|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:
|*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
feet of RG-213 coax with the antenna installed at 55 feet:
|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
73's, Rod, K0DAS