RF Hardware 101 - Bullets, Barrels, Connectors, Cable Choices
If you are new to the radio frequency space, you may find the content of this document useful. The RF discipline uses a number of terms that may be unfamiliar. Here we will list a small subset of those terms and describe what they mean. Moreover, we will discuss the different types of cable (for connecting the reader to an antenna) in the marketplace, along with their general characteristics, and how to choose the appropriate cable.
First, if you need to know about the different types of connectors, please click here to view this very helpful picture catalog of common RF connectors used in UHF RFID. At some point in your RF adventures, you'll likely come across every one of these connector types.
In addition to the connectors listed in that linked document, you may find RP (or Reverse Polarity) versions of these types. Typically, RP variants of connectors look like their Normal Polarity counterparts, with the exception that the center pins change (if present in NP, then not present in RP, and vice versa) Here's a quick example of NP SMA Male and a RP SMA Female. For clarification purposes a 'male center' has a pin whereas a 'female center' does not.
The quick takeaway is this: gender is always dictated by whether there are outer threads (female) or inner threads (male). Specifying the RP version of a particular connection only refers to the center pin. Stated otherwise, RP solely means either add or subtract the center pin based on the opposite center pin state of Normal.
As you probably already know, the Impinj Revolution R420 and R220 readers have RP-TNC connection types for the antenna ports. In contrast, the Impinj Antenna Hub uses Normal Polarity SMA ports. These facts are important when choosing the antennas and cables in your system.
Sometimes you may need to interface two cables that have the same gender ends. To accomplish this you'll need either a bullet or a barrel. A barrel refers to an adapter with two male ends, while a bullet refers to one connecting two female ends.
Finally, we often are asked what type of cable to use. This question is hard to answer uniformly as a couple of variables routinely impact the decision. Those variables are: (1) tolerable insertion loss (or, the amount of signal that can be lost as it travels down the wire without adversely affecting system performance) from the length/type of cable, (2) cost and (3) any insertion loss arising from adapters or external switching equipment such as the Antenna Hub.
Variable 1) Insertion Loss Inherent to Cable Type/Length
All wire conductors have some degree of signal loss. The longer the signal must travel down a guided path, the more loss (also known as signal attenuation) occurs. Cable composition can either magnify or minimize the amount of loss. RG58 is fairly lossy in runs of up to 100', whereas LMR400 is significantly less lossy, but higher cost. In runs of 8ft or less, it's common to use RG58 since the loss is tolerable and the cable is inexpensive compared to other types. The following table is a useful general* guideline to show the relationship b/w cable type and the amount of loss you might expect to see.
To achieve the best performance in most cases, you'll want to ensure as much power leaving the reader actually gets to the antenna before being deliberately radiated. This is especially true in many supply chain and asset tracking applications where getting enough power to the tags proves challenging sometimes. To illustrate by way of example, assume you have an antenna cable which is 50' long. The cable is standard RG58. Referencing the table above means ~8.25 dB of power (.5 x 16.50) has been lost via the cable before the signal even gets radiated into the air by the connected antenna. For other cable types you can calculate your loss using this transmission line loss calculator site.So what the figures tell us is that instead of sending the antenna close to 1W (assuming a loss free cable), the original 1W of power has been reduced to only ~150 mW by the time it reaches the antenna. Remember, loss is specified in dB scale, which means every ~3 dB decrease is equivalent to a power drop of roughly one-half. Depending on your application, that ~8.25 dB of loss could be the difference between an excellently-performing system and one that barely works, if at all. Again, the particulars of your installation will determine what's tolerable loss.
Variable 2) Cost
Generally, the less lossy a cable, the more expensive it is due to manufacturing and material complexities.
Variable 3) Loss from Adapters, External Components
RF components such as bullets, barrels and connectorX-to-connectorY adapters (where X is one type and Y a different type) also result in some loss of signal. Again, manufacturing quality is an issue here. Really inexpensive versions may be (but not necessarily so) very lossy. It's always best to get the datasheet for the adapters you choose so you'll know what the nominal loss should be. Typically, .3 to .5 or less dB is normal for most adapters. If in doubt, it's always a good idea to buy from a reputable RF supply company. We'll provide a non-exhaustive list of some suppliers at the end of this document. The listing should not be conferred as our endorsement, either explicit or implicit, of those co's and their products. Other devices may introduce additional loss. The Impinj Antenna Hub is a fantastic separately purchased multiplexer deeply integrated with the low-level operations of the Revolution R420 4-port reader. Impinj Antenna Hubs --when the maximum number of 4 are used for a single reader-- empower a user to take a normal R420 4-port reader and fan it out into an impressively scaled 32-port reader. The switching circuitry in the Antenna Hub is highly efficient, but not loss-free. As such, you should expect up to 1.3 dB of loss as the signal moves through the Hub. This is just one example of a loss you should account for when designing your RF system and choosing the correct cable and power settings.
Supplier WebSites (just a few) - just for representative sample; not to be considered an endorsement