Estimating RF Power Loss


Estimating power loss is an important task to complete before purchasing a coaxial cable. Loss can typically affect the read range of your antenna, so it is recommended that an antenna is deployed as close to a reader as possible. If your application does not allow short cables, low loss cables are a good substitute. This article will briefly show you how to test how much power loss is acceptable within your application by reducing your reader’s power to simulate power loss.

Link to A Guide to Cables, Connectors, & Adapters:

Testing for Acceptable Loss Within Your Application

For the purposes of this article, we will assume you are aiming to achieve a 9 ft read distance.

The following table shows loss for various cable lengths:

To approximate the amount of loss you will need to account for in your application, follow the steps below:

1.    Power your reader to full power (for example, 31.5 dB on an Impinj R420 Speedway reader if operating in the FCC frequency range), and connect your antenna directly to your reader. Try testing an intended read distance of 9 feet, and if you are able to read the tag at this distance, proceed to the next step.

2.    Now, you should estimate the cable length you would need for antenna deployment. Say your estimate is about 10 feet, and you decide to use the LMR-240 10ft cable for your application. At a length of 10 feet, this cable has an estimated loss of 0.8 dB.

3.    To simulate loss without the cable, reduce the RF power on your RFID reader by 1 dB, bringing the transmit power in our example to 30.5 dB. Most readers’ power can only be adjusted in 1 or 0.5 dB, so adjusting by 0.08 will not be possible.

4.    Check your tag’s read distance with the reduced power. If you can read the tag at this power, it is okay to purchase this type of cable for your application. If you are unable to read the tag, try increasing the power to find the minimum power needed by the antenna to read the tag at 9 feet. If you were to find that you require 31 dB to read the tag, then the best option is to opt for a LMR-400 cable, which has an estimated loss of 0.4 dB at 10 feet.