CHAPTER 7

LINES

This is the most mysterious chapter. Of all the pieces in a duplexer, the lines that connect the cavities and the external lines that connect the duplexer to other devices, generate the most questions. They also spawn many old wives tales. Fortunately, we are going to learn that determining the lines is easy, especially if we take a practical approach.

In this chapter we are going to look at the types of cable to use, the equivalent circuit of the lines and loops, why it is difficult to calculate line length, an easy way to determine the length of lines, and what to do with the external lines.

Line Type

Like almost everything else in a duplexers, my experiments have demonstrated to me that the type of coax that you use to couple the cavities together is not critical. Only three things matter. The first is shielding. All the coax that you use inside of a repeater must be 100% shielded. It is frankly more or less of a necessity outside, as well. Otherwise all of the work that you do to isolate your receiver from your transmitter and from the neighbors with cavities can easily be lost by direct pickup through the shield of the coax.

Ordinary coax, with a single braided shield, the kind we normally use, is inadequate. It is too leaky. It works fine for common transceiver applications, but not for duplexers. Only two types of coax are satisfactory for repeater use, double-shielded flexible coax or hard line. Most of the flexible coax types that we commonly use can be obtained in double-shielded versions. The only difference is that it has two layers of braided wire loom as the outside conductor instead of one. This effectively makes it 100% shielded.

Rigid hard line is naturally totally shielded. Its outer conductor is rigid copper or aluminum tubing. Generally it is impractical to use inside of a repeater, however. You'll use it mostly for external feedlines. Its main benefit in this case is its ability to withstand deteoration from weather. Flexible feedline with either a single or a double shields allows moisture to pass through over a period of time. Losses increase rapidly in exposed flexible cable at VHF and UHF. Inside a repeater, however, there is no problem.

The second factor to consider in the type of cable that you select, is the connectors. Especially for the home builder, this can alter costs considerably. You can find connectors to fit almost any type of cable, but the cost varies dramatically. After all, you do not use very much cable to build a duplexer, but you do use quite a few connectors. Shop your suppliers carefully.

The exact type that you use is not critical, as long as it is correct for the cable that you are using. I prefer to use crimp-on connectors for convenience and durability, but the tool to attach them is expensive. The screw on type are totally satisfactory, even those obtainable from your local bargain retail electronics outlet.

I recommend two types of connectors, N and BNC. If you are running 100 watts or less, BNC connectors are the best choice. They are moderately priced and have good RF characteristics. N connectors are better for higher power and also have good RF characteristics. Do not, however, be tempted to use SO/PL 239 connectors. They are unfortunately common on many RF devices, but often have poor RF characteristics. One or two in a repeater is not usually a problem, but try to avoid them is you can. Many commercial duplexer manufacturers use them, but they select the type that they use carefully. The kind you'll buy at your local store can cause you grief.

The third consideration in selecting cable is power handling capacity. As with connectors, for power levels under 100 watts, double-shielded cable similar in size to common RG-58 is quite satisfactory. I use an economical variety of foil double-shielded RG-58 that is similar to the cable developed for cable TV applications. Most cable manufacturers now offer it. The point is, almost any type will work if it is double shielded.. If you wish to use the expensive silver plated double braid varieties, that's fine, but less expensive types work just as well.