Friday, March 4, 2011

The W8NX trap dipole antenna


This antenna was designed for amateurs with limited space who also wanted to operate the low bands.It was first described in July 1992 QST by A. C. Buxton, W8NX, and features innovative coaxial-cable traps.

Fig  shows the antenna layout; it is resonant at 1.865, 3.825, and 7.225 MHz. The antenna is made of #14 stranded wire and two pairs of coaxial traps. Construction is conventional in most respects, except for the high inductance-to-capacitance (L/C) ratio that results from the unique trap construction.

The traps use two series-connected coil layers, wound in the same direction using RG-58 coaxial cable’s center conductor, together with the insulation over the center conductor. The black outer jacket from the cable is stripped and discarded. The shield braid is also removed from the cable (pushing is easier than pulling the shield off). No doubt you will want to save the braid for use in other projects. RG-58 with a stranded center conductor is best for this project.

The 7-MHz traps have 33 µH of inductance and 15 pF of capacitance, and the 3.8-MHz traps have 74 µH of inductance and 24 pF of capacitance. The trap Qs are over 170 at their design frequencies. These traps are suitable for high-power operation. Do not use RG-8X or any other foam-dielectric cable for making the traps. Breakdown voltage is less for foam dielectric, and the center conductor tends to migrate through the foam when there is a short turn radius. Loading caused by the traps causes a reduced bandwidth for any trap dipole compared to a half-wave dipole. This antenna covers 65 kHz of 160 m, 75 kHz of 80 m, and the entire 40-m band with less 2:1 SWR.

Quarter-wave vertical antenna for VHF


Quarter-wave vertical antennas are useful for local communications when size, cost and ease of  construction are important.

The antennas shown are built on a coaxial connector. Use UHF or N connectors for the fixed station antennas. BNC connectors are good for mobile and portable antennas. BNC
and N connectors are better than PL-259 connectors for VHF/UHF outdoor use because: (1) they provide a constant impedance over the frequencies of interest, and

(2) they are weatherproof when the appro priate connector or cap is attached. The ground-plane antennas require a panel-mount connector (it hasmounting holes to hold the radials).
If the antenna is sheltered from weather, copper wire is sufficiently rigid for the element and radials.

Antennas exposed to the weather should be made from 1/16- to 1/8-inch brass or stainless-steel rod.Radials may be made from 3/16-inch aluminum rod or tubing and mounted on an aluminum sheet. Do not use aluminum for the antenna elementbecause it cannot be easily soldered to the coaxial-connector center pin.Where the figures call for #4-40 hardware, stainless steel or brass is best. Use cadmium plated hardware if stainless steel or brass is not available.


Off-Center-Fed (OCF) dipole for 3.5, 7 and 14 MHz


Fig  shows the off-center-fed or OCF dipole. It is not necessary to feed a dipole antenna at its center, although doing so will allow it to be operated with a relatively low feed-point impedance on its fundamental and odd harmonics. (For example, a 7-MHz center-fed half-wave dipole can also be used for 21-MHz operation.) By contrast, the OCF dipole of Fig, fed  1/3 of its length from one end, may be used on its fundamental and even harmonics. Its free-space antenna-terminal impedance at 3.5, 7 and 14 MHz is on the order of 150 to 200 Ω. A 1:4 step-up transformer at the feed point should offer a reasonably good match to 50 or 75-Ω line, although some commercially made OCF dipoles use a 1:6 transformer.   At the 6th harmonic, 21 MHz, the antenna is three wavelengths long and fed at a voltage loop (maximum), instead of a current loop. The feed-point impedance at this frequency is high, a few thousand ohms, so the antenna is unsuitable for use on this band.

10M Rectangular Loop Antenna.


With the large number of operators and wide availability of inexpensive, single-band radios, the 10-m band could well become the hangout for local ragchewers that it was before the advent of 2-m FM, even at a low point in the solar cycle.

This simple antenna provides gain over a dipole or inverted V. It is a resonant loop with a particular shape. It provides 2.1 dB gain over a dipole at low radiation angles when mounted
well above ground. The antenna is simple to feed— no matching network is necessary. When fed with 50-Ω coax, the SWR is close to 1:1 at the design frequency, and is less than 2:1 from 28.0-28.8 MHz for an antenna resonant at 28.4 MHz.

The antenna is made from #12 AWG wire and is fed at the center of the bottom wire. Coil the coax into a few turns near the feedpoint to provide a simple balun . A coil diameter of about a foot will work fine. You can support the antenna on a mast with spreaders made of bamboo, fiber glass, wood, PVC or other non conducting material. You can also use aluminum tubing both for support and conductors, but you’ll have to readjust the antenna dimensions for resonance.

This rectangular loop has two advantages over a resonant square loop. First, a square loop has just 1.1 dB gain over a dipole. This is a power increase of only 29%. Second, the input
impedance of a square loop is about 125 W. You must use a matching network to feed a square loop with 50-Ω coax. The rectangular loop achieves gain by compressing its radiation pattern in the elevation plane. The azimuth plane pattern is slightly wider than that of a dipole (it’s about the same as that of an inverted V). A broad pattern is an advantage for a general-purpose, fixed antenna. The rectangular loop provides a bidirectional gain over a broad azimuth region.

Mount the loop as high as possible. To provide 1.7 dB gain at low angles over an inverted V, the top wire must be at least 30 ft high. The loop will work at lower heights, but its gain advantage disappears. For example, at 20 ft the loop provides the same gain at low angles as an inverted V.

40M and 15M Dipole antenna


Two popular ham bands, especially for Novice and Technician class operators, are those at 7 and 21 MHz. As mentioned earlier, dipoles have harmonic resonances at odd multiples of their fundamental resonances. Because 21 MHz is the third harmonic of 7 MHz, 7-MHz dipoles are harmonically resonant in the popular ham band at 21 MHz. This is attractive because it allows you to install a 40-m dipole, feed it with coax, and use it without an antenna tuner on both 40 and 15 m.

To put this scheme to use, first measure, cut and adjust the dipole to resonance at the desired 40-m frequency. Then, cut two 2-ft-long pieces of stiff wire (such as #12 or #14 house wire) and solder the ends of each one together to form two loops. Twist the loops in the middle to form figure-8s, and strip and solder the wires where they cross. Install these capacitance hats on the dipole by stripping the antenna wire (if necessary) and soldering the hats to the dipole about a third of the way out from the feedpoint (placement isn’t critical) on each wire.

W5LAN's 2-meter mobile antenna


Brass tubing is available in some hardware and hobby stores. It comes in sizes from 1116 to 21/32 inch outside diameter(OD), in l/32 inch steps. Each size slip fits within the next larger size. It is usually sold in 12- or 36-inch lengths.The antenna is made from two 12-inch lengths of 5/32-inch tubing and two 12-inch lengths of 118-inch tubing. A V-shaped horizontal dipole is formed when the tubes are mounted through a short piece (6 inch or so) of 7/8-inch OD plastic pipe (see Fig). It is V shaped to reduce the overall size and provide a better match to 50 R coax.

Begin by drilling two 5/32-inch holes through the plastic pipe at right angles to each other. (Position one hole slightly below the other so that the dipole elements cross inside the plastic pipe without touching.) Enlarge the holes of two solder lug and force each over one end of the S/32-inch tubes and solder them in place.

Push the other end of those tubes through the holes in the plastic pipe until the solder lugs are flush against the pipe. Strip the end of a length of coax, then solder the braid to one solder lug and the center conductor to the other. Use sealant to weatherproof the coax end and feed point.

The antenna is adjusted to resonance by sliding the 118-inch tubing in and out of the larger tubing to achieve minimum SWR. If the fit is too loose, nick the end of the smaller tube slightly with diagonal cutters, and force it into the larger tubing. After performing the adjustment, cut the smaller tube to a length that leaves about an inch inside the larger tube and solder it to the larger tube. The element lengths on my antenna are about 20.5 inches each, and the SWR was near unity over most of the 2-meter band, with a slight rise at the high frequency end.