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Radio Frequency - Antenna

An antenna is an RF component used to transform an RF signal, traveling on a conductor, into an airbourne wave and vice versa. Antennas are passive devices that radiate and pick up radio frequency energy (RF). Antennas are typically designed so that they work with the desired operation frequency, have a wanted radiation pattern and are matched to the cable connected to them (most often 50 ohm coaxial cable, can also be 75 ohm coax or 240-300 ohm flatline).

Antennas do not create RF energy. In transmitting applications antennas focus the energy in a pecific area or direction, which increases the signal strength in that direction or area. This is specified as Gain in units of dBi. An antenna with 0dBi gain is one which radiates in all directions equally. An antenna with 12dBi gain, has a direction in which the signal is 12db stronger than in another direction. In reception the antenna gain helps to the antenna to pick up signals from one direction stronger than from other directions. This directivity is very important if you need to receive weak signals in noisy environment.

Every antenna and every antenna feed-line have a characteristic impedance, or opposition to electrical current. In an ideal situation, the impedances of line and antenna match perfectly, and 100 percent of the electrical energy sent to the antenna is converted to radio energy and radiated into the atmosphere. In a less than ideal case, when the impedances aren't perfectly matched, some of the electrical energy sent to the antenna won't be converted to radio energy, but will be reflected back down the feed-line. The energy reflecting back from the antenna causes standing waves of electrical energy in the feed-line. The ratio of highest voltage on the line to lowest is the standing wave ratio. In the perfectly matched system, the SWR is 1:1. Typical radio equipment (transitters and receivers) are designed for 50 ohm impedance (many consumer radio receivers and TVs are designed for 75 ohms impedance). An ideal antenna solution has an impedance of 50 ohm all the way from the transceiver to the antenna, to get the best possible impedance match between transceiver, transmission line and antenna. Since ideal conditions do not exist in reality, the impedance in the antenna interface often must be compensated by means of a matching network, i.e. a net built with inductive and/or capacitive components. Antenna matching is essential in transmitting circuits. A poorly matched antenna connected to a transmitter means that some part of transmitting power does not get to the antenna, but is lost somewhere else, for example on radio equipment output stage (poor matching or missing antenna can lead to transmitter damages on high power transmitting systems). In receiving antennas poor impedance matching causes signal attenuation, meaning poorer radio reception.

To radiate efficiently, a transmitting antenna has to be resonant. If the antenna is not suitable for the transmitted frequencyand transmitter impedance, the result is very much reducedperformance and even a transmitter damage (usully with highpower transmitters). At first sight the radiation resistance of an antenna has no influence on the radiated power, as long as you match your transmitter to this resistance. But unfortunately the radiation resistance is not the only resistance that is consuming the transmitter power, there are also the loss resistances. These losses occur within the antenna (+ the antenna matching system) and in the environment of the antenna (ground, objects near the antenna). In receiving the antenna quality is not so critical if maximumperformance is not needed. If the antenna is not optimal, thereceived signal is just weaker than with optimal antenna. Antenna operation and coverage are the same whether the antenna is transmitting or receiving.

The oldest antenna structure is the dipole, or Hertz, which is usually fed by a transmission line at the antenna's center point. It is self-resonant at a length of one-half the operating wavelength, with an impedance of 72(ohm). Ideally, an antenna should be one-half the wavelength of the transmitting frequency. Maximum current flow at the center of the half wave, maximum voltage at the ends. The impedance at the center happens to work out at about 72 ohms, which matches standard 75 ohm coaxial cable very nicely. Thus, the half wave antenna is most usually broken into two equal quarter waves and fed by coaxial cable at the center. This type of antenna is known as a half wave dipole, and is the fundamental type by which the performance of other types of antenna are judged. Half wave dipole antenna is a single band antenna that offers 2dB of gain in a relatively narrow frequency range.

Slightly younger than the dipole is the monopole, also called the "whip", "quarter wave ground plane" or Marconi, antenna. It is constructed as a system where one leg of a half wave dipole is replaced by a sheet of metal at right angles (this acts like a reflector). The monopole is a vertical dipole; however, the phantom reflection of a conductive ground plane underneath the antenna replaces one leg of the dipole. This antenna is one-fourth-wavelength long, and its impedance is 36(ohm), one-half that of a dipole. The roof or trunk of a car, or body of a walkie talkie acts as a good reflector. The feed impedance of a quarter wave ground plane is around 40 ohms, sufficiently close to 50 ohm coaxial cable to form a potential match. This antenna has theoretically circular azimuth radiation pattern. Unfortunately, the ideal full conductive plane under the antenna usually is nonexistent or erratic. The actual azimuth pattern, thus, depends heavily on installation and use, in contrast to its theoretical circular pattern. The elevation radiation angle is also a function of the ground-plane situation and antenna's height above ground.

The third type of antenna, the loop, can be rectangular or circular and resonates at a perimeter length of one wavelength; it is fed by simply breaking anywhere into the loop. Although loops are often mechanically difficult to support at long wavelengths, they are practical when frequencies get up to hundreds of Mhz.

Discone Antenna is a relative of the 1/4 wave ground plane antenna optimized for wide frequency bandwidth reception. It typically offers 0dB of gain, on frequencies from about 120-1300 MHz, and with a vertical element on top, it is usable down to about 30 MHz. Gain is achieved by compressing the radiation pattern into a donut shape with little of the signal radiating upwards or downwards, concentrating the pattern perpendicular to the vertical axis of the antenna. This antenna type is called a discone because it is comprised of two parts, the disc, a group of elements parallel to the ground around the top, and the cone, the diagonal radial elements around the bottom. These could be made from a solid metal disc and a cone shaped sheet metal radial.

There are also many other antenna constructions. Many of the more complicated antennas are antennas that have more controlled directivity than those simple basic antennas. Directional antennas are used for example for point-to-point communications applications, cellular base stations and TV signal reception. Those antenna consist typically of a large number of antenna segments placed at suitable distance from each other. Quater wave length segments are very common and useful. The most well known antennas of this kind of are Yagi and Log Periodic antennas. The most useful feature of this kind of beam antenna in reception, is that the can be rotated to null out a signal you do not want or maximizing the one you do want. In transmitting applications you can point your signal to where you want to send it.

Yagi antenna is named after it's inventors Mr Yagi and Mr Uda. Yagi antenna is a single band antenna that offers typically 10-20dB of gain and 10-30dB of front-to-back isolation in a relatively narrow frequency range. A yagi antenna is built out of a group of dipoles all the same length, connected to a boom, to hold them a specific distance apart. It offers excellent gain, and front-to-back isolation, and a narrow beam width that it will receive from. The gain is determined by how many elements are used as directors, and is achieved by limiting how many directions a signal can be received from. The down side is, it will only have gain in a narrow frequency range of about +/-1% of the center frequency. Yagi antenna is most commonly used by commercial and amateur operators, since it is an inexpensive and very efficient type of antenna for single band.

Log Periodic Antennas are remarkable antennas that exhibite relatively uniform input impedances and radiation characteristics over a wide range of frequencies. Log-periodic (LP) antenna is a broadband, multielement, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The length and spacing of the elements of a log-periodic antenna increase logarithmically from one end to the other. The Logarithmicly Periodic Dipole Array (LPDA) is a beam antenna optimized for wide frequency bandwidth. It offers 5-15dB of gain with a moderate 10-15dB of front-to back ratio; the beam width is fairly wide when compared to a Yagi. It is a group of dipoles of decreasing size (with the longest in back and the smallest in front), connected to a boom, to hold them a specific distance apart. The tapering of the elements is what gives it the wide frequency range, by always providing an element that resonates near the frequency that your operating on. It is most commonly used in TV antennas, where operation on many frequencies is required.

Thare are also antenna types that can be integrated easily to circuit board. The patch antenna is a conducting surface separated from an underlying ground plane by a dielectric; a double-sided circuit board often works as a dielectric. Each edge is one-half wavelength at resonance, or you can use a circular patch with a radius of 0.3[lambda]. You feed the antenna through a small hole in the ground plane.

Antennas in mobile applications are often smaller than the free-space or ideal-ground self-resonant dimensions indicate. In addition, the antenna is near other electronic circuitry, a user's body, an enclosure, power circuitry, and structures. Fortunately, antennas that are smaller than resonant size can still be effective radiators or energy receivers. Pagers, for example, use loop antennas that are about (1/10)[lambda]. However, the impedance-matching circuitry between the antenna and the power amplifier or front end causes losses and, thus, wasted power, reduced coverage, or weaker received-signal strength.

TV antennas are antennas that are optimized for the TV bands reception. If you look closely at a TV antenna you will notice that the taper of the elements is not uniform. There will be several long ones (Chan 2-6 at 54-88MHz) then several medium long ones, usually interspersed with the long ones (Chan 7-13 at 175-216MHz), and then a bunch of short ones, all the same length (UHF 470-812MHz). UHF elements on a TV antenna are almost alwasys a Yagi design, and the reception range that they advertise is only on one channel or few TV channels. There are also antennas with wider response. A typical 4-bay bow tie, it has about 6dB of gain, a 15dB front-to-back ratio and resonates across a wide frequency range. Nowadays there are also quite good wideband TV antennas that use Logarithmicly Periodic Dipole Array (LPDA) design. Broad band LPDA TV antennas are always optimized only TV frequencies, and do not typically receive other frequencies well.

It's relatively easy to build an antenna that covers one specific frequency. It's a lot harder to make one that covers a wide range of frequencies well. There are also special antenna constructions for special applications. When you need to flood a wide but defined area with RF energy, such as for perimeter security systems, tunnels, and cellular- or 802.11-system interior zones, one approach is to use an RF-leaky feeder cable to provide controlled radiation.

Ideal free-space antennas have a purely resistive impedance. Smaller antennas usually have a lower resistive component to their impedance, and most part capacitance and/or inductance. For example, an antenna with several ohms of resistance, fed by matching circuitry with a comparable resistance, wastes half the transmitted or received power in the matching circuitry. The lower antenna resistance causes higher antenna currents and ohmic losses through matching components. Short dipoles and monopoles have a capacitive impedance. Therefore, the matching circuitry that transforms the antenna's complex impedance into an apparent resistance must introduce inductance to compensate. You implement this inductive loading in monopoles as a discrete wire coil at the antenna base, a coiling of the antenna whip at its base, or a continuously wound helix around a flexible core�the common, rugged, bendable "rubber-duck" antenna. Most pagers and wireless wands use loop antennas. Unlike the dipole and monopole, the smaller-than-resonant loop antenna is inductive and needs capacitive compensation to yield the resistive result.

Impedance matching is necessary to keep the VSWR low enough for your application. Relatively low-power mobile units can often accept VSWR values as high as 1.5 or 2, although higher power base-station transmitters usually need VSWRs lower than 1.5 to prevent output-stage damage. You should also filter the transmitted RF signal to minimize interference and intermodulation.

A good general rule for antennas is as big as practical, as high aspractical, as clear of obstructions as practical, and watch out forpower lines.There's really no substitute for a decent rooftop antenna on TV and radio reception. When installing and using antennas that are outside, please pay attention to a proper lightning protection. At basic this is a good grounding of all metal parts in the antenna with a grounding system that can survive lightning strike. In addition ther could be need to have some overvoltage protectors on the antenna lines (if you need those or not can vary depending on the enviroment and value of equipment connected to antenna).

The cabling between antenna and the transmitting/receving equipment cause also losses. Those need to be taken into consideration when designing the antenna positioning. It doesn't matter how good your antenna is, if you are feeding it with lossy coaxial cable. The loss that a coax has, is determined by many factors, most having to do with the density and effectiveness of the shield and the dielectric, and the length of the cable. Frequency is the other major contributing factor in determining your losses. The higher the frequency, the higher the loss. Here is a chart of some common 50 ohm coax and their loss at different frequencies for comparison: 

                         Losses in dB per 100 feet (30m)        

            50MHz          100MHz         500MHz        900MHz  

          ----------    -----------    -----------   -----------

RG-58A/U      3.3          4.9           13.3           20.0    

RG-8/U        1.2          1.8            4.7            6.7    

Belden 9913   0.9          1.4            2.9            4.2    

1/2" Heliax   0.56         0.83           2.0            2.8
The losses scale proportionally with length. Half as long, half the loss in dB. Double the length causes double the loss.

An antenna system needs to be correctly constructed to work well. If you have an antenna system that once worked well, but is not working well anymore, here are few tips to find and fix receiving antenna problems (most tips apply also to transmitting antennas as well). First visually inspect every inch of you antenna system. Look for loose connections, corrosion, cut or burnt cable, cracked insulation, foreign metallic objects or birds nests on the antenna, bent antenna elements, antenna aiming, and problems with splitters in line. Next unhook the cable at the antenna and place a short across it. The measured resistance should be "low" depending on cable length. Then remove the sort and measure again. Now the resistance should be high. If you are using a coaxial line (as opposed to twinlead) check the balun at the antenna... or just replace it. they don't cost much. Look for a "blob" inline near the antenna that might be an inline amplifier, check that is is working correctly and getting the operating power it needs (could be separate powerinc cable or powered through the antenna coax cable). If the antennas you have are many years old, consider replacing the antenna, because many cheap typical consumer antennas just don't seem to hold many years in hard environment.

 

 

 

    Antenna cabling issues

    Even the best antenna and the most expensive receiver will not produce an acceptable output (audio or picture) if the transmission line has not been carefully selected and correctly installed. The transmission line from antenna to receiver is more important than most people realize. Proper transmission line from transmitter to receiver is also important. There are three basic types of transmission line used for antenna connection: 300 ohm twinlead, 75 ohm coaxial cable and 50 ohm coaxial cable. 300 ohm twinlead and 75 ohm coaxial cable are typically used for antenna connections in consumer TV and radio reception application. 300 ohm twin-lead has a characteristic impedence which allows the signal to be best transfered from the 300ohm antenna to the 300 ohm input connections on the TV (on those TVs that has those). Using a different cable could reduce the signal level but it may not be a factor if you have a high signal strength. 75 ohm coaxial cables are typically very low loss coaxial cables that are used to transport signals from antenna to TV in applications where shielded cable is needed and the signal input is matched to 75 ohms (usually the antenna itself has different than 75 ohm impedance, and it is matched to 75 ohm cable impedance with suitable matchign network/balun built into antenna). Common antenna network wiring is typically built using 75 ohm coaxial cable and coaxial antenna signal inputs on TVs and FM radio receivers are matched to 75 ohms. The 50 ohm coaxial cable is the type used on on radio transmission applications, and the most often used coaxial cable type in professional radio applications. You will see 50 ohm coaxial cable in almost all radio transmitters, cellular phone antenna cabling, radiophone antenna wiring, etc.

     

    • 50 ohms versus 75 ohms in antenna cable  
    • Attenuation Losses in dB/100 ft (30 meters) of Coaxial Transmission Line and Maximum Average Power Handling Capabilities for Coaxial Transmission Line - This table give the data for coaxial cable type RG-8, RG-11, 9913, RG-58/6, 3/8" Foam, 1/2" Foam, 7/8" Foam, 7/8" Air and 1 5/8" Air.  
    • Microwave connector reference - This document has pictures and some basic information on BNC, TNC, N, UHF, C. SMA, SMB, SMC and APC-7. 
    • Most Often Asked Questions About Power Splitter / Combiners 
    • Putting a Balun and a Tuner Together - A balun is a transformer that converts an AC signal from balanced to unbalanced, or vice versa. This document has information on antenna baluns and their use.  
    • Quarter-Wave Power Divider - use this to phase two antennas together for increased gain  
    • RF Coaxial Connectors - RF coax(ial) connectors are a vital link in the radio spectrum. Coax connectors are often used to interface two units such as the antenna to a transmission line, a receiver or a transmitter. Coax connectors come in many impedances, sizes, shapes and finishings. There are also female and male versions of each. As a consequence, there are thousands of models and variations, each with its advantages and disadvantages. The proper choice of a coax connector will facilitate this interface.  
    • RF Directional Couplers - The equations that describe the performance of transformer based directional couplers are derived. The best theoretical performance available from a directional coupler, using ideal transformers, is a function of the turns ratio, and the terminating impedances. At VHF and UHF frequencies, wire gauge and core material can be chosen to closely approximate the response based on the solution of these equations.  
    • Signal Cabling - quite often the humble feed line is to blame if the communication does not work   
    • The purpose of a balun - information on benefits of balusn in eadio reception  
    • Weatherproofing Andrews Connectors - Andrew connectors are designed to be pretty weatherproof by using tight tolerances and O-ring gaskets in a couple of places. It is ALWAYS a good idea to weatherproof any connector joint; in fact, it is done practically 100% of the time by professional installers. 
    • Coaxial Cable -- The Neglected Link - Is a better grade of coaxial cable worth the price difference? This analysis of the importance of shielding in coax lines explains why the answer is "Yes!" 
    • Selecting and Installing Transmission Line - Transmission line or downlead, is the wire that carries the signal from the antenna output terminals to the receiver input terminals. Even the best antenna and the most expensive receiver will not produce an acceptable picture if the transmission line has not been carefully selected and correctly installed. The transmission line is more important than most people realize. Color television reception is sensitive and highly susceptible to interference from many different sources. Transmission line that is carefully chosen and neatly run by an installer who knows what he is doing will reward the customer with clear, distortion free color TV reception.  
    • Transmission Lines - Transmission lines are the link between your station equipment, transmitter, receiver, transceiver, and the antenna. There are many different varieties but two major types of line predominate for frequencies in general use by radio amateurs. 

 

 

    Active antennas

     

    • Active Antenna II - This circuit is designed to be used on receivers that use untuned wire antennas, such as inexpensive units and car radios.
    • Active Antenna III - This circuit is designed to be used on receivers that use untuned wire antennas, such as inexpensive units and car radios.  
    • AM/FM/SW active antenna - an active antenna that can be used for AM, FM, and shortwave, on the shortwave band this active antenna is comparable to a 20 to 30 foot wire antenna  

 

    Antenna amplifiers

    Antenna amplifies can work in helping weak radio signal reception - within their limits. Most TV/FM boosters are simple, broadband VHF amplifiers. They provide an extra amplification stage for the tuner. This kind of amplifiers amplify anything entering to them that is within their operation frequency range. This means that they amplify the signal, but they will also amplify the noise. Most designs have pretty good noise characteristics, but they may belacking in other areas. In particular, some are easily overloaded by stronglocal signals (e.g. TV stations and public service band stations caninterfere). When this happens, the FM signals can become badly distorted.If you are subject to multipath problems, the booster make make them worse.The first step to improving radio reception is always to check, and possiblyimprove, the antenna. Make sure that your antenna is properly connected toyour tuner and that the feedline impedance matches the tuner antenna imputimpedance. Make sure the feedline is properly connected to the antenna. Ifyou do not have a good outdoor antenna, get one. I have never seen abooster help a simple, indoor antenna.Radio receivers and tuners vary a lot in sensitivity. Some work better with weak signals than some other. For some less sesitive radios, extra amplifier can be very helpful. If you have a very snsitive radio, you might not benefit at all from the antenna amplifier. Besides pure signal amplification need, the antenna amplifiers are often used to compensate the cable losses from the antenna signal source to receiver that long cables can cause. In this kind of applications, it is the best idea to put the amplifier as near the signal source as possible to get best results.

    • FM-Band Preamplifier - Here is a high performance RF amplifier for the FM band which can be successfully built without any special test equipment. The grounded-gate configuration is inherently stable without any neutralization if reasonably good layout techniques are employed. The output transformer is designed to resonate with the FET's drain capacitance at about 92 MHz giving the amplifier the highest gain at the low end of the band where the weaker stations operate. No tuning capacitor is needed as long as the transformer is built precisely as described. The performance of the amplifier is quite good. The noise figure is below 2 dB and the gain is over 12dB.   
    • Hacking The Original 915 MHz WaveLAN (NCR 915 MHz WaveLAN 2 Mbps DSSS) - amplifier circuit, datasheets, antenna designs, etc.  
    • UHF Preamplifier - This circuit is designed to work at UHF frequencies in the range 450-800MHz. It has a gain of around 10dB and is suitable for boosting weak TV signals. 

 

    Antenna adapters and couplers

     

    • The Care and Feeding of the R.F. Isolator - The application of suitable R.F. isolators with complementary filtering devices now becomes more and more important as the number of receivers and transmitters on one site increase all the time. The purpose of this bulletin is to review the characteristics of R.F. isolators and their operation and discuss the ways in which these devices are applied to control interference due to intermodulation products and to provide other benefits. A better understanding of R.F. Isolators, their benefits, limitations and short comings and the best ways to employ them will be covered in this bulletin. 
    • A Happy Tune: Tiny ATUs for the Trail - A tiny homemade QRP antenna tuning unit - ATU or transmatch - is inexpensive to build and is easily customized for special needs.  
    • Simple Inductive Coupling - This document shows a simple inductive coupler used to improve reception on the PMR446 band. A short length of coax cable, some coated copper wire, a PL connector (Male or female), solder and a soldering iron.
    • RF isolation, cheap and easy Isolation may be best for eliminating AM broadcast signals and induced electrical hum in some communications base station - There are times when the signal from the local AM broadcast station gets into your station equipment. You don't want it for reasons other than not liking the program content. Communications towers at a height nearly equal to the quarter wavelength of the AM station frequency not only influence the radiation pattern of the broadcast station, but also provide considerable energy in the ground return of the tower. 

 

    Antenna baluns

    Baluns are sonverters which convert the unblanced antenna signals from coaxial cable to a blaanced format suitable for antenna types which need balanced signals (for example dipole antennas). Besides this conversion the baluns will quite often do some form of impedance conversion in the process. Different blauns are needed in different applications. In some applications only balanced-unbalanced conversion is needed, while in some oother applications also impedance conversion is needed. From a technology viewpoint, alot of baluns are also based upon 1:1transformations (in differing configurations.) Another popular balun with antennas is 4:1 balun. A typical 4 to 1 balun acts as a center tapped auto transformer. Theunbalanced signal uses one end of the winding and the grounded centertap, while the balanced signal uses the ends of the winding. Usingcoax or twisted pair transmission line for the windings is a way toget very good coupling and wide bandwidth. If the transmission linewinding is cut to the right length, it can make a narrower band balunwith no magnetic core. Sometimes 4:1 balun configuration is built witgh with a natural 1:1 impedance transformer wired in the right way.

 

    Impedance matching

     

    • How RF Transformers Work - RF transformers are widely used in electronic circuits for impedance matching, DC isolation, voltage/current up/down conversion and balanced/unbalanced conversion  
    • Impedanssin muunto nelj�nnesaaltomuuntimella - how to change impedance with quarter wave microstrip adapter, text in Finnish  
    • Antenna SWR Tuning - The SWR, or Standing Wave Ratio, of an antenna is a measure of how efficiently your radio is radiating the energy it produces when you transmit.  

 

 

    Signal attenuators

     

    • Attenuator Pads - homebrew attenuation pads  
    • Do-It-Yourself Power Splitters - Plans for making low-cost power splitters   
    • Fixed Attenuators - Fixed attenuators can be designed to have either equal or unequal impedances and to provide any amount of attenuation (theoretically) equal to or greater than the configuration's minimum attenuation - depending on the ratio of Z1/Z2. Attenuators with equal terminations have a minimum attenuation of 0 dB. Unequal terminations place a lower limit on the attenuation.  
    • Most Often Asked Questions About Electronic Attenuators  
    • Step Attenuator - This attenuator is designed for 50 ohms impedance and provides switches for 20, 16, 8, 4, 2 and 1 dB attenuation.  

 

 

 

    Other antenna related circuits

     

 


 
Created by Maman Nurohman,
Nurohman's Site, 2006