Home | Site Map | Calendar | e-mail | Links | Meetings

The Modified Slinky Antenna
or the High-Q-Coil
By John Wendt WA6BFH

The above drawing is a schematic symbolic representation of a directly tunable portable multi-wavelength antenna. This antenna by its design should be capable of wavelength band operation on all High Frequency (HF) 3.5 through 28 MHz frequency bands, as well as the Medium Frequency (MF) band of 160 Meters or 1.8 MHz.

The key to the tunability of this antenna is primarily accomplished by use of the coils at the tip ends of the antenna. These coils are fabricated from "Slinky's," the spring-like toy you may remember from your childhood. Another important aspect of this antenna is the use of a somewhat conventional dipole as an "impedance moderator." This moderator works and functions as listed below.

The simple straight wire dipole elements represent a capacitive leaning half-wavelength at 40 Meters. The length of these wires is leaning toward the "C" side of the L/C resonant circuit, because each is only 85, rather than 90 degrees in electrical length, or 30.908 feet in length (for 7.150 MHz). Both halves of the wire elements combined would be 170 electrical degrees in length, or 61.81 feet. We can round out, measure and cut each wire to 31 feet, for a combined length of 62 feet between the inductor coils.

The coils as I said are fabricated from toy Slinky's, and would be supported by and slide along 10 foot lengths of white PVC tubing. The PVC tubing also of course serves as end insulators. The configuration allows the antenna to be readily retuned to any band, by compressing or expanding the coils via fishing line lanyards. After some experience, you may find that you can shorten the insulators by a few feet. The coils can be stretched or compressed by tying off lengths of plastic fishing line to their far ends. It is also recommended to place painted "hash marks" at one foot increments along the PVC pipe, as this will later allow the antenna to be more quickly and easily tuned.

The feed-line utilized may be either 50 Ohm coaxial cable such as RG-58A/U, or Ladder-line twinlead. In either case the feedline is connected to the center of the antenna. Upon wavelength bands where the antenna performs as near a half-wavelength dipole on 40 Meters, or odd order harmonic dimension such as on 15 Meters, the antenna will represent a low impedance close to 50 Ohms. If the reactive impedance of the antenna is too high, such as perhaps on 10 or 20 Meters, the coils will have to be adjusted to conform the antenna to some better harmonic length.

The center insulator of the antenna is fabricated from Plexiglas plate stock. On this are mounted both two Brass binding post screws with wing-nuts, and also an SO-239 coax connector. This configuration easily allows the antenna to be best efficiently used for certain desired types of operation. In most instances, coaxial cable will probably be the preferred feedline!

Parts Needed for the Modified Slinky Antenna:

• 62 feet of #14 or #12 AWG stranded antenna wire
• 2 toy Slinkys
• 2 (10 foot lengths) of ½ inch diameter 40 schedule white PVC pipe
• Approximately 100 feet of 1/8^th inch diameter black nylon camping rope
• 1 small spool of plastic fishing line (to be used as control lanyards for the coil adjustments)
• 1 piece of Plexiglas plate stock 3/8^ths inches thick and 3 inches wide by 5 inches long
• 2 Brass machine screws 3/16^ths inch by 2 inch
• 2 Wing-nuts for above screws
• 4 appropriate size lug terminals for the binding post screws (and also to connect the SO-239)
• 1 SO-239 chassis mount female coaxial connector
• 4 small 6-32 machine screws, nuts, and lock washers to mount the SO-239

Directions:

Drill two ¼ inch diameter holes 1 inch down the long axis of the Plexiglas plate, and ¾ inches from its side edges. These will be to secure the wire antenna elements through.

Drill two 3/16^ths inch diameter holes directly 1.5 inches below these holes, for the Brass screws and wing-nuts.

Drill one 5/8^ths inch diameter hole 1.5 inches blow the 3/16^ths inch holes, and on the long axis center-line of the plate. Insert the SO-239 in this hole, and mark the smaller mounting tab holes for drilling.

Plexiglas center insulator plate, with dimensions for hole centers