The 10-m "Bionic Death Ray" by K0DAS

This antenna seemed to be really "hot" in the 1999 10-m contest, so I thought it was only just to present it to the world on my "ham antenna page"--  W0FMS.  [HTML formatting by W0FMS]

Here is the yagi design data:
Boom diameter is 2-in.
Element diameter is 1-in. with end pieces of 7/8 in to adjust length by sliding inside the 1 in main tube.
Boom-to-element clamp is a plate with two U-bolts about 3-in apart holding the elements down.

Element Lengths and Location from the rear are in inches:
Element Length: Element Location:
Reflector: 206.0 inches 0.0 inches
Driven element: 208.0 inches* 43.0 inches
Director #1: 191.0 inches 95.5 inches
Director #2: 186.5 inches 201.5 inches
Director #3: 179.0 inches 300.0 inches
*Note:  DE is longer than Reflector.  This is ok as it centered VSWR better when the folded dipole match was in place.  The computer initially said DE should be 198.0-inches.  We stuck a 5-in extension on the ends of the DE to center the SWR curve at about 28.5 MHz.

Folded dipole DE is constructed using AWG #22 wire spaced 3-inched from the EDGE of the DE tubing (3.5-in center-to-center).  It attached to the DE tubing with an aluminum bar 0.25x0.50x4.5 inches.  One end of bar clamped to the DE tubing with a single U-bolt.  A tapped hole held a screw and the wire on the other end.  The bar is clamped 5-inches from the end of the DE.  Six stand-off insulators were made of 1/2 in teflon bar stock.  One end drilled to pass the wire.  The whole affair secured to the DE tube with high quality electrical tape.  Duct Tape is an acceptable substitute. [If the women can't find you handsome, at least they can find you handy!]   For a permanent installation, I recommend something better than tape.

The two ends of the wires are fed in the center with a 4:1 transmission line balun constructed with a 12 ft. 1 in. length of RG-142 teflon insulated coax cable.  For this cable type, 12 ft. 1 in. is a half wavelength at 28.5 MHz.

Connect it per the antenna handbooks to give you a balanced 4:1 impedance transformer.  I checked this one with a 200 Ohm resistor and an impedance noise bridge and it is right on the button.  I used large ceramic posts on a plate mounted on the DE center to hold the wire ends, the balun ends and a type N coax connector.
One more tip:  Years ago, I helped a ham put up a similar HB yagi and after it was up we noticed the elements would "sing" in the breeze.  They would vibrate in a mechanical resonance at certain wind speeds.  About 6 months later, he found half an element laying in his yard - broken off due to metal fatigue.  The solution? - we re-built the yagi and put a piece of rope inside each element tube to dampen the resonance.

This yagi exhibits the same problem.  On Death Ray, Sr. [7-ele 40 foot boom!] we slipped 1 foot pieces of foam pipe insulation on the ends.  It made the antenna look like an elaborate trap tri-bander and did nothing to stop the vibration.  Looks like the rope trick (no pun intended) is the best solution.

Here is the SWR data measured on a Bird 43 on the end of approximately 85
feet of RG-213 coax with the antenna installed at 55 feet:
28.0 MHz SWR 2.0:1
28.2 MHz SWR 1.5:1
28.4 MHz SWR 1.2:1
28.5 MHz SWR 1.2:1
28.6 MHz SWR 1.2:1
28.8 MHz SWR 1.5:1
29.0 MHz SWR 1.8:1

Another tip:  Fred, W0FMS loaned me his Antenna noise bridge made by Palomar Engineering.  It is a very simple but very effective method of matching the antenna.  It took a lot of the guess work out of the process I had been using with a directional coupler.  [W0FMS note:  Rod used it correctly-- note that for the 200 Ohm impedance he was matching he first "calibrated" the noise bridge using the external resistance for comparison.  As he noted during the contest, the calibration of the noise bridge isn't perfect, and it is best used for these types of relative readings.]
And a final tip:  I recommend lashing a newly minted beam to a pole (or the side of your tower) pointed skyward.  Get the reflector off the ground at least 4 feet (on 10 meters).  This method gives the least interaction with the ground and yet makes the driven element accessable for adjustments for matching.

73's, Rod, K0DAS

Last updated: 14 December 1999, FMS