TARA NEWS - March, 1998 Page 3


Fig 1.00—A simple groundplane antenna for the 144, 222 and 440-MHz bands. The feed line and connector are inside the mast, and a hose clamp squeezes the slotted mast end to tightly grip the plug body.

Easy to Homebrew 1/4-l Ground Planes

The ground-plane antenna in Fig 1.00 uses female chassis-mount connectors to support the element and four radials. If you have chosen large-diameter wire or tubing for the radials refer to Fig 1.02. Cut a metal sheet as shown (size is not critical, and the mounting tab is optional). Drill the sheet to accept the coaxial connector on hand (usually 11/16 inch) and the 4-40 hardware for the radials and connector. Bend the plate or radials as shown with the aid of a bench vise. Mount the coaxial connector and radials to the plate.

Small diameter (1/16-inch) radials may be attached directly to the mounting lugs of the coaxial connector with 4-40 hardware. To install 3/32- or 1/8-inch radials, bend a hook at one end of each radial for insertion through the connector lug. (You may need to enlarge the lug holes slightly for 1/8-inch rod.) Solder the radials (and hardware, if used) to the connector using a large soldering iron or propane torch.

Solder the element to the center pin of the connector. If the element does not fit inside the solder cup, use a short section of brass tubing as a coupler (a slotted 1/8-inch-ID tube will fit over an SO-239 or N-receptacle center pin).

Once the antenna is mounted and tested, thoroughly seal the open side of the coaxial connector with RTV sealant, and weatherproof the connections with rust-preventative paint.

Length: 144MHz = 19-1/4" - 222Mhz = 12-1/2" - 440Mhz = 6-5/16"

where l = length, in feet - f = frequency, in MHz

Fig 1.02—Methods of mounting 3/16-inch aluminum-rod radials to a VHF/UHF groundplane antenna. At A and B the radials are made approximately 1-1/2 inches longer than l, then bent (45) and attached to a flat aluminum sheet. At C and D, the radials are somewhat shorter than l, and the corners of the aluminum sheet are bent to provide the 45 angle. In both cases, l is measured from the radial tip to the element. The size of the aluminum sheet is not critical. The mounting tab shown at D is optional; it could be added to the sheet in A and B if desired.