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Clear Springs Press |
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Clear Springs Press |
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Portable Wire Antennas
Third Edition - September, 2018 The third edition contains many edits, better organization and about 20 percent more content than the previous edition. This is not a book on antenna theory. It does provide clear easily understood explanations on the principles of antenna operation, transmission line considerations, impedance matching, baluns, tuners, and the pros and cons of different types and configurations of wire antennas. This book is written with the intention of enabling a new comer to amateur radio to understand the concepts and practical considerations of this subject without getting lost in mathematics and complex theory. Portable, reliable HF communication is required for emergency management, expedition communications and recreational uses. This book is about selecting and constructing portable HF wire antennas that will provide maximum performance with low power, light weight and low bulk. All the information required to assemble antennas and antenna kits that can be deployed in multiple configurations is provided. Special attention is given to wire antennas deployed at low heights above ground, a situation confronted by operators on the move. Special attention is also given to NVIS (Near Vertical Incident Skywave) communications, which is also optimum with low antenna elevation. NVIS propagation enables communications in the 50-500 mile range regardless of terrain and independent of repeaters. With one to three low suspension points (8-20 ft.), one can deploy wire antennas that cover the 160-30 meter bands (those capable of NVIS propagation) with performance optimized for NVIS communication. With a single high suspension point that can be improvised from one or two tall trees, buildings or cliff faces and one of the antenna launch systems described here, one can deploy multiple antenna options that can be configured from the modular antenna kit. These include inverted V resonant or random wire dipoles, long wire antennas, sloper dipoles and inverted L antennas for efficient all-band regional use. It also covers long wire, two suspension point rhombics, terminated V beam and inverted half rhombic beam antennas optimized for 20-10 meters while being broad band (no tuner required), low noise and gain equal to or greater than a high dipole. With two medium to high suspension points (15-80 ft. depending on the band), one can install Extended Double Zep or half square wire antennas that produce low angle radiation that is optimum for DX operation and yield 3 dBd of gain over a dipole. Chapter 1 PreviewBackpack Portable CommunicationsBackpack portable HF radios have been important to expedition communications, military operations and emergency operations since the invention of radio. This has not changed despite the increasing use of satellite based communications technologies. Today, there is an additional interest in using HF communications as a recreational activity. Backpack portable radios are, by necessity, small, lightweight and low power. Effective back country communications often requires the use of the HF bands, especially the lower HF bands. The challenge becomes in designing and building a portable, light weight antenna that is efficient enough to use effectively with the low power radios on the lower frequency HF bands. The lower bands, in general, require large dimensions and higher elevation. While there are a number of antenna kits that are self-supporting, they often come with performance compromises. Shortened dimensions often involve loading coils and lower radiation efficiency. An alternative is hanging wire antennas that are full length. Wire antennas are often (but not always) dipoles or some variation of the basic dipole. Some wire antenna designs offer improved gain or multi band performance or both. Wire antennas can be relatively light, compact and easily carried. Even so, deployment is dependent on the availability of supports. Fortunately, NVIS optimized antennas work best when deployed at low heights and NVIS is a preferred mode for many EMCOMM and expedition support functions. Rarely can an antenna be deployed at an optimum DX height of one half wavelength or more on the lower HF bands whether in a fixed or portable location. Performance compromises have to be made. Making the most of what is available includes understanding the tradeoffs and having a variety of antenna choices to apply to a specific situation. This presentation minimizes the use of technical language, and concepts. It is intended to be as clear and non-technical as possible while providing sufficient detail to enable anyone to build their own wire antennas. We would all like for our portable antennas to be one hundred percent efficient, weigh nothing, be self-supporting and be deployable in seconds. A backpack portable station is, by weight requirement, low power. It has to be low power because batteries and the solar panels and other means for charging them are heavy. QRP is generally defined as 5 watts or less. Working with QRP power levels can be very challenging and may not be reliable under some band and traffic conditions. CW and digital modes can be successful at power levels well below that required for SSB phone contacts. Digital modes require more gear which adds more weight, but with modern pads and netbooks, may be practical. A number of commercial HF transceivers are small and light enough for portable use. However, pay close attention to the battery drain while in receive and standby mode. The battery packs, chargers, solar panels, etc. add weight. The weight of the power system is the reason that 100 watt transceivers are not carried by many backpackers. Military and commercial manpack HF radios generally have a 20 watt transmitter capability. This seems to be the minimum power level that affords generally reliable SSB phone communications. Deployment of the antenna is a major consideration with portable operation. The lightest weight antenna with superior performance characteristics is the wire antenna. The challenge is getting it up in the air at an effective height and being able to find suitable deployment sites repeatedly while on the move. In this regard, it makes a major difference whether you are interested in DX or regional communications. DX communication requires the maximum height attainable. Regional communications, defined here as 50-500 miles, utilizes NVIS, (Near Vertical Incident Skywave) propagation. The deployment of an antenna for optimum NVIS performance requires a lower height above ground. Heights of 10-15 ft. above ground in the lower HF bands is optimum for a dipole antenna. These heights are much easier to achieve in a variety of terrains on a repeatable basis. Some operators have achieved results with wire antennas suspended only inches or a few feet above ground. There is a tradeoff, however, because close proximity to the ground results in signal loss and lower antenna efficiency, precious commodities with low power transmitters. Chapter 3 PreviewAntenna Performance ComparisonWhen comparing antenna performance specifications and reports, understanding the meaning of decibels and the nature of the S meter is helpful. >The Decibel Power and antenna gain measurements are expressed in dB. The decibel (dB) is a logarithmic unit that indicates the ratio of a power measurement relative to a reference level. A ratio in decibels is ten times the base 10 logarithm of the ratio of two power quantities. dB=10xlog(P1/P2) A change in power ratio by a factor of 10 is a 10 dB change. A change in power ratio by a factor of two is a 3 dB change. Comparison of antenna gain is generally referenced either to a standard resonant dipole (dBd) to an isotropic antenna (dBi). An isotropic antenna is a theoretical antenna in free space which uniformly distributes energy in all directions. A dipole in free space has a figure 8 radiation pattern. The dipole's gain in the direction of its pattern is 2.15 dBi compared to 0 dBi for the isotropic antenna. 0 dBd = 2.15 dBi Real world dipoles don't live in open space, they live above a ground. The ground affects the antenna radiation pattern and relative gain. So taking a ground into account, the gain of a real world dipole can be as high as 8.5 dBi. This matters because it is sometimes a source of confusion when comparing antenna specifications. Mixing up dB and dBi reference points and free space versus over ground can cause some confusion. Signal Strength Measurement Most transmitters and transceivers have an "S" meter built in. Its purpose is to provide a relative indication of the strength of a received signal. The signal report ranges from S1 which is faint and barely perceptible to S9 which is very strong. The meter is supposed to be calibrated to read S9 when a 50uv signal is received. In practice, this standard is not always followed. Each S unit down from S9 is supposed to mean a reduction of 6 dB of signal strength. The S meter will read in S units and dB above S9. Signal strength reports are given in S units or the number of dB's over S9, like "10 over 9" or "10 by 9" meaning 10 dB over S9. S9 is generally in the middle of the scale. Because there is apparently a lot of variation in the way S meters are implemented and calibrated, one cannot accurately compare the S meter readings on different transceivers. Accurate measurement and comparison of received signal strength requires more test equipment than many amateurs have available. The S meter is still useful for rough comparison of signal strength between stations and antennas using the same receiver. Chapter 6 PreviewAntenna System ComponentsAn antenna system consists of:
Matching the source and load impedance between the different components is very important because:
The maximum power transfer theorem states that, to obtain maximum external power from a source with a finite internal impedance, the impedance of the load must be equal to the impedance of the source as viewed from the output terminals. While this yields the maximum power transfer, it is only at a 50% efficiency. In this situation, half the power is dissipated in the source and half in the load. In an electrical power system this would be highly inefficient and uneconomical. The efficiency increases as the load impedance increases relative to the source impedance. In a communications system getting the most power to the antenna is more important than doing it efficiently. So, in a communications system, the best result is achieved when the output impedance of the transmitter is equal to the transmission line impedance and the transmission line impedance is equal to the antenna impedance. Impedance has a resistance component and a reactance component. Z=R + jX Antennas and transmission lines have inductance and capacitance distributed along their length as well as resistance. The characteristic impedance of a uniform transmission line is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. At resonance, only the resistive components of an antenna are seen by the transmission line. Antenna Resistance = Ohmic resistance + radiation resistance The radiation resistance represents the loss due to RF energy being radiated off of the antenna into space. Resonant antennas have reflected or standing waves along the antenna length. The radiation is reflected between the antenna ends until all of the energy is dissipated either as radiation or ohmic losses. In a traveling wave antenna, radiation resistance and radiated energy are higher with longer wire lengths. Long means long relative to the wavelength of the signal. When not at resonance, a complex impedance is seen. The further from resonance, the higher the impedance becomes. When the transmission frequency doesn't match the resonant frequency of the antenna, the antenna impedance increases and creates an impedance mismatch between the antenna and the transmission line. The impedance mismatch causes reflections and some of the energy delivered by the transmission line is reflected away from the antenna, back toward the source. It is the interaction of these reflected waves with the incident waves that causes standing wave patterns in the transmission line. Reflected power has two main implications in radio transmitters. Transmission line losses increase and the transmitter finals can become stressed or damaged by the higher operating temperatures that result from having to dissipate the reflected power. The standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in a transmission line. When the standing wave ratio is expressed as a voltage rather than power ratio it is VSWR. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value. Impedance mismatches between the transmission line and load (antenna) causes a reflection of some of the RF energy back toward the source end of the transmission line, preventing all the power from reaching the antenna. An ideal transmission line would have an SWR of 1:1, with all the power reaching the destination load and no reflected power. An infinite SWR represents complete reflection, with all the power reflected back down the cable. It is obvious that it is desirable to match the impedance of the transmitter, transmission line and antenna. In real world operations, this becomes difficult because changing frequency changes the impedance of the antenna and affects the transmission line as well. So, what happens when we have significant impedance mismatches between the transmission line and antenna? Standing waves develop as a result of the impedance mismatch. High SWR increases the transmission line losses because the higher effective current that is created in the transmission line increases the line loss from both the resistive and reactive components of the transmission line. Another way to visualize this is to consider this analogy. Pause for a moment and consider an analogy of wave propagation in a wave tank (a rectangular tank of water). When a displacement is created at one end a wave is created and it propagates down the tank and is reflected off of the far end. But then it travels back down the tank toward the origin where it is reflected again. This continues until the wave loses all of its energy to the environment. In the transmission line, a similar thing can be imagined with the reflected wave losing some of its energy to line loss each time it travels down the line. If the transmission line was lossless, all of the energy would get delivered to the antenna and radiated despite the impedance mismatch. Sadly, transmission lines do have losses. Some of the signal traveling from the transmitter to the antenna is lost. Then a portion of the reflected signal is lost to the transmission line before it gets back to the antenna. When the SWR values are very large and reflected energy is very high, transmission line losses can become a major problem. Not all transmission lines are created equal. For example, an open wire line may have a loss of 0.01 dB per 100 ft. while a coaxial line may have a loss of 1.5 dB per 100 ft., depending on the frequency. The losses increase with frequency and higher SWR. The point is that in a high SWR system a coaxial transmission line might eat a significant portion of your transmitter power while open wire line, ladder line or even twin lead will eat only a negligible amount of your transmitter power. So, you can use resonant antennas and carefully matched transmission lines which doesn't give you much operating flexibility, or you can use non-resonant antennas with high SWR and low loss transmission lines. In order to get the high SWR scenario to work, particular attention has to be paid to the impedance match between the transmitter and transmission line. Before looking at this, there is one more characteristic of transmission lines that one should be aware of. Transmission lines also act as a transformer. A transmission line terminated in its characteristic impedance, and of a length that is a multiple of a half wave length, acts as a 1:1 transformer. It does not change the terminating impedance into something else. However, when any other length of line is not terminated in its characteristic impedance, it does change the impedance at the transmitter end. The transformed impedance is a function of antenna impedance, line loss, frequent, and the length and characteristic impedance of the transmission line. Lower loss transmission lines tend to have a high characteristic impedance, typically 300 (twinlead), 450 (ladder Line) or 600 ohms (open line) as compared to the 50 ohm output impedance of the transmitter. Add to this the fact that the impedance of the antenna is changing drastically as you change frequency and band and the transmission line is acting as a transformer that changes its characteristics as a function of frequency. From the transmitter's point of view, this is a disaster. The consequence of a high feed point impedance mismatch is that the transmitter finals have to dissipate a lot of reflected power. Solid state final amplifiers overheat and "blow" under such conditions. The older vacuum tube transmitters had no problem with this scenario because they had a built in matching network that could be manually tuned to match the impedance seen at the transmitter output. Modern equipment lacks this feature. Modern transmitters have a fixed 50 ohm output impedance and are not very tolerant of mismatches. The solution is the antenna tuner. An antenna tuner consists of a network of inductors and capacitors. The sole purpose of the antenna tuner is to match the output impedance of the transmitter to the input impedance of the transmission line. Another way to say it is that the antenna tuner transforms the impedance of the transmission line - antenna combination into 50 ohms. Automatic antenna tuners are fast and convenient in creating a good match. By matching the impedance of the transmitter and transmission line, two important things are accomplished:
The role and function of the antenna tuner are apparently frequently misunderstood. To be clear:
Unfortunately antenna tuners do introduce signal loss. High Q capacitors are compact and available. Sadly, high Q coils want to be large while compact design requirements want them to be small. Because actual signal losses in an antenna tuner are affected by multiple variables there is no standard technical specification on this parameter. Generally speaking, the losses should be lower when the SWR is lower. Having an antenna system that presents a good impedance match removes the losses of a tuner and reduces the weight and bulk of a portable station. On the other hand, a good tuner makes it possible to use random wire and other non-resonant antennas on all bands. There is also some common apparent confusion regarding transmission line characteristics and losses. To be clear:
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