U.H.F. Transmission Lines and Converters
June 1954 Radio-Electronics

June 1954 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

About the only kind of transmission line homeowners use today is 75 Ω coaxial cable for television and Internet connections. Up until the turn of this new century (two decades old by now - ugh!), a fair percentage of TV owners still had an antenna mounted on the roof or maybe rabbit ears sitting atop the set. Twin lead transmission line was still a common sight as at least the "last mile" (last few inches, actually) connection via an impedance converter to the pair of screws on the back of a TV or FM radio. By then, most TVs also had an "F" coaxial connector for direct attachment without a converter. Internet connections at the time were telephone cords connected to the modem board inside your computer. It's hard to believe that was just a relatively few years ago that we early World Wide Web (WWW) users got to cross our fingers when dialing up a local phone number for AOL, Earthlink, Compuserve, Prodigy, Mindspring, etc., and hoping those handshaking squeals resulted in a successful connection rather than hearing the number get dialed again for another try. Aside: When I made a household move in 1996, it took me two months to get basic telephone service to the new house because the WWW craze resulted in so many people having second lines added just for online service that no lines were available for new service. That really sucked.

Pointers on using these elements to improve performance

By Matthew Mandl and Edward Noll

The proper type of transmission line and its installation are important factors in getting the most from a u.h.f. installation. Capacitance losses are greater at u.h.f. Therefore the transmission line should be spaced at least six inches or more from the mast or other metal objects. Even with a wide spacing it is best not to run the line parallel to rain pipes or other metal conduit for any great distance. The line should be as short as possible, and sharp bends should be avoided.

Line having the least loss should be used, though installation factors may alter such a choice. The open-wire line, for instance, has much lower losses than other types, but is most difficult to install, particularly where it enters the home. The new Gonset u.h.f, open-wire line has an impedance of 375 ohms with a 2-db loss at 500 mc. Imperial has a 250-275-ohm open-wire line which matches 300-ohm loads and has a loss of about 0.8 db per 100 feet at 500 mc when dry. As shown in the table on page 39 of the January issue, other lines have a greater loss for this frequency. Since losses are calculated on the basis of 100-foot lengths, the other lines can be used for ease of installation, provided too great a length is not required.

Losses increase for the higher u.h.f. channels and too lengthy a run will cut down the signal appreciably. Thus, if the antenna is raised by 100 feet the additional length of the ribbon lead would introduce over 3 db loss around channel 19 and almost 5 db for channel 83. A 5-db loss is greater than the gain realized by stacking an antenna (3 db for each bay added to an existing antenna). The tubular type line is preferable over the flat because of its lower loss, but precautions must be taken to seal the ends of the line to keep out moisture. Some technicians form a reverse loop in the tubular lead-in at the antenna to keep out moisture, and also provide a drain hole by cutting a slot in the bottom of the line where it enters the home. When the top is sealed, however, a loop is avoided and a neater installation results.

The American Phenolic Corp. manufactures a polyethylene end seal plug which provides a quick and positive closure for tubular lead-in. It seals the inside of the tubular lead and also provides a protective cap and seal for the outside. (Amphenol, 66-213 Twin-Lead and seal.)

The shielded wire lines are not recommended for u.h.f. installations. Their excessive losses more than overcome their advantages-low noise (shielding), plus unchanging impedance in wet weather.

U.H.F Transmission Lines

The table lists the common advantages and disadvantages for various types of u.h.f. transmission lines.

Uhf converters

Many u.h.f. converters have a gain of 1 and therefore do not increase the signal strength during the conversion process. Thus it is essential that the converter operate at peak efficiency. Otherwise signal strength will decline during the conversion process. When there is reason to doubt the performance of a converter, the tubes should be checked and the mixer crystal changed. It is also a good idea to try several local-oscillator tubes as well as mixer crystals, because certain combinations deliver greater signal output.

Some converters have provisions for both the v.h.f. and u.h.f. antennas. The respective transmission lines are attached to the terminals and automatically switched into position as the selector knob is turned to either u.h.f. or v.h.f. In some converters such switches introduce capacitive losses. In weak-signal areas such losses can be severe enough to degrade picture quality considerably. A simple check consists of setting the converter for v.h.f. reception and tuning in a weaker station. Remove the v.h.f. antenna transmission line from the converter and attach it directly to the antenna terminals of the receiver. If there is an increase in picture quality and contrast, the converter is introducing losses. In such an instance it would be preferable to install a separate d.p.d.t. switch. The switch within the converter can also be removed and the u.h.f. antenna coupled directly to the input terminals, thus bypassing the switch losses.

Reception can be improved considerably by using tin foil on the transmission line. This has always been helpful on the upper v.h.f. channels to minimize the effects of standing waves and antenna mismatching. Results for u.h.f. are also excellent. Best results are obtained if two sections of foil are used, as shown in Fig. 1. Such foil is not as effective on the tubular or heavier insulated transmission lines. For best results with such types, add 12 inches of ordinary flat line between the u.h.f. antenna terminals and the u.h.f. transmission line.

A 2-inch section of line works well prior to the converter, while a section from 6 to 8 inches should be used on the line that runs between the converter output and the antenna terminals of the receiver. A slight adjustment of each section of foil for the weakest u.h.f. stations will make a considerable difference in reception.

Posted March 3, 2021