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

6 METERS
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

17 METERS
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.


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