J-pole construction. Judd reported that the Slim Jim produces a lower takeoff angle and better electrical performance than a ⁠ 5 / 8 ⁠ wavelength ground plane antenna, however others' test and analyses show Slim Jim antennas to have no performance advantage over a conventional, single-wire J-pole antenna. Slim Jim antennas made from ladder transmission line use

3792-409: The current path and concentrates the current at peaks, thus increasing ohmic loss. The insulating jacket can be made from many materials. A common choice is PVC , but some applications may require fire-resistant materials. Outdoor applications may require the jacket to resist ultraviolet light , oxidation , rodent damage, or direct burial . Flooded coaxial cables use a water-blocking gel to protect

3871-430: The current would flow at the surface, with no penetration into and through the conductor. Real cables have a shield made of an imperfect, although usually very good, conductor, so there must always be some leakage. The gaps or holes, allow some of the electromagnetic field to penetrate to the other side. For example, braided shields have many small gaps. The gaps are smaller when using a foil (solid metal) shield, but there

3950-467: The dimensions of the cable and connectors are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a transmission line. Coaxial cable was used in the first (1858) and following transatlantic cable installations, but its theory was not described until 1880 by English physicist, engineer, and mathematician Oliver Heaviside , who patented the design in that year (British patent No. 1,407). Coaxial cable

4029-454: The electric and magnetic fields are both perpendicular to the direction of propagation. However, above a certain cutoff frequency , transverse electric (TE) or transverse magnetic (TM) modes can also propagate, as they do in a hollow waveguide . It is usually undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes with different phase velocities to propagate, interfering with each other. The outer diameter

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4108-732: The elevation (or E plane ) reveals a slight elevation of the pattern in the direction of the J element while the pattern opposite the J element is mostly broadside. The net effect of the perturbation caused by quarter-wave stub is an H-plane approximate gain from 1.5 to 2.6 dBi (-0.6 dBd to 0.5 dBd). Like all antennas, the J-pole is sensitive to electrically conductive objects in its induction fields (aka reactive near-field region ) and should maintain sufficient separation to minimize these near field interactions as part of typical system installation considerations. The quarter wave parallel transmission line stub has an external electromagnetic field with strength and size proportional to

4187-403: The existing parallel conductor for the folded dipole element, but in the copper pipe variation, the Slim Jim requires almost twice as much material, for which it returns no performance benefit. The approximate gain in the H-plane of the Slim Jim is from 1.5 to 2.6 dBi (−0.6 dBd to 0.5 dBd). The Super-J variation of the J-pole antenna adds another collinear half-wave radiator above

4266-522: The feedpoint impedance of a half-wave dipole, mounted approximately a half-wave above "normal" ground (ideally 73 Ω, but reduced for low-hanging horizontal wires). RG-62 is a 93 Ω coaxial cable originally used in mainframe computer networks in the 1970s and early 1980s (it was the cable used to connect IBM 3270 terminals to IBM 3274/3174 terminal cluster controllers). Later, some manufacturers of LAN equipment, such as Datapoint for ARCNET , adopted RG-62 as their coaxial cable standard. The cable has

4345-448: The fields before they completely cancel. Coax does not have this problem, since the field is enclosed in the shield. However, it is still possible for a field to form between the shield and other connected objects, such as the antenna the coax feeds. The current formed by the field between the antenna and the coax shield would flow in the same direction as the current in the center conductor, and thus not be canceled. Energy would radiate from

4424-404: The following section, these symbols are used: The best coaxial cable impedances were experimentally determined at Bell Laboratories in 1929 to be 77 Ω for low-attenuation, 60 Ω for high-voltage, and 30 Ω for high-power. For a coaxial cable with air dielectric and a shield of a given inner diameter, the attenuation is minimized by choosing the diameter of the inner conductor to give

4503-488: The form "RG-#" or "RG-#/U". They date from World War II and were listed in MIL-HDBK-216 published in 1962. These designations are now obsolete. The RG designation stands for Radio Guide; the U designation stands for Universal. The current military standard is MIL-SPEC MIL-C-17. MIL-C-17 numbers, such as "M17/75-RG214", are given for military cables and manufacturer's catalog numbers for civilian applications. However,

4582-477: The gain slightly on the side of the J stub element and reducing the gain slightly on the side opposite the J stub element. At right angles to the J-stub, the gain is closer to the overall average: about 2.2 dBi (0.1 dBd). The slight increase over a dipole's 2.15 dBi (0 dBd) gain represents the small contribution to the pattern made by the current imbalance on the matching section. The pattern in

4661-406: The image; multiple reflections may cause the original signal to be followed by more than one echo. If a coaxial cable is open (not connected at the end), the termination has nearly infinite resistance, which causes reflections. If the coaxial cable is short-circuited, the termination resistance is nearly zero, which causes reflections with the opposite polarity. Reflections will be nearly eliminated if

4740-399: The inner conductor and inside of the outer conductor that restrict the signal's electric and magnetic fields to the dielectric , with little leakage outside the shield. Further, electric and magnetic fields outside the cable are largely kept from interfering with signals inside the cable, if unequal currents are filtered out at the receiving end of the line. This property makes coaxial cable

4819-426: The inner conductor so that the two voltages can be cancelled by the receiver. Many senders and receivers have means to reduce the leakage even further. They increase the transformer effect by passing the whole cable through a ferrite core one or more times. Common mode current occurs when stray currents in the shield flow in the same direction as the current in the center conductor, causing the coax to radiate. They are

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4898-451: The loss. Supports shaped like stars or spokes are even better but more expensive and very susceptible to moisture infiltration. Still more expensive were the air-spaced coaxials used for some inter-city communications in the mid-20th century. The center conductor was suspended by polyethylene discs every few centimeters. In some low-loss coaxial cables such as the RG-62 type, the inner conductor

4977-423: The low to high impedance points is available. The J-pole design functions well when fed with a balanced feed (via balun , transformer or choke) and no electrical connection exists between its conductors and surrounding supports. Historical documentation of the J antenna suggests the lower end of the matching stub is at zero potential with respect to earth and can connect to a grounding wire or mast with no effect on

5056-504: The lowest capacitance per unit-length when compared to other coaxial cables of similar size. All of the components of a coaxial system should have the same impedance to avoid internal reflections at connections between components (see Impedance matching ). Such reflections may cause signal attenuation. They introduce standing waves, which increase losses and can even result in cable dielectric breakdown with high-power transmission. In analog video or TV systems, reflections cause ghosting in

5135-591: The most compact resonant antenna, may be the most widely used antenna in the world. The five-eighth wave monopole, with a length of 5 / 8 = 0.625 {\displaystyle 5/8=0.625} of a wavelength, is also popular because at that length a monopole radiates maximum power in horizontal directions. Common types of low-gain omnidirectional antennas are the whip antenna , "Rubber Ducky" antenna , ground plane antenna , vertically oriented dipole antenna , discone antenna , mast radiator , horizontal loop antenna (sometimes known colloquially as

5214-407: The mounting structure. The antenna consists of two parallel straight metal conductors, one ⁠ 3 / 4 ⁠ of a wavelength and the other ⁠ 1 / 4 ⁠ of a wavelength long at the operating frequency, shorted together at the bottom. Typical construction materials include metal tubing, ladder line , or twin-lead . Since the matching section must act as a transmission line,

5293-457: The opposite of the desired "push-pull" differential signalling currents, where the signal currents on the inner and outer conductor are equal and opposite. Most of the shield effect in coax results from opposing currents in the center conductor and shield creating opposite magnetic fields that cancel, and thus do not radiate. The same effect helps ladder line . However, ladder line is extremely sensitive to surrounding metal objects, which can enter

5372-451: The optimal 3 dB over a conventional J-pole or halfwave antenna. The approximate gain in the H-plane of the Super-J antenna is from 4.6 to 5.2 dBi (2.4 dBd to 3.1 dBd). The collinear J antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil. The resulting gain is closer to the optimum 3 dB over a conventional J-pole or halfwave antenna. The approximate gain in

5451-414: The parallel conductors should be no more than .02 wavelength apart. The J-pole antenna and its variations may be fed with balanced line. A coax feed line may be used if it includes a means to suppress feed-line RF currents. The feed-point of the J-pole is somewhere between the closed low-impedance bottom and open high-impedance top of the J stub. Between these two extremes a match to any impedance between

5530-454: The primary lobes above the horizon reducing antenna effectiveness for terrestrial service. J-pole antennas with electrical connection to their supports often fare no better, and often much worse, than the simpler monopole antenna . A mast decoupling stub reduces mast currents. A variation of the J-pole is the Slim Jim antenna, also known as G2BCX Slim Jim , that is related to the J-pole in

5609-482: The radiator above the stub. All involve the use of a high impedance choke at the first voltage loop. These methods fall short of the goal as choking a high impedance point with a high impedance allows energy to pass the choke. Omnidirectional antenna In radio communication , an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis ( azimuthal directions), with power varying with angle to

5688-414: The signal back toward the source. They also cannot be buried or run along or attached to anything conductive , as the extended fields will induce currents in the nearby conductors causing unwanted radiation and detuning of the line. Standoff insulators are used to keep them away from parallel metal surfaces. Coaxial lines largely solve this problem by confining virtually all of the electromagnetic wave to

5767-413: The signal exists only in the space between the inner and outer conductors . This allows coaxial cable runs to be installed next to metal objects such as gutters without the power losses that occur in other types of transmission lines. Coaxial cable also provides protection of the signal from external electromagnetic interference . Coaxial cable conducts electrical signals using an inner conductor (usually

5846-470: The simplest practical antennas, monopole and dipole antennas , consisting of one or two straight rod conductors on a common axis. Antenna gain (G) is defined as antenna efficiency (e) multiplied by antenna directivity (D) which is expressed mathematically as: G = e D {\displaystyle G=eD} . A useful relationship between omnidirectional radiation pattern directivity (D) in decibels and half-power beamwidth (HPBW) based on

5925-453: The spacing between the parallel conductors. The parallel conductors must be kept free of moisture, snow, ice and should be kept away from other conductors including downspouts, metal window frames, flashing, etc. by a distance of two to three times the spacing between the parallel stub conductors. The J-pole is very sensitive to conductive support structures and will achieve best performance with no electrical bonding between antenna conductors and

6004-687: The surface of the Earth because they radiate equally in all horizontal directions, while the power radiated drops off with elevation angle so little radio energy is aimed into the sky or down toward the earth and wasted. Omnidirectional antennas are widely used for radio broadcasting antennas, and in mobile devices that use radio such as cell phones , FM radios , walkie-talkies , wireless computer networks , cordless phones , GPS , as well as for base stations that communicate with mobile radios, such as police and taxi dispatchers and aircraft communications. The most common omnidirectional antenna designs are

6083-627: The two separated by a dielectric ( insulating material); many coaxial cables also have a protective outer sheath or jacket. The term coaxial refers to the inner conductor and the outer shield sharing a geometric axis. Coaxial cable is a type of transmission line , used to carry high-frequency electrical signals with low losses. It is used in such applications as telephone trunk lines , broadband internet networking cables, high-speed computer data busses , cable television signals, and connecting radio transmitters and receivers to their antennas . It differs from other shielded cables because

6162-413: The wrong voltage. The transformer effect is sometimes used to mitigate the effect of currents induced in the shield. The inner and outer conductors form the primary and secondary winding of the transformer, and the effect is enhanced in some high-quality cables that have an outer layer of mu-metal . Because of this 1:1 transformer, the aforementioned voltage across the outer conductor is transformed onto

6241-505: Was manifested when trying to send a plain voice signal across the transatlantic telegraph cable , with poor results. Most coaxial cables have a characteristic impedance of either 50, 52, 75, or 93 Ω. The RF industry uses standard type-names for coaxial cables. Thanks to television, RG-6 is the most commonly used coaxial cable for home use, and the majority of connections outside Europe are by F connectors . A series of standard types of coaxial cable were specified for military uses, in

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