Antenna Topics W
Total Page：16
File Type：pdf, Size：1020Kb
antenna topics W. Clem Small, KR6A, CET The T2FD Antenna Some readers have asked about the termi- should be rated to handle 35 percent of the milk jug folded double. Thread the antenna into nated, tilted, folded dipole (T2FD) or W3HH transmitter's final -stage input power. Carbon the insulator and add the nylon nuts and bolts antenna first described by Countryman in 1949. film resistors are noninductive at HF. High - (your hardware store has them) as indicated in This antenna is moderately nondirectional and, power noninductive resistors are sometimes fig. 1. as it is oriented somewhat vertically, it will available through surplus outlets such as Fair perform well for DX. Being a sloper it requires Radio Sales, 1016 E. Eureka St., Lima, OH 5. Trim the insulation from both ends of the relatively little mounting space compared to many 45802.. antenna for about 1-1/2 in. At each end add an wire antennas. Some reports indicate that it ex- The antenna's length is determined by the insulator (fig.1), twist the wire ends together and hibits gain over a dipole across its bandwidth; formula: solder them to hold the insulator in place. however, other reports (including my own expe- rience) indicate that its performance is somewhat Length (in feet) = 328/lowest operating frequency 6. At one of the middle cuts you made in step 3 below that of a halfwave dipole's within the in MHz remove insulation from both ends ofthe cut wire dipole's bandwidth. On the other hand, the T2FD and solder a 390 -ohm resistor (375 to 400 ohms has a bandwidth that far exceeds that of the Length (in meters) = 100/lowest operating frequency acceptable) between these ends (fig.1). Leave a dipole: about 5 or 6 times its design frequency. in MHz bit of slack in the resistor leads. Perhaps more pertinent to receive -only appli- cations is the fact that broadbandedness in an HF For example, the length of a T2FD cut to 7. Clean the insulation from the ends of the antenna is of more interest when one uses an cover from 7 MHz to 35-42 MHz length would be other middle -cut wire and from the conductors at antenna for transmitting as well as receiving. 46.9 ft. one end of the twinlead lead-in. Thread the lead- This is due to the fact that signal -to -received - in into the insulator as in fig. 1 and add the nylon noise level, rather than simply signal level Let's Make One nut and bolt as shown. Twist and solder the two delivered by the antenna, will determine recep- twinlead wires to the two antenna wires. tion success over most, or usually all, of the HF 1. Determine the length of your antenna from a band as well as for frequencies below HF. Thus, the formula above. Add 2 inches to this length. 8. Attach one end of the antenna to high point even random -length wires can give considerable and the other end to an anchor point nearer the "apparent broadbandedness" in HF and lower - 2. Get enough 300 -ohm TV twinlead to equal ground. Remember the 30° angle you are striving to achieve. frequency receive -only applications. the length found in step l , plus enough to go from The T2FD (fig. 1) should ideally be slanted the antenna to wherever you mount the balun or 30 degrees (20 to 40 degrees acceptable) from the antenna tuner mentioned below. 9. Run the lead-in cable to a balun or to an vertical. At this tilt the antenna is moderately antenna tuner that can handle balanced lead-in. If is to its nondirectional but it does have a number of lobes 3. Cut a piece of twinlead to the length found in a balun used, attach the twinlead lead-in high -impedance terminals. A 4-to -1 ratio balun is and nodes in its directivity pattern, at least when step 1. Find its middle and cut each wire conduc- mounted near a conductive structure. tor there. Don't cut the center part ofthe insulation. correct for 75 ohm coax and a 6 -to -1 ratio balun The resistor (fig. 1) is noninductive and, for (to be covered in next month's column) would be correct for 50 -ohm coax; however, the 4 -to -1 receive -only installations, its power rating can be 4. Make a center insulator as shown in fig. 1 acceptable in applications. Run 1/2 watt or larger; for transmitting installations it from a sheet of plastic. I used the side of a plastic should be most coax from the low -impedance terminals of the balun, or from the tuner, to your rig. RnPE ANTENNA ANTENNA 1 For receive-only HF installations you can INSULATOR usually omit the tuner or balun and still have satisfactory results by connecting one twinlead ANTENNA wire to the center of your antenna input and one to the outer shell of the connector. 10. If you live in lightning country don't forget lightning protection. The minimum here is to disconnect and ground the antenna when it is not irS in use and never use the antenna in weather likely /-I ANTENNA to produce lightning. FEEDLINL Your antenna is now to - -- M'` tPASUI.ATUR_ - ready use. Happy ',._. ï monitoring! ._. -- ROPE.- ti FEEDLINE sTAKb FIG. 1. A 17FD ANTENNA WITH CENTER -INSULATOR DETAIL SHOWN. 110 May 1994 MONITORING TIMES www.americanradiohistory.com.