This antenna is especially interesting now that high-band propagation conditions are improving.
The K7EFX Multiband "3-Half-Wave 6-Meter,"
"Half-Wave 17-Meter," Rotatable Antenna
Author: John A. Brackemyre, K7EFX, Stockton, California © June 1, 2010
 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. All tubes 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. |
SWR |
R (Ohms) |
X (Ohms) |
| 50.000 MHz |
1.6:1 |
25 |
0 |
| 50.257 MHz |
1.5:1 |
26 |
0 |
| 50.408 MHz |
1.4:1 |
27 |
0 |
| 50.875 MHz |
1.3:1 |
28 |
0 |
| 50.992 MHz |
1.4:1 |
29 |
0 |
| 51.612 MHz |
1.5:1 |
30 |
0 |
| 51.920 MHz |
1.6:1 |
33 |
9 |
| 52.153 MHz |
1.7:1 |
35 |
12 |
| 52.395 MHz |
1.8:1 |
38 |
18 |
| 52.680 MHz |
1.9:1 |
40 |
20 |
| 52.765 MHz |
2.0:1 |
44 |
22 |
17 METERS
| Freq. |
SWR |
R (Ohms) |
X (Ohms) |
| 18.000 MHz |
1.9:1 |
42 |
22 |
| 18.036 MHz |
2.0:1 |
44 |
15 |
| 18.073 MHz |
2.1:1 |
45 |
16 |
| 18.173 MHz |
2.2:1 |
45 |
17 |
| 18.278 MHz |
2.3:1 |
43 |
17 |
| 18.464 MHz |
2.4:1 |
42 |
8 |
| 18.717 MHz |
2.5:1 |
47 |
9 |
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 M2 beam 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
Technical Forum.
This page was last modified: Sat, 04 Sep 2010 22:14:07 GMT
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