Development and technology of coaxial cables in high-frequency technology
From wire to coax: the development of high-frequency feed lines
When radio technology was still in its infancy at the turn of the 20th century, the common method of “transporting high frequency” was the asymmetrical single-wire line and the symmetrical two-wire line. They were used as adapted or tuned feeder lines for a long time, both in the commercial sector and in amateur radio. Although W. Siemens proposed and patented a coaxial cable for low-frequency applications as early as 1884, there was still a long way to go before we could recognize what we understand by a coaxial cable today. It took almost half a century and numerous inventors and patents before the development of the coaxial cable and 50-Ohm technology was complete. In amateur radio, the use of coaxial feed lines only began in the 1950s and 1960s. Today, matched twin-lead lines are still used advantageously only for the symmetrical feeding of multi-band dipoles.
High-tech in copper and plastic
A coaxial cable is axially symmetrical, but viewed against ground it is an asymmetrical feeder line. The outer conductor usually consists of a dense copper braiding; high-quality cables are double-shielded and have a thin copper foil underneath. The inner conductor of more flexible coaxial cables is designed as a copper strand. Coaxial cables with a larger outer diameter have a core made of one or more solid copper full wires. The insulating material between the inner and outer conductors - the dielectric - is preferably made of low-loss polyethylene (PE), either as a solid material, foamed or in a web construction with a predominantly air dielectric. The outer protection of the cable is a plastic sheath made of polyvinyl chloride (PVC), which is colored black for good UV resistance.
How mechanical properties shape electrical behavior
The technical data of coaxial cables are determined by their mechanical properties such as the diameter of the inner and outer conductors, the distance between them and the nature of the insulating material (the dielectric) between the inner and outer conductors. These factors essentially determine the electrical characteristics of the cable: the impedance, also known as the wave impedance, the frequency-dependent attenuation, and the voltage and power strength. Coaxial cables have a fixed characteristic impedance, which is independent of frequency and is 50 ohms for the usual cable types. The characteristic impedance is determined by the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor, i.e. by the size and distance between the conductor surfaces facing each other.
A little line theory
Coaxial cables are normally designed for use as a traveling wave line, which means that the current and voltage curves are the same along the entire line, i.e. no minima or maxima occur. However, this requires that the coaxial cable is terminated at both ends with its impedance of 50 ohms. The transmitter has a 50-Ohm output and the antenna and its matching also fulfill this condition in the resonance case. Only the two-sided termination of the cable with 50 Ohm ensures an optimal power transmission, in this case one speaks of power matching. Then the standing wave ratio is SWR = 1.0.
The difference is made by the line attenuation
Unfortunately, the fact that every RF line has a more or less significant attenuation is unavoidable. The attenuation of a fully insulated coaxial cable is minimal at a diameter ratio of 3.6. This is the theoretically optimal value. In practice, there are slight deviations. Well-known cables such as RG58 or RG213 have diameter ratios of around 3.28. The optimum is not very pronounced, but more widely dispersed.
The nature of the intervening dielectric, whose properties are characterized by the dielectric constant, is equally important. In addition to cables with a solid dielectric, there are also cables with a dielectric made of foamed insulating material or a predominant air insulation. The latter cables have lower losses than those with a solid insulation. They offer less attenuation, but unfortunately they are also more mechanically sensitive.
The attenuation of a cable is specified in dB per 100 meters for certain frequencies. The dB value of the specified frequency closest to the operating frequency is divided by 100 and multiplied by the individual cable length in meters to give the approximate attenuation value of the cable used.
Thicker coaxial cables have lower attenuation than thinner cables, so they are better suited for higher frequencies and higher power levels. Last but not least, coaxial cables with increasing diameter have a higher dielectric strength due to the thicker dielectric and are therefore better suited for transmitting higher power levels.
Cable attenuation not only reduces the power traveling to the antenna, but also attenuates the return. Replacing it with a higher quality coaxial cable with lower attenuation does not improve the standing wave ratio, but instead increases it slightly.
Why tuners and preamplifiers are directly connected to the antenna
Antenna tuners should be placed at the feed point of the antennas if possible. If the tuner is located at the bottom of the station, the high standing wave ratio causes additional losses on the line due to the cable attenuation that is already present. Receive preamplifiers should be installed at the top, directly at the antenna if possible. There they receive a stronger signal level without the cable attenuation, which is higher than the noise level!
How to find the perfect cable
Which coaxial cable is the right one? This question cannot be answered so easily. There are significant differences between the various cable types. The right cable - the cable that makes the most sense to use - depends largely on the respective application, the length, the transmission power and the frequency. Before deciding to buy a particular coaxial cable, you should systematically check the requirements placed on the cable.
These requirements can vary greatly from one individual to the next, because they depend on the configuration of the radio system. If a coaxial cable of the type RG58CU seems sufficient for just five meters to the antenna, a change to a higher-quality cable may be urgently required for longer cable lengths, higher frequencies and higher power levels. But caution is advised when buying cables, because in addition to the original designs with printed type designations, there are also similar-looking “no-name” products of lower quality on the market. The following criteria should definitely be considered beforehand:
- Is it the original cable with the type and manufacturer's name printed on it? Cables without this information have different electrical properties and external diameters that deviate from the standard.
- Will the cable be used in the shortwave, VHF or UHF range? For long cable lengths and/or higher transmission powers, select a higher-quality coaxial cable with low attenuation values.
- Do I need a cable with the greatest possible flexibility and an inner conductor made of strands, or will a cable with a solid inner conductor suffice? At the rotor, the cable must be particularly flexible due to the mechanical stress of the rotary motion!
- Is the outer shielding braid sufficiently dense and double-shielded, with an additional aluminum foil underneath?
- Cables with a solid inner conductor have lower attenuation, but are stiff and therefore mainly suitable for fixed installation.
- Solo i cavi flessibili con conduttore interno a trefoli sono adatti all'alimentazione di antenne a filo con linee di alimentazione sospese liberamente. Anche il peso gioca un ruolo importante con i cavi sospesi liberamente al punto di alimentazione. I cavi pesanti aumentano la flessione dell'antenna a filo.
- Which plug versions and plug standards are available for the selected cable?
From the hot road to outdoor adventure
Coax cables in vehicles must be resistant to heat (keyword Heatex), be firmly installed and be sufficiently flexible at movable transition points, such as the trunk lid. A double shielding is sufficient for the EMC problem and prevents interference from and to the on-board electronics (see also WiMo blog “Amateur Radio in Cars”).
The requirements for a cable for fixed installation and for portable radio operation could not be more different. In the first case, a rigid cable with a solid inner conductor is sufficient, but for portable radio operation such as field days, SOTA and POTA, a cable is needed that is UV-resistant, crush-resistant and flexible, as it is wound and unwound multiple times. For better visibility in the field, you should choose a color that is as light as possible. For fixed cables, it can make sense to match the color to the surroundings for camouflage. Heat and saltwater-resistant cables are particularly suitable for use near the coast and in shipping. Finally, don't forget the different weights of the various cables! Among the aspects of “portable radio, air luggage and sagging of wire antennas”, the particularly light “Airborne” with an aluminum sheath but a rigid inner conductor should be emphasized here.
Often no longer up to date...
RG58 and RG213 are probably the best-selling coaxial cables and thus familiar to every radio amateur. However, they are only acceptable for shorter cable lengths, lower power and low frequencies in the lower shortwave range. One or other of the no-name products among the simple coaxial cables often suffers from insufficient shielding, which means that it picks up all kinds of interference from the surrounding area at close range. This is also the cause of unwanted radiation from the cable at the frequency of the useful signal. If you must use a simple coaxial cable, don't use the simple RG58 or RG213, but rather the better shielded types RG58CU or RG213U in the MIL standard.
This step alone pays off with reduced attenuation over longer cable lengths in the shortwave range. However, there are much better alternatives - please read on!
What is the military standard MIL C17F?
Originally, the military standard was intended exclusively for the military sector. In the meantime, this standard has long been recognized internationally as a quality standard in the entire field of RF technology. Coaxial cables stamped with the MIL standard guarantee compliance with the required properties and values, such as shielding density, resistance to aging and accuracy of fit for the standardized connectors. The traditional RG213 U and the RG58 are also manufactured in the MIL-Norm C17F and can still be described as common coaxial cables for applications in the (lower) shortwave range in this quality. A very dense shielding braid ensures an improved shielding effectiveness of approximately 60 dB. A good RG213 U must have a copper number of 80, which means that at least 80 kg of copper must be used in one kilometer of cable. However, there are also RG-213 cables on the market that do not bear this designation. These versions are usually significantly lighter than the original, at only 60 kg, and have a diameter that differs from 10.3 mm, so that the installation of standard plugs is problematic.
Inner conductor (core)
The centrally arranged inner conductor made of solid copper wire or strand can have different cross-sections depending on the cable.
Dielectric
Insulating layer made of plastic (polyethylene), keeps the inner conductor centered at a distance from the shielding, is mainly responsible for the impedance and the attenuation value of the cable.
Shielding
Electrically conductive copper braiding to protect the inner conductor from electromagnetic influences.
Cable sheath
Outer protection of the cable against moisture, mechanical damage and UV radiation.
Impedance (wave impedance)
Geometric addition of ohmic resistance and reactance. Coaxial cables for applications in high-frequency technology have an impedance of 50 ohms. In audio and television/video technology, those with an impedance of 75 ohms are used.
Attenuation
The reduction of the signal level during transmission in the cable. This value is frequency-dependent and is usually specified in dB per 100 m.
Shielding effectiveness
This value characterizes the cable's shielding ability both inward and outward and is given in dB.
Traveling wave line
Primarily the intended operation of a 50-Ohm cable, at which both ends are terminated with an impedance of 50 Ohm (signal source and load). This case is called power adjustment. The standing wave ratio is 1.0. There is no ripple on the line; current and voltage have the same amount everywhere.
Matched feeder line
It is the opposite of a traveling wave line, usually an open symmetrical two-wire line (chicken ladder) which is tuned to resonance together with the symmetrical dipole connected to it via an antenna coupler that is also symmetrical. In this case, the radiator and the feed line form a resonant overall system, whereby the two-wire line is a non-radiating component of the antenna when it is completely symmetrical. The RF currents on the two conductors are in antiphase, which means that their fields largely cancel each other out.
Sheath waves and sheath wave trap
If a symmetrical antenna is fed at the feed point without a symmetrical element (1:1 balun unsym./symm.) with an asymmetrical coaxial cable, there may be problems with sheath waves depending on the length of the feed line. In these cases, the coaxial cable, with its shielding sheath, becomes part of the antenna, as a result of which the station carries HF. An ideal electrical cable length of ʎ/2 (or a multiple thereof) x velocity factor of 0.66 may help. This cable length at least transforms the conditions from the feed point of the antenna in a 1:1 ratio to the lower end of the cable. It is usual to insert a sheath wave trap (also sheath wave choke), preferably at the top in front of the feed point. However, this only combats the symptoms of the problem for the time being, but not the actual cause (see first sentence of this section). The power component “stuck” in the sheath waves is converted into heat in the sheath wave trap. In extreme cases, it can heat up considerably. If a coaxial cable is laid underground to the feed point of the antenna, the external damping of the shielding is intentional and helpful.
Current high-quality cable comparison
Aircell 7
Aircell 7 is a coaxial cable for the frequency range up to 3 GHz. In relation to its diameter, the low attenuation and high flexibility make this cable suitable for numerous applications in radio technology, for example as a longer antenna feeder cable in the shortwave and especially in the VHF and UHF range. The low attenuation is achieved by an extremely low-loss PE compound dielectric. This is a foamed material with a low density and a high air content of over 50%. The inner conductor of Aircell 7 is a stranded copper wire and gives the cable a particularly high flexibility. The outer conductor consists of an overlapping copper foil with a shielding braid over it. This ensures that the shielding effectiveness reaches a value of almost 100%. The copper foil is PE-coated on its inner side and thus protected against tearing if the bending radii are too small. The outer insulation of the cable is made of UV-stabilized PVC and is therefore highly resistant to aging.
Aircom Plus
Aircom Plus is a coaxial cable with good electrical and mechanical properties. The low attenuation values for a cable of this dimension make it particularly suitable for use in the VHF, UHF and SHF ranges. Aircom Plus has an elastic PVC insulating jacket and resembles the well-known RG-213 in appearance and diameter. The outer conductor of Aircom Plus consists of a copper foil with an overlying shielding braid. The copper foil is plastic-coated on the inside and thus protected against tearing when the cable is bent with too small bending radii. The overlying shielding braid has a coverage of 75% and thus contributes significantly to the mechanical stability of the cable. The centering of the inner conductor is achieved by the use of a continuous immovable plastic that permanently protects the inner conductor against corrosion. This means that Aircom Plus cannot be displaced by bending and stretching. Fully assembled cables can be bent as required without the inner pin of an N-connector being pushed out of the connector housing. If the loop radii are sufficiently dimensioned, Aircom Plus can be used in rotatable antenna systems. Aicom Plus is therefore rotor-compatible!
Ecoflex 10
Ecoflex is a flexible coaxial cable with very low attenuation for the frequency range from DC to 4 GHz. State-of-the-art production processes and the use of a low-loss PE-LLC dielectric ensure very low attenuation values. The high flexibility of Ecoflex 10 is made possible by a stranded inner conductor. The cable's EMC shielding effectiveness of >85 dB/3 GHz is achieved by double shielding with overlapping copper foil and an outer copper braiding. The foil is PE-stabilized and protected against cracks when bent at too small a radius. This coaxial cable is suitable for applications in high-frequency technology. It is low-loss, flexible, resistant to interference radiation and can be used up into the microwave range.
Ecoflex 15
Ecoflex 15 is a new type of coaxial cable for the frequency range from DC to 6 GHz. The special design of this cable combines the excellent attenuation values of rigid 1/2-inch cables with a solid inner conductor with the well-known flexibility of RG coaxial cables with a stranded inner conductor. The result is a highly flexible and very low-attenuation cable for demanding applications in telecommunications and information technology. An EMC-compliant shielding attenuation of >90 dB/1 GHz is achieved by double shielding with overlapping copper foil. As with Ecoflex 10, the foil is protected against cracks by a PE coating. A high-quality solderless N-connector has been developed for Ecoflex 15 that can be quickly and easily mounted without the use of special tools.
Hyperflex coaxial cable
Hyperflex-5 is a coaxial cable with an outer diameter of 5.4 mm. The dielectric is foamed, which results in very low attenuation in relation to the diameter. The inner conductor is constructed as a strand and consists of 19 very fine individual wires. This makes the Hyperflex cable very flexible, well suited for situations where the cable is frequently moved. Typical applications are short connection cables in measurement technology, VHF radio (amateur radio, private mobile radio), LTE and GSM for short cable lengths, Wifi at 2.4 GHz. The attenuation at 144 MHz is 9.4 dB/100 m. This cable is also available in 10 and 13 mm versions.
Pota-Flex coaxial cable
The lightweight and flexible Pota-Flex 6 coaxial cable has an outer diameter of 5.9 mm, with 19 individual copper wires forming the inner conductor. The shielding consists of three layers: two braids of copper wire with 120 wires each, and copper foil with a PE layer surrounding the dielectric. This not only provides ideal RF shielding, but also protects the cable from mechanical damage. The dielectric is foamed, which results in very low attenuation in relation to the diameter. It was developed especially for outdoor applications such as POTA (Parks on the Air), where extremely robust cables that are highly visible in the field are in demand. The yellow sheathing of the cable is highly visible and thus offers additional protection against tripping and unintentional “getting caught” in busy places. This cable is also available in a 7 mm version.
The right plug for a perfect connection
Of course, you can find the right connectors for the cable you have chosen here and there. However, it is best to buy high-quality coaxial cables in their original designs, with the type and manufacturer's name printed on them, from specialized companies or from a trusted amateur radio dealer who, in addition to all common and less well-known cable types, can also supply the matching connectors from a single source. Incorrect or unsuitable connectors can quickly negate the good properties of a high-quality cable. Connectors and cables must match. Each soldered connector must fit perfectly, both mechanically and electrically.
Apart from the effects of mechanical damage, such as sharp bends, coaxial cables are extremely sensitive to the penetration of moisture. Once moisture has entered the cable, it cannot be removed. Due to the capillary effect of the braiding, the moisture will eventually permeate the entire cable, rendering it completely unusable. And where does it start? Right, at the connector! Using the original connectors designed for the respective cable types ensures an optimal, weather-protected cable connection. For all coaxial cables available from WiMo, the original, matching connectors are also available!
Customized coaxial cables
You can also order pre-assembled coaxial cables with connectors from WiMo. These are high-quality cables of different types and diameters, with the most common connector combinations and in the lengths often requested for connecting equipment within the radio station. Of course, we can also assemble an antenna cable of any length with the connectors of your choice. To do so, you can use the Cable Configurator to order the cables.
Whether you assemble it yourself or we assemble it for you according to your specifications, we wish you every success and good connections, both with our coaxial cables and via radio!
Source reference:
Klüß, A.; DF2BC: Koaxkabel & Stecker, FUNK 5/2004, S. 44-48, Verlag für Technik & Handwerk, Baden-Baden
November 2024, Alfred Klüß, DF2BC
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