2014 Tech study guide: types of feedline, connectors

In the 2010 study guide, I also included questions about SWR and antenna measurements in this section. For 2014, however, I’ve decided to make that a separate section…Dan

Feedlines connect radios to antennas. There are many different types of feedlines, but coaxial cable is used more often than any other feedline for amateur radio antenna systems because it is easy to use and requires few special installation considerations. (T9B03) A common use of coaxial cable is carrying RF signals between a radio and antenna. (T7C12) Note, however, that the loss increases as the frequency of a signal passing through coaxial cable is increased. (T9B05)

When choosing a feedline, it is important to match the impedance of the feedline to the output impedance of the transmitter and the input impedance of the antenna. Impedance is a measure of the opposition to AC current flow in a circuit. (T5C12) Ohms are the units of impedance. (T5C13)

Most amateur radio transmitters are designed to have an output impedance of 50 ohms. Because that is the case, the impedance of the most commonly used coaxial cable in typical amateur radio installations is 50 ohms. (T9B02)

RG-58 and RG-8 are two types of coaxial cable often used in amateur radio stations. Both have an impedance of 50 ohms, but there are important differences between the two. One electrical difference between the smaller RG-58 and larger RG-8 coaxial cables is that RG-8 cable has less loss at a given frequency. (T9B10) The type of coax that has the lowest loss at VHF and UHF is air-insulated hard line. (T9B11)

Moisture contamination is the most common cause for failure of coaxial cables. (T7C09) One way that moisture enters a cable is via cracks in the cable’s outer jacket. The reason that the outer jacket of coaxial cable should be resistant to ultraviolet light is that ultraviolet light can damage the jacket and allow water to enter the cable.(T7C10) A disadvantage of “air core” coaxial cable when compared to foam or solid dielectric types is that it requires special techniques to prevent water absorption. (T7C11)

PL-259 connectors are the most common type of connectors used on coaxial cables in amateur radio stations. One thing that is true of PL-259 type coax connectors is that they are commonly used at HF frequencies. (T9B07)

One problem with PL-259 connectors is that they are not the most suitable connector when operating at higher frequencies. Instead, a Type N connector is most suitable for frequencies above 400 MHz. (T9B06)

No matter what type of connector you use, coax connectors exposed to the weather should be sealed against water intrusion to prevent an increase in feedline loss. (T9B08) Also make sure to tighten connectors well. Also make sure that your antenna connections are tight and the connectors are soldered properly. A loose connection in an antenna or a feedline might cause erratic changes in SWR readings. (T9B09)

 

Comments

  1. Is there room for information that is not on the test? For example, BNC connectors are also suitable above 400 MHz, even though they are not one of the answers in the question pool.

    K6WRU

  2. Dan KB6NU says:

    The question is always what to put in and what to leave out. I usually err on the side of leaving things out, but in this case, I might just include it with the caveat that BNC connectors are not designed to handle as much power as N connectors. Do you know if they’re waterproof?

  3. I think BNC are can be weatherproof. I’m not sure all of them are.

    The power handling is interesting. I can’t find figures on the resistance of the contacts, but here is the peak voltage from Amphenol datasheets:

    UHF: 500V
    BNC: 500V
    Type N: 1500V

    http://www.amphenolrf.com/products/bnc.asp
    http://www.amphenolrf.com/products/typen.asp
    http://www.amphenolrf.com/products/uhf.asp

  4. The problem with including information that is not useful for passing the exam, is that it goes against the ‘No Nonsense’ description. The problem I have with most study guides is that they have so much material that’s not useful for passing the exam, that it’s no longer a study guide and it decreases the likelihood that the student will complete the study guide and pass the exam! I recommend Dan’s study guide to my students, but recommend that they read other ‘study guides’ only after they’ve passed the exam. I provide two follow-up classes, *after* students have passed the exam – that’s where we get to talk about using BNC connectors up to 400MHz and being waterproof. We also show the new hams how to use a BNC adaptor on SMA connectors, to avoid stripping the threads. Again, all good stuff, but it doesn’t belong in the study guide.

  5. Dan KB6NU says:

    Thanks for the info, Walt, and your comment, Norm.

    I think that what I’m going to do as soon as I finish updating the study guide is write the book, What Every New Ham Should Know. It will cover in more detail what is not in the study guide.

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