Signal drops and high standing wave ratio (SWR) are often caused by the use of an unsuitable or damaged feeder rather than the antenna itself. Exactly radio antenna cable is a critical link in the energy transmission path, and ignoring its parameters leads to a loss of up to 50% of the transmitter power on the way to the emitter. An incorrect choice of characteristic impedance or feeder length can cause a mismatch, which will not only degrade the communication range, but also lead to overheating of the transceiver output stage.
The main task of a coaxial cable is to transmit a high-frequency signal with minimal losses and without radiation from the line itself. Unlike low-frequency wires, geometry is important here: the ratio of the diameter of the central core and the internal diameter of the braid must strictly comply with the standard 50 ohm. Any deviation from this value, for example, the use of a 75-ohm television cable, creates a reflected wave that returns to the transmitter and can damage it if it is transmitting for a long time.
To ensure stable operation of the radio station, it is necessary to take into account not only the characteristic impedance, but also the signal attenuation per linear meter, which increases with increasing frequency. On the bands VHF and especially UHF Losses in a thin or low-quality cable become significant even over a length of several meters. Therefore, the choice is between cheap RG-58 and thicker RG-213 or Aircell-7 It often becomes a question of the efficiency of the entire radio system, especially if the antenna is mounted high on the mast or roof of the vehicle.
Coaxial cable design and dielectric types
A coaxial cable consists of a central copper core, a dielectric, a braided shield, and an outer sheath. Quality and material dielectric directly affect signal attenuation and mechanical strength of the product. RF cables most often use polyethylene (PE), polyethylene foam, or Teflon (PTFE). Foamed dielectric reduces signal loss because it contains more air, which is an excellent insulator, but requires special care when cutting.
The shielding braid can be made in the form of a mesh of tinned copper threads or a combination of foil and mesh. For radio stations where protection from external interference and minimization of feeder radiation is important, dense copper braiding with a high coverage factor is preferable. Aluminum foil, often used in television cables, conducts current less well at high frequencies and is less resistant to repeated bending, which is critical for mobile installations.
⚠️ Attention: The use of cables with a central copper-clad steel core (CCS - Copper Clad Steel) is only permissible for stationary antennas with a fixed feeder. On cars, such a cable quickly breaks due to vibrations, since steel does not have sufficient flexibility.
The outer shell also plays an important role, especially for outdoor use. Polyvinyl chloride (PVC) is suitable for indoors, but in the cold it hardens and cracks. For external work, polyethylene is required (PE) black color, which is resistant to ultraviolet radiation and temperature changes. There are also cables with additional protection against rodents or an oil and petrol resistant sheath for specific operating conditions.
The secret to low losses
Why is air dielectric better? In premium cables (for example, Heliax), the dielectric is the air itself, and the central core is held in place by spiral washers. This provides the lowest possible attenuation, but such cables are rigid, expensive and require special connectors to seal against moisture.
Basic types of cables for radio communications
The market offers many brands of cables, but for amateur and professional radio communications a certain standard nomenclature has developed. The most common de facto standard is the series RG (Radio Grade), although modern foam cables often outperform them. The choice of a specific brand depends on the frequency range, feeder length and installation conditions.
For short connections inside a car or portable radios, they often use RG-58. This is a thin cable with a diameter of about 5 mm, which bends easily and has a small bending radius. However, its attenuation is high: at 430 MHz, losses can be more than 30 dB per 100 meters, making it unsuitable for long VHF links.
- 📡 RG-58: Diameter 5 mm, resistance 50 Ohm. Ideal for patch cords up to 1-2 meters long. High attenuation on VHF.
- 📡 RG-213 / Aircell-7: Diameter about 10mm. "Gold standard" for base stations. Low attenuation, but requires quality N-type connectors.
- 📡 LMR-400: Modern analogue of RG-213 with foamed dielectric. It is more flexible and has less losses, but requires special tools for cutting.
- 📡 RK-75: TV cable. Strictly not recommended for power transmission, only for signal reception (GPS, DVB-T).
Series cables LMR and their analogues (for example, Ecoflex) are becoming increasingly popular due to their combination of flexibility and low attenuation. They are often used in difficult installation conditions where repeated bending is required.
Attenuation calculation and influence of feeder length
Signal attenuation in a cable is measured in decibels per 100 meters (dB/100m) and depends on the frequency of the signal and the quality of the materials. The higher the frequency, the stronger the skin effect manifests itself, displacing current onto the surface of the conductor, which increases active resistance and losses. Therefore for ranges UHF (430 MHz and above) the requirements for cable quality are significantly higher than for HF (short waves).
The length of the feeder also matters. If for a short 2-meter tail in a car you can use almost any 50-ohm cable without a noticeable difference, then for a base with a 20-meter mast the choice becomes critical. A loss of 3 dB means half the transmitter power is lost. For example, if you put 50 W into the cable, and 25 W reached the antenna, the efficiency of the system dropped twofold.
Below is a comparison table of attenuation for popular cable types at different frequencies. Data are averaged for quality samples.
| Cable type | Diameter (mm) | Attenuation at 145 MHz (dB/100m) | Attenuation at 435 MHz (dB/100m) |
|---|---|---|---|
| RG-58 | 5.0 | 18.0 | 32.0 |
| RG-213 | 10.3 | 8.5 | 15.0 |
| Aircell-7 | 10.3 | 6.5 | 11.5 |
| LMR-400 | 10.3 | 6.0 | 10.5 |
When calculating the energy budget of a communication line, it is necessary to take into account not only the passport data, but also real conditions. An old, oxidized cable can have losses many times higher than the nominal ones. It is also worth considering that connectors introduce additional attenuation, usually 0.1-0.3 dB for each high-quality joint.
Main conclusion: At frequencies above 400 MHz and a feeder length of more than 10 meters, saving on high-quality cable (such as Aircell-7 or LMR-400) will lead to a loss of communication range, which cannot be compensated by increasing the transmitter power.
Rules for installation and cable laying
Correct installation of the cable for the radio antenna is the key to the longevity of the system and stable SWR. The basic rule is: avoid sharp corners and kinks. The minimum bending radius is usually 10 cable outer diameters for a static position and 15 diameters for a dynamic position. Violation of this rule leads to deformation of the dielectric, a change in wave impedance and the appearance of a reflected wave.
When laying through metal surfaces (car body, building walls), it is necessary to use rubber bushings or oil seals. When the metal edge vibrates, it will rub the braid and sheath, which will lead to a short circuit between the central core and the screen. In automotive installations, the cable should be secured with plastic ties in increments of 30-40 cm to prevent looseness.
Sealing external connections is a mandatory installation stage. Water that gets into the connector causes corrosion and dramatically changes the electrical parameters of the line. For protection, special sealing tapes (self-vulcanizing) or heat-shrinkable tubes with an adhesive layer are used. Regular electrical tape is not suitable for these purposes, as it dries out and slips over time.
☑️ Antenna cable installation checklist
Connectors and their effect on the signal
The quality of the cable connection to the antenna and radio station is determined by the connectors used. The most common standard in power radio communications is the connector N-type, which ensures excellent matching and tightness. Often used for portable equipment and car radios SMA (SubMiniature version A) and their variations.
It is important to distinguish between connectors SMA-Female (with thread inside) and SMA-Male (with thread on the outside). Radios usually use a "Female" type connector, and "Male" is soldered on the cable. There is also a standard RP-SMA (Reverse Polarity), where the central contacts are reversed. Trying to connect a regular SMA to an RP-SMA is mechanically possible, but there will be no electrical contact and the force may damage the thin center pin.
Soldering connectors requires care. Overheating of the central core can melt the dielectric, changing the geometry and creating a matching "failure". For cables with foam dielectric (Aircell, LMR) soldering is often impossible or undesirable, since heat destroys the structure of the foam. In such cases, crimping (crimp) or screw-on (screw-on) connectors that do not require heating.
⚠️ Attention: Never pull the cable by the connector when disconnecting. This leads to separation of the central core from the contact group. Always grasp the connector body.
Diagnostics and measurement of SWR
After installing the antenna system, it is mandatory to check the standing wave ratio (SWR). This parameter shows the degree of matching between the antenna, cable and transceiver. The ideal SWR value is 1.0, which is unattainable in practice. A good result is considered to be an SWR in the range of 1.2–1.5. Values above 2.0 require immediate attention.
For measurements, an SWR meter is used, which is connected to the gap between the radio station and the antenna cable. The procedure is carried out at several frequencies in the operating range. If the SWR minimum is offset from the operating frequency, adjust the length of the antenna's radiating element, not the length of the cable (unless the cable is part of a matching device).
A sharp change in SWR over time may indicate problems in the cable: moisture ingress, fracture of the core, or oxidation of the contacts. Regular monitoring of this parameter helps prevent failure of expensive equipment. If the SWR is high, check the integrity of the braid and the reliability of the contacts in the connectors.
Helpful Hint: When tuning your antenna, always check the SWR at the operating frequency and at the edges of the band. This will help you understand where you need to shift the resonance - shorten or lengthen the antenna.
How to distinguish a quality cable from a fake?
A high-quality cable has clear markings every meter, an even round cross-section and a central core made of pure copper (not steel). Counterfeits often have uneven braiding, a thin center core, and markings that can be erased with a finger. The weight of a quality cable is always higher due to the use of copper instead of aluminum or steel.
Is it possible to extend the antenna cable?
It is possible to extend the cable, but it is not advisable. Each connection introduces additional losses and a potential point of failure. If extension is necessary, use only high-quality adapters or soldering while maintaining wave impedance. The joint must be carefully insulated. It is better to replace the entire cable with a longer one.
Why does the cable heat up during transmission?
Heating of the cable indicates large losses of power, which turns into heat. This occurs when the SWR is high (high reflected power) or when using a cable that is too thin for the given transmitter power. Prolonged heating will lead to destruction of the dielectric and a short circuit.
Does the color of the cable affect its properties?
The color of the insulation itself does not affect the electrical parameters. However, a black polyethylene shell provides better protection from the sun's ultraviolet radiation than a white or clear one. For outdoor installation, select only black cable marked “for outdoor installation.”