Understanding Cables for RF

Understanding Cables for RF

Share this Facebook Twitter LinkedIn

Understanding Cables for RF

Facebook Twitter LinkedIn

Whiteboard Session

Shure Applications Engineer, Tom Colman answers common questions about cable components for your RF setup.

RF cables are an often overlooked aspect of  wireless systems. You can have everything else set up perfectly: you've  got the right kit; you've coordinated frequencies; your antennas are  placed correctly, but when all is said and done, it's all too easy to  shoot yourself in the foot by selecting the wrong antenna cables.

Why are RF cables required?

RF antenna cables are a crucial piece of kit when:

A: Your wireless receivers are too far away from the performance area  to achieve correct antenna placement, meaning you'll need to cover some  distance with cables.

B: Your receivers are installed in a metal rack and you want to avoid  poor performance by removing the antennas from inside the metal  housing.

Considering signal loss

RF cables should be treated differently to standard audio cables. For  instance, with balanced audio cables, we can operate successfully over  relatively long cable runs. With RF cables, however, signal loss is a  much greater issue.

There are three core elements when considering signal loss: 1) The  build quality of the cable, 2) The frequency at which you're  transmitting, and 3) The length of your cable run.

Additionally, while all the components of your wireless system should  ideally have the same impedance, your RF cables are the component most  likely to fluctuate. Always use high-quality cables to reduce risk.

What cables should you use?

Standard RF cables are essentially coaxial cable with BNC connectors  at each end. These cables are very similar to those used by the video  industry, with one key difference: their signal resistance  characteristics. Video coaxial cable is typically 75 Ohm, whereas  wireless system operators use 50 Ohm coaxial. (Ohm's Ω – refers to the  amount of electrical resistance in the cable, where the greater the  number of ohms, the greater the resistance will be). If you were to use  mistakenly a video coaxial cable at 75Ω, you would receive very little  signal, so please do take extra care.

Let us now consider the chart illustration below to demonstrate the varying signal loss in two different 50Ω RF cables...


For the sake of example, let's say our radio microphone kicks out 10  milliwatts (mW) of RF power and is operating at 600MHz. Also, let's  assume there's no path loss (i.e there is no signal loss from the  transmitter to the antenna). In this [perfect] example, our antenna is  seeing all 10 mW of power, which allows us to demonstrate the signal  loss of two common RF cables.

Knowing that 3dB of signal loss results in a power loss of roughly  half, we can use the two examples (RG-58 & RG213) to approximate the  amount of loss through each cable. RG-58 would take the signal strength  down from 10 mW to around 3 mW, which is quite a lot of loss. RG213 on  the other hand only loses 1.9 dB – translating roughly to a loss of  approximately 3 mW (7 mW output).

In an ideal world, we would have no signal loss across our wireless  setup, so it's important to compensate for any loss using a signal  booster or an active antenna. See the screenshot below for an example of  how active antennas can boost the signal significantly and compensate  for loss.



Learn more 

To learn more about RF cables and other best practice wireless  principles, consider attending one of our Wireless Mastered or Wireless  Workbench training sessions.