It is used with 75 ohm cables for UHF TV, but it does not present a constant impedance to the cable. Thus you can buy set-top boxes and even RTL-SDR sticks with one fitted. If you are an American you may never encounter this connector, because it survives only as the antenna connector on European and Australian TV sets. Starting with the oldest of our selection, the Belling-Lee has its roots in the 1920s. Welcome to the Coax Connector Jungle Left to right: BNC, UHF, Belling-Lee Fortunately of that huge catalogue only a few styles have become popular enough that you might commonly encounter them, so when taking a look at individual connectors in detail we only need to consider a few examples. Some of them have very specific applications while others are intended for general-purpose use. There are a myriad coax connector styles, both obsolete and still on the market. Or in the case of the “RCA” phono connector, a similarly aged design whose roots lie in audio and whose RF properties were of no interest to its originators. Popular connectors have a habit of staying in use for many decades, so a few that you’ll encounter such as the Belling-Lee antenna socket on European TV sets or the now-inappropriately-named “UHF” you’ll see on HF radios are really old designs conceived before the mechanism of characteristic impedance was fully understood. It’s worth mentioning though that sometimes this goal of constant impedance has not been pursued. If you were to saw a typical high-performance RF connector in half lengthways then you would see it in action, great care has been taken to achieve this aim. To achieve this perfect impedance match the designer of an RF connector must ensure that the internal diameter of the outer shell, the diameter of the centre conductor, and the dielectric properties of the insulator are such that at all points during the signal path the characteristic impedance is the same. Electrically it must appear as though the connector is not there: an unbroken piece of coax at the desired frequency of operation. There in a nutshell is the problem faced by designers of RF connectors: to ensure that their product appears throughout all its parts as exactly the same impedance as the coax it is connecting. Connector impedance matching illustrated in US patent US 2540012, a precursor to the BNC, granted in 1951 to Octavio M Salati. From the RF signal’s point of view then, anything you connect to it should look electrically like just more coax. If a different impedance is presented then some of the RF will be reflected back down the coax, the total impedance of the system will be changed, and signal loss will occur. If you connect anything to the coax in the previous paragraph, it must be designed to present the same impedance as the characteristic impedance of the coax. In a perfect piece of theoretical coax this impedance figure will be the same no matter what the RF frequency or voltage is it is dependent on the physical properties of the coax itself, the dimensions of the conductors and the dielectric properties of its non-conducting components. If you were to non-invasively measure the RF voltage and current, you could then calculate an impedance in ohms in a similar way as you might a resistance in a DC circuit. So, imagine an infinitely long piece of coax with RF flowing though it. And for that we need to talk briefly about characteristic impedance without descending too far into the mathematical proofs that detain first-year electronic engineering students. Form Follows Functionīefore looking at individual connectors it’s worth looking at the much broader picture, of what an RF connector does and why it is designed in a particular way. This article will look at some of the different types of connector and try to explain some of those choices. You might imagine that the most important would be the physical and electrical specification of the connector itself, but other factors such as company design policy, the accepted norm of a particular field, and the personal preferences of the designer come into play. There are a huge variety of factors that lie behind the choice of RF connector on a piece of equipment. Don’t worry, your secret is safe with me. In making all these disparate devices talk to each other you probably have a guilty past: at some time you will have created an unholy monster of a coax interface by tying several adaptors together to achieve your desired combination of input and output connector. You’ll need them, because the chances are your bench will feature instruments, devices, and modules with a bewildering variety of connectors. If you do any work with analogue signals at frequencies above the most basic audio, it’s probable that somewhere you’ll have a box of coax adaptors. Just a selection from the author’s unholy assortment of adaptors.
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