RF Site Survey

The RF site survey is an essential part of the whole site survey and analysis. Either perform an RF site survey yourself or have a trained RF engineer perform one for you. The RF site survey utilizes an RF spectrum analyzer. Because an RF spectrum analyzer is a very costly device, you have to decide whether to invest in it or to hire the service of a specialized company that already owns a spectrum analyzer and performs the analysis for a fee. Most companies choose to outsource these survey services to wireless network companies because they have developed expertise in the area. The initial design phase should have identified the radio frequency of the system that will be used. An RF site survey identifies potential sources of interference in or near this frequency range. For this portion of the survey, network and spectrum analyzers are needed, and as mentioned previously, it must be decided whether the analysis will be done internally or through a hired contractor.

Place the analyzer and its antenna in and around the proposed interrogation zones and record the results. If interference occurs at or around the proposed frequency range, it needs to be identified. Shielding or other solutions need to be deployed so that the interference does not degrade the capabilities of the interrogation zone. The steps of performing the RF site survey are as follows:

1. Obtain a facility blueprint. If no blueprint is available, have an “as built” drawing prepared.

2. Visually inspect the facility. Many drawings do not include stored goods, metal shelving, or other potential barriers to RF signals, for example. Note these elements on your blueprints for future reference.

3. Identify the interrogation zones on the blueprints. Knowing the ambient RF noise throughout the entire facility is not required, but knowing those levels in your operational areas (in and around the interrogation zones) is essential to your design. Do not forget the RF spectrum analysis and interference identification in a site survey. You must know why it is important and what to do when RF interference is found.

4. Using the spectrum analyzer, measure the ambient RF in and around the interrogation zones. Set your analyzer with the center frequency as the proposed frequency for the system and allow for one channel frequency width to either side of the total range your systems will cover (for example, 901.5–928.5 for an analysis of a 902–928 installation). If high levels of RF noise are found, record the findings and then look for the source of the interference.

To find sources of interference, follow these steps:

1. Move the analyzer antenna 10 feet in any direction and take a measurement reading of the performance again. Repeat this process until the source of the interference is found and mapped. Remember this rule: The closer to the source of interference, the stronger the signal becomes.

2. When the source is identified, shield it using an absorptive or reflective material on the side toward the impacted interrogation zone. Sometimes, shielding of the interference is not possible. When this occurs, either the design has to be changed, or the interfering device has to be replaced with one that does not interfere (for example, a 2.4 GHz cordless phone to replace a 915 MHz cordless phone).

3. Measure the zone again to ensure that the interference has been removed. The whole cycle can be repeated as needed.

Finally, the site analysis needs to determine the correct interrogation zone configuration based on the types of tracking the application requires. They may include all or some of the following:

Item-level Tracking—The tag is placed on an individual item within a case (or can be incorporated into the product itself) to ensure complete visibility of the item throughout the supply chain. This type of tracking is used in the pharmaceutical industry as well as in retail and government (DoD) applications for tracking high value items.

Case-level Tracking—The tag is placed on the case. The case may contain several items of the same product; however, it may also be a combination of products. This type of tracking is currently used in retail today.

Pallet-level Tracking—The tag is placed on a pallet and can provide information about the products on the pallet. This type of tracking is used often in retail, where the tags on pallets are associate with the tags on the cases.

Trailer-level Tracking—The tag identifies a shipping trailer and can provide information about the trailer, pallets, and cases on the trailer, as well as shipping information. Depending on the type of tag, it can also provide information about the environmental conditions the trailer goes through during the transport of goods.The RFID design must include the types of tracking that each zone will use. This ensures that the right interrogators, antennas, portals, tunnels, and tags are used.

These steps guide you through the process, but experience is the best teacher. You have to do several of these surveys and go through a lot of trial and error before you will be comfortable that you are doing them correctly. In the meantime, it is best to hire an experienced professional or firm to perform the RF site survey for you.