Thursday, June 23, 2011

Dimensions for a Fixed 3el and a 7el 40M WIRE Yagi

Brent was interested in dimensions for a 40M Yagi, so I figured I might as well post mine in the antenna section for future use.

These dimensions are for semi-inverted vee elements. The els are shortened about 2.7% as per my experiments. (cut for 7275 kc, about 150kc higher, but work on 7150) These are computer generated beams that have been adjusted to the real world. The height of the element apexes are about 80' and the ends about 60'-65'. Average height at 70' per element - a good 1/2 wavelength above ground for best sweet spot pattern and best high angle suppression.

The 7el Yagi has an input impedance of about 25-30 ohms, so will need a match of some kind. Look up my techniques for using coaxial ununs to go from 25: 50 ohms or 50 ohms to 75 ohms, etc. A coil of about 6 turns of coax at the feedpoint on a 4" ABS tube is for the RF choke. Use the tube as the center insulator. The 3el is about 50 ohms for a broader coverage of the band so a direct coax match using the RF choke is FB.

#10 or #12 wire OK - Use a steel cable for the boom - use a separate rope and pulley for each element for easy access, for tuning, if desired:

Dimensions for 3 element 40M wire Yagi in inches :

El Position on boom 1/2 Length of el (total length is X2)

Ref 0 406"
Dr 360" 392"
D1 720" 360"

Total boom length: 60'

7 Element 40M wire Yagi in inches:

El Position on boom 1/2 Length of el (total length is X2)

Ref 0 398"
Dr 305" 388"
D1 601" 375"
D2 876" 369"
D3 1213" 375"
D4 1550" 360"
D5 1887" 350"

Total boom length: 157'

The above dimensions are for inverted vees. If using flat elements, then increase all element lengths by 2.7%. Keep positions on boom the same. Use a reference dipole to A/B compare the f-b and if desired, move the WHOLE beam's element lengths up or down the band as a unit. About 5" per side/leg is a good change, but do them ALL at the same time. The model is optimized and will vary according to height above ground and other factors. You need to fine tune the eles as a unit on your favorite portion of the band (like 7160) to acheive 30+db f-b numbers or else settle for less right out of the box.

No comments: