Coaxial Cable & Line loss

Radio signals require careful consideration when it comes to cabling and connectors, there are very specific rules, and if these are ignored then signal losses can be such that devices may simply not function at all.

The cables used to carry RF signals, such as those between a VHF and its antenna, are called 'Coax' cables short for coaxial.

The term coaxial is derived from the inner conductor and outer conductor sharing the same axis; cross sectionally the centre of the outer conductor shares the same centre as the inner - as covered in the Level 2 course.

As with powering DC Equipment we have to deal with voltage drop, based on the amps and cable lengths, there is a similarity with coax cable in that we have to deal with signal degradation based on the frequency, cable length and number of and type of connectors. Signal strength is measured in decibels, and there are acceptable limits that we must observe.

When using standard cables to power equipment we have to pay careful attention to voltage drop in volts, when dealing with Coaxial cables we have to contend with Signal loss in Decibels.

Each time a radio signal travels down a coaxial cable some of the RF energy is lost, there is no perfect coaxial cable, although better quality cables exhibit substantially less loss than some of the 'budget' offerings, suffice to say coaxial cable is not an area to scrimp on.

Avoid any coaxial cables that have a hollow dielectric, these are not suitable for use in the marine environment as water can track up the cable and this causes massive signal losses.

Signal loss relates to range loss, and that could place the vessel in danger should communications be required for an emergency.

Key learning points from this lesson are to fully understand how to calculate signal loss, and what is an acceptable level of signal loss for the various types of equipment found onboard. In order to complete the video lesson below you will need to print the PDF form above.

Video Lesson

In order to complete the quiz for this section you will need to complete the following questions, and due to the safety critical nature of this topic, the quiz is graded. Please do not start the quiz until you have answered the questions below and thoroughly understand this subject.

  • 1) You have a Satellite antenna running at 1900Mhz, what would the losses be for a 65' length of RG8X with one connector?

  • 2) Your VHF radio is not working as well as expected and you notice that there are 2 different types of cable in use. From the radio to the mast base it's a run of 15' of RG8U, and from the base of the 40' mast to the antenna the cable is thinner RG58U. Whats the signal loss, and is this acceptable?

  • 3) You've had a new AIS system fitted and it is not receiving any targets, it has a cable run of RG58U of 10 feet but the cable needed to be cut and joined with a connector. Calculate the signal loss, to see if this is acceptable.

Complete and Continue