![\"antenna](\"https:\/\/www.hamlinux.it\/wp-content\/uploads\/2025\/11\/antsys.png\")
<\/a><\/p>\nThe antenna<\/strong><\/em>: it is the interface between the radio waves in the air and the electric current in the cable. In transmission, it converts the electrical signal into radio waves, while in reception it collects the radio waves and returns them as an electrical signal.<\/p>\n For the antenna we consider at this moment two elements:<\/p>\n Resonance:<\/strong> it is the electromagnetic equivalent of a tuning fork, which when struck oscillates at a specific frequency. Every antenna has one or more frequencies (called resonance frequencies) at which conversion efficiency is maximum. Moving away from that frequency the efficiency progressively decreases, as does the impedance.<\/p>\n<\/li>\n The transmission line<\/strong><\/em> connects the antenna to the equipment. It is normally made of coaxial cable, which, like the antenna, must have a characteristic impedance of 50 ohms<\/strong>, defined by the physical characteristics of the cable. It is what is called an unbalanced<\/em> line, in which the central conductor carries the signal, while the outer metallic braid acts as return and shield, and is connected to ground. It should be remembered, however, that, as we will see, balanced<\/em> lines are also used, in which there are two conductors that are both active with respect to ground.<\/p>\n We must not forget ground<\/strong><\/em>, which in radio frequency is rarely just a stake in the ground: every antenna system should be properly grounded.<\/p>\n The first reason is safety<\/strong>. Even if modern transmitting equipment is powered at low voltage, at the transmitter output \u2013 or at the resonance points of the antenna \u2013 very high voltages<\/strong> can be generated. In some types of antennas with a high quality factor (Q)<\/strong>, such as tuned magnetic loops<\/strong>, it is easy to reach several thousand volts<\/strong>, with currents sufficient to cause potentially dangerous shocks.<\/p>\n But grounding is not only for safety: it also helps to improve the overall efficiency of the system<\/strong>, reduce local noise pickup<\/strong>, and discharge electrostatic charges<\/strong> that accumulate due to wind or atmospheric activity. In a radio station, it is important to distinguish between two types of ground:<\/p>\n Safety ground<\/strong> is that required by civil electrical regulations: it connects the metallic masses of the equipment and the domestic electrical system to a ground rod (or network of ground electrodes). It serves exclusively to protect people<\/strong> from possible leakage or insulation faults.<\/p>\n<\/li>\n RF ground (radio frequency)<\/strong>, instead, is an integral part of the antenna system. It is designed to provide a low-impedance return for radio frequency currents, improving efficiency and reducing noise. It can consist of radials, copper braids, plates or metal meshes<\/strong> placed in the ground or near the base of the antenna.<\/p>\n<\/li>\n<\/ul>\n The two grounds must not be completely separate<\/strong>, but connected at a single point<\/strong> (generally at the station entry point), to avoid dangerous potential differences and unwanted return currents. A single, short, robust connection \u2013 ideally with flat strap or copper braid \u2013 ensures both electrical continuity<\/strong> and a common reference<\/strong> for all equipment.<\/p>\n Now that we have the components, let\u2019s see what happens when they are not perfectly tuned. This is where the important element comes into play: Standing Waves<\/strong>.<\/p>\n In a matched<\/em> chain, transmitter, transmission line, and antenna will all have equal impedance. Under these conditions, all the energy produced by the transmitter is transferred to the antenna and converted by it, more or less efficiently, into radio waves.<\/p>\n If there is no impedance uniformity, part of the energy is not transferred, but essentially remains in the transmission line, and is dissipated as energy, and is defined as standing waves.<\/em><\/p>\n It is a value that is often measurable directly from the transceiver, or through a simple external instrument, and is expressed as a ratio, called standing wave ratio<\/strong> (SWR) or, in Italian, ROS<\/strong> (rapporto onde stazionarie).<\/p>\n In the ideal situation, the ratio will be 1:1, which indicates that all the energy is effectively transferred to the antenna. Up to 1.5:1 we are in full normality, 2:1 is overall acceptable, but one should never operate with SWR greater than 3:1. The untransferred energy is dissipated as heat, and therefore operating at full power with too high ratios can seriously damage the equipment.<\/p>\n SWR and efficiency<\/strong><\/p>\n One might be led to think that a low SWR indicates good antenna operation. In reality, this is not the case: it is true that a 1:1 ratio shows that all the energy is transferred to the load<\/em>, that is, to the antenna, but it provides no information on the efficiency of the latter. If we connect the transmitter to a dummy load<\/em>, a power resistor used for equipment tuning that dissipates all the energy as heat, we will have SWR 1:1 but zero or negligible radiation.<\/p>\n Antenna tuners<\/strong><\/p>\n A non-optimal standing wave ratio can be corrected using an antenna tuner (which should more correctly be called an impedance matcher<\/em>). This device does not improve<\/em> the efficiency of the antenna, but serves to reduce or avoid losses due to load mismatch. We have a resonant antenna at 14 MHz with 200 \u03a9 impedance, connected via 22 m of 50 \u03a9 coaxial cable to a 50 \u03a9 transmitter. In this configuration there is an evident mismatch, which generates an SWR of 4:1 and a loss of about 2 dB.<\/p>\n The ideal insertion point of the tuner is directly under the antenna<\/strong>. At this point the tuner can perfectly match the 50 \u03a9 feed impedance to the 200 \u03a9 load. Line losses due to mismatch<\/em> disappear, because reflections are practically eliminated. The antenna receives all the power available from the transmitter, and reception also improves, because the signal along the line no longer suffers losses due to reflections.<\/p>\n Placing the tuner near the transmitter<\/strong> is not optimal, but I still have a benefit. The 22 m of cable in fact behaves like an impedance transformer, and at the cable connector we would measure an effective impedance of about 80 -70j \u03a9 (80 \u03a9 resistive and 70 \u03a9 capacitive reactance). Without a tuner, since the TX has 50 \u03a9 impedance, this mismatch produces an SWR of about 3:1, which causes an additional loss of about 1.34 dB, which we round to 1 dB considering also the intrinsic losses of the cable. Obviously the values reported are indicative numbers, useful only to show the order of magnitude of the phenomenon. They serve to highlight how even on the transmitter side the effects of mismatch are felt, albeit to a lesser extent than the ideal positioning of the tuner directly under the antenna.<\/p>\n The cable<\/strong><\/p>\n The transmission line, except in special cases, consists of a coaxial cable with a characteristic impedance of 50 \u03a9. There are many types of cable, identified by codes: the most common are those defined at the time of World War II by the U.S. military (RG- [radio group]), which today no longer correspond to official military specifications, but have remained as commercial and technical convention; other nomenclatures also exist, such as \u2013 for example \u2013 LMR by Times Microwave.<\/p>\n Coaxial cables are characterized by a central conductor, which can be solid or stranded, an insulating cylinder called the dielectric<\/em>, a shield consisting of a braid of copper wires, and an insulating jacket. The geometric proportions of the various elements determine the cable\u2019s impedance.<\/p>\n In amateur use, the three most commonly used cable types are generally: RG-58\/U, RG-8X\/U and RG-8\/U (the final \/U stands for utility<\/em>, identifying the civilian-use version). Each cable has its technical characteristics; of these we are synthetically interested in two: physical dimensions and attenuation.<\/p>\n Attenuation is the first element to consider:<\/p>\n In this table I have summarized the typical attenuations in decibels (dB) per 100 m of cable for the main amateur radio bands. It is worth clarifying the two concepts.<\/p>\n Attenuation<\/strong><\/em> is the loss of the signal as it travels through a transmission medium, in our case: the cable. It is caused by resistances, dielectric losses, shielding anomalies, mechanical deformations, and also by the fact that a part, albeit minimal, of the signal is still radiated. This loss is indicated in decibels<\/strong><\/em>, which is not a quantity, but a logarithmic unit that expresses a ratio of power or voltage. It therefore measures how much of the energy remains downstream of the transmission line. The fact that it is logarithmic means that small attenuation numbers imply large differences in absolute terms. 3 dB of attenuation is equivalent to half the power lost in the transmission line, 6 dB to a quarter.<\/p>\n It is a value to keep in mind, but without worrying excessively. On the reception side the measurement is made in voltage, and in this case, it is 6 dB<\/strong> that corresponds to 50%. 3 dB lost in transmission corresponds to half an S point<\/strong> lost in terms of the reading on the receiver. Consequently, doubling or halving the transmitted power produces a minimal variation in the perceived received signal. This means that, beyond the physiological cable attenuations, the practical difference in listening is not decisive: what really matters is the overall quality of the system (antenna, positioning, background noise), rather than chasing every decibel of loss.<\/p>\n The other aspect of the cable is its dimensions: RG-58 and RG-8X are smaller and more flexible: RG-58 has a diameter of 4.8 mm, RG-8X of 6.1 mm. Cables such as RG-8 (or even RG 213) are decidedly larger and stiffer, with a diameter of 10.3 mm. Obviously, a consequence of the combination of cable size and related attenuation is also the greater power-handling capability.<\/p>\n Generally, larger and higher-performance cables are used for the main runs from the antenna to the station, and thinner cables for the interconnection jumpers inside the radio station. The final choice must balance electrical performance, mechanical dimensions, and material cost.<\/p>\n Connectors<\/strong><\/p>\n An important role in transmission lines is played by connectors, which are not just simple joining elements, but an integral part of the system and can significantly influence its performance. The elements to consider are these:<\/p>\n In the amateur radio field, four types of RF connectors are mainly used:<\/p>\n The type of connector is normally dictated by the equipment or antennas to be connected, but it must be borne in mind that the connector should be chosen based on the type of cable used. PL-259s were born for use with RG-8 \u2013 they are screwed onto the cable \u2013 but there are adapters to use them with smaller-section cables. For the other connectors there are specific versions for different types of cable.<\/p>\n\n
\nAt the resonance frequency the reactive component is zero, and therefore only the resistive part remains.
\nRadio equipment normally uses a standardized impedance of 50 ohms (\u03a9)<\/strong>.<\/li>\n<\/ul>\n\n
Dynamic interaction<\/h3>\n
<\/a>
\nThe role of the tuner is often misunderstood: some consider it useless and others indispensable. The truth, as always, lies in the middle.
\nLet me give a practical example.<\/p>\n<\/a><\/p>\n
\nBy inserting the tuner, this second mismatch is eliminated, thus recovering about 1 dB.<\/p>\n<\/a><\/p>\n\n\n
\n <\/td>\n RG\u20118<\/td>\n RG\u20118X<\/td>\n RG\u201158<\/td>\n<\/tr>\n \n 3.5<\/td>\n 0.9<\/td>\n 1.2<\/td>\n 1.5<\/td>\n<\/tr>\n \n 7<\/td>\n 1.7<\/td>\n 2.4<\/td>\n 2.6<\/td>\n<\/tr>\n \n 14<\/td>\n 2.6<\/td>\n 3.6<\/td>\n 4.0<\/td>\n<\/tr>\n \n 21<\/td>\n 3.4<\/td>\n 4.7<\/td>\n 5.0<\/td>\n<\/tr>\n \n 28<\/td>\n 3.8<\/td>\n 5.2<\/td>\n 6.0<\/td>\n<\/tr>\n \n 50<\/td>\n 4.2<\/td>\n 6.8<\/td>\n 7.0<\/td>\n<\/tr>\n \n 144<\/td>\n 7.5<\/td>\n 14.0<\/td>\n 14.5<\/td>\n<\/tr>\n \n 432<\/td>\n 14.5<\/td>\n 26.0<\/td>\n 27.0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \n
<\/a><\/p>\n\n