Because it’s commonly used in radio work, every ham should be familiar with coaxial cable, often simply called coax.
Coaxial cable is most often used between the transceiver (or T/R switch) and antenna. In this application coax acts as the feed line (AKA transmission line) to carry transmitted and received RF signals between the antenna and radio. Other types of feed line can be employed but coax is used by many hams because it is easy to work with and readily available.
Coax is a type of electrical cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Most coaxial cables also have an insulating outer sheath or jacket. The term coaxial comes from the inner conductor and the outer shield sharing a geometric axis.
To be useful coaxial cable must be terminated with mating RF connectors. An experienced ham may terminate their own coax; at greater cost they may purchase ready-made and tested assemblies.
A wide variety of coaxial cable and assemblies are available with different characteristics. A quick summary of the important features:
- Characteristic impedance
- Signal loss
- Power capacity
- Environmental resistance
A seventh important characteristic of coax is velocity factor but that is a more advanced topic of lesser importance so we’ll simply mention it here.
Coaxial cable selection for each installation may be a compromise between features, requirements, and cost. The ham has to factor in what he needs or wants, what is available, and what it costs.
A quick look at these features of coaxial cable:
Characteristic impedance– Like all other cable, coax is specified at so many ohms impedance based on its physical and electrical properties. In radio work the characteristic impedance is usually needed to match that of the radio and antenna. Hence 50Ω is commonly used, although 75Ω cable may sometimes be used with an impedance matching device.
Signal loss– RF signal strength will attenuate as it passes through coax. This is an undesired characteristic which must be considered for each installation.
Different coax types have different losses so material selection (type) can be important. Online and manufacturer calculators are helpful here to select the best cable. Links to a couple are given at the end of this post.
Cable loss depends on diameter of the cable and dielectric (insulation) used between the inner and outer conductors.
Length is also a major factor; loss is specified in dB per 100ft, so it’s apparent that short runs will have less loss. Conversely, a long run between the radio and antenna demands careful consideration of coax cable type.
Additionally, loss is proportional to frequency.
For VHF and UHF application, a ham is advised to choose a lower loss cable, particularly for long runs.
Whatever frequency you operate, recall that a 3dB loss represents half power so make careful consideration of loss.
Power capacity– Different coax types have different power ratings. For most hams with 100W maximum power, there are many common cables to choose from. Hams blessed with RF amplifiers running 500 to 1500W transmit power will need to be more selective about their coax. For receiver only situations or low power transmit (QRP), coax power rating is not a concern.
Here again online calculators are very helpful for cable selection based on power requirements. As with all kinds of wire and cable, higher current demands thicker conductors so high wattage coax will be larger in diameter and less flexible.
Diameter/weight– Coaxial cable comes in a range of diameters and proportional weight:
Cable diameter should be a minor consideration, as other factors such as power handling and loss are much more important. However, there are occasions when weight or size do matter. For example, this LMR400 coax is low loss for VHF but not practical to mate with a handheld radio:
While this skinny RG-174A coax is more appropriate (but rather lossy):
Flexibility– Coax is available with solid or stranded center conductors. Solid conductors make the overall cable more rigid but easier to terminate (solder) should you choose to roll your own. Stranded center cable is more flexible but also more expensive.
Environmental resistance– The outer jacket determines the suitability of a coaxial cable use in various conditions. The biggest concern is resistance to ultraviolet (UV) light from the sun when coax is run outdoors.
Other considerations are chemical resistance, if so exposed, and ability to bury coax under ground (direct burial), as some hams do.
All of these features can be found in the manufacturer’s spec sheets and/or sales info for every particular coaxial cable type.
One of the more important considerations with coaxial cable use is to prevent moisture from entering the dielectric insulator between the center conductor and outer shield. Water intrusion will dramatically alter the characteristic impedance and increase loss of the cable, affecting performance. This is why there are a few license exam questions on the topic:
Obviously we should be concerned about cuts and abrasions in coaxial cable, as this also allows water an entry point.
A few good references
ARRL topic on feed lines (includes coax)