Antenna Design
This antenna is especially interesting with improving high-band conditions.

The K7EFX Multiband "3-Half-Wave 6-Meter," "Half-Wave 17-Meter," Rotatable Antenna

Author: John A. Brackemyre, K7EFX, Stockton, California © June 1, 2010

K7EFX Multiband Antenna Photos

Design Scope
The purpose of this build was to replace my 12/6-Meter inverted-vee with a rotatable antenna. Being a Special Effects designer and machinist I had all of the parts so I designed around what was in my collection of tubing, clamps and hardware.

Using the 468/F MHz half-wave antenna formula I calculated the 6-meter half-wave length and multiplied by 3 to arrive at the total length for 3 half-waves on 6-meters and a half-wave on 17 meters. I had black plastic tube clamps for one-inch tubing and also two feet of 1-1/8 inch fiberglass tubing (greenish tubing that I used inside for structural support). Inside each driven element clamp is a one-foot piece of the fiberglass tubing. A length of 1-1/8 inch .060 aluminum was put over that and sandwiched between the center and outer insulator clamps. (See the photos)

The outer clamps have a one-inch aluminum tube inside which is inserted inside of the 1-1/8 inch center tube. A sheet metal screw makes electrical connection about 6-inches from the center. Alltubes are inserted at least eight-diameters for strength. All tubes are hacksaw-slitted in two directions and debured inside, so hose clamps can tighten and held them in place. All tubing has a light coating of Copper Shield high-current conductive copper paste.

See photos of the RG-58 50-ohm feed-line connecting to the copper 6-32 brass studs and nuts. I only had to lengthen the outer 5/8-inch diameter tube one time after erection to resonate the antenna at the 6-Meter design center frequency. Initial tuning was done on the ground with a 6-foot piece of PVC supporting pipe stuck in a picnic table umbrella stand as a supporting mast and with a 50-length of coaxial feed-line.

The 6-Meter design center frequency was 51 MHz. The calculated half-wavelength at 51 MHz was 9.176 feet, so the calculated length for three half-wavelengths was 27.529 feet. The finished tuned and measured element length is 27.958 feet, which is only slightly longer. At the 51 MHz center frequency the VSWR is 1.3:1 and the feed resistance is 29 Ohms with no reactance.

Analyzer results

Freq.SWRR (Ohms)X (Ohms)
50.000 MHz1.6:1250
50.257 MHz1.5:1260
50.408 MHz1.4:1270
50.875 MHz1.3:1280
50.992 MHz1.4:1290
51.612 MHz1.5:1300
51.920 MHz1.6:1339
52.153 MHz1.7:13512
52.395 MHz1.8:13818
52.680 MHz1.9:14020
52.765 MHz2.0:14422

Freq.SWRR (Ohms)X (Ohms)
18.000 MHz1.9:14222
18.036 MHz2.0:14415
18.073 MHz2.1:145 16
18.173 MHz2.2:14517
18.278 MHz2.3:14317
18.464 MHz2.4:1 428
18.717 MHz2.5:1479

On-Air Results
I received an honest S8 report from W9IMS, the Indianapolis Motor Speedway Special Events station Friday, May 21, 2010 at 5:45 PM PDST on 18.140 MHz (17-Meters). The operator was Bill.

Saturday, I worked VE7DAY on Vancouver Island in British Columbia on 6-Meters. He was using a 7-element M2beam at 50-feet to my three-halfwave rotatable dipole at 30-feet. Signals were S8 to S9 both directions with 100-watts of RF output on both ends.

If there is a lot of interest I can prepare build-able drawings, a list of components, vendors, build notes, and tuning instructions.

Discuss, debate and ask questions about multiband antennas in the Ham Radio Forum.

Search other ham radio sites with Ham Radio Search