The W7GJ "Big Stealth" (B.S.) Antenna - A 13 Element Yagi for 50.1 MHz

The main boom is 2" diameter aluminum irrigation tubing (.050" wall), vertically braced in two places both front and rear with 1/8" aircraft guy cable. Phyllistran 1200 non conducting guy cable is used for side-to-side bracing two places in each the front and rear halves of the antenna.  Additional 8' fiberglass braces later were added out each side from the center of the antenna to provide connections for additional Phyllistran side-to-side bracing lines that ran out to the front-most brace connection position (just to the rear of the second element from the front of the yagi).  It was found that this greater angle was required to keep the front of the antenna from flexing excessively during high winds.

(Where the "Big Stealth" antenna eventually failed, however, was in the rear half of the yagi, right where the PVC center stiffener ended.  Apparently, extremely high winds caused uplift of the rear of the antenna, creating metal fatigue at that point.  The aluminum irrigation tubing used for the Big Stealth boom is not designed to withstand the repeated flexing as well as the 6061T6 aluminum alloy used in commercial antennas.  Big Stealth was replaced in November 1998 by a commercial equivalent - the M Squared model "6M35WL", which is constructed using much sturdier and heavier materials.  The 6M35WL has an overall length of 69.5').

A 40' long piece of irrigation tubing was used from the center of the yagi to the front of the array, and a 30' long piece of irrigation tubing was cut to 28'-4" to form the rear half. A spreadsheet was created to incrementally analyze the projected antenna weight at every inch along its entire length ( in order to select the lengths and locations of PVC plugs, hardware locations, etc.), so the joint would be supported near the center of gravity. The antenna is mounted at the center of gravity, which is only 2" to the rear of the joint between the two pieces of aluminum tubing forming the boom. PVC plugs (made for PVC plumbing pipe) are inserted in both the front and rear ends to close off the irrigation tubing. Each plug adds 1" to the length of the antenna, making the overall length 68.5'.

1.5" PVC schedule 80 high pressure pipe was also used to provide additional strength to the boom where elements holes were drilled. 3" long pieces were inserted and centered at D1-D4 locations, and 6" long pieces were centered at D7-D10 locations. An 8" long piece was positioned under the U bolts holding the driven element mounting plates in place. A 73" long piece was inserted in between elements D5 and D6 (and extending 3" past the holes on either side), not just to provide extra strength where the elements are mounted, but to prevent collapse of the tubing where the 5 central U bolts hold it to the mounting channel.

The elements are 3/8" diameter 6061T6 aluminum tubing (.035" wall), mounted through the boom (insulated with nylon bushings manufactured for bed casters). The driven element is a folded dipole constructed of 1/2" diameter 6061T6 aluminum tubing (.058" wall), and was designed to provide a feed impedance of 75 ohms. The top and bottom of the folded dipole sections are spaced 4" apart (center-to-center), and shorted with 1/2" wide aluminum straps on the ends. The driven element sections are clamped to 1/2" thick HDPE plastic plates, which in turn are bolted above and below the boom using muffler clamp parts. To strengthen the driven element pieces where they were clamped to the plastic mounting plates, 12" long x 3/8" diameter reinforcing inserts were inserted into the middle of the top and bottom of the folded dipole elements (aluminum tubing in the continuous top section, and nylon in the split bottom section).

At the feed, 75 ohm hardline is directly attached (using stainless steel hardware, and completely potted in non-acetic acid silicone caulking) and immediately followed with a quarter wavelength decoupling sleeve (built using 2" irrigation tubing and PVC plumbing fittings for spacers), followed by 8 ferrite toroids (heat shrink wrapped) around the hardline. The antenna is fed with 185' of 75 ohm 3/4" CATV hardline (not including the 25' section of hardline mounted on the boom, between the driven element and the flexible 10' piece of RG-11 around the mast to the top of the tower).

The antenna boom was mounted on a 3.5' long piece of 4" wide structural aluminum channel. This same aluminum channel was used to construct a "box" into which the piece with the antenna on it tightly fit inside. On the rear and bottom of this box, the channel holding the antenna was supported by a pivot assembly (which takes all the weight when the antenna is elevated). The pivoting shaft permits elevation of the antenna up to 50 degrees (and is shown in the photo at an angle of 45 degrees). The pivot assembly is a shaft of 1.5" IPS steel pipe inside a sleeve of 2" IPS schedule 80 steel pipe. This leaves a small space between the two pipes, which was shimmed with a single layer of thin teflon sheet, to serve as a bearing between them. A smaller piece of aluminum channel was clamped onto the pivoting sleeve. The 4" wide piece attached to the antenna boom fits over this shorter and upside-down pivoting section of channel, and the two mating channel sections (one fastened to the antenna boom and the other to the mount) are secured together with two hardened 7/16" through bolts.

Elevation indication is achieved through a weighted shaft on a potentiometer installed in the round film can mounted under the channel holding the boom. The potentiometer is part of a Wheatstone Bridge arrangement, which indicates elevation in the shack. Elevation was incorporated into the antenna for two reasons. The usable window for EME contacts with various stations overseas is greatly expanded by providing elevation at at least one station. Second, the local horizon, especially to the north, is impaired by a 3000' higher mountain ridge peaking 5 miles from the antenna; the effective horizon is 10-15 degrees toward the north, and does not drop down to zero until around 240 and 120 degrees azimuth. A surplus Luxor TVRO actuator easily provides the necessary mechanical force to raise the front of the boom to elevate the antenna.

The stacked 6m halos, more important now that a very directional antenna is being employed most of the time, were lowered down the tower, to be 18' below the boom just out of the pattern so there is no interaction between the antennas.

Big Stealth Element Layout

Reflector 116.2699 0
Driven Element 106.5 34.2202
Director 1 108.5531 73.543
Director 2 105.0389 117.9233
Director 3 105.5161 173.6502
Director 4 104.6083 248.058
Director 5 103.4279 328.3095
Center of Balance and Location of Mount
Director 6 99.5753 396.7612
Director 7 101.3621 460.3414
Director 8 102.1781 543.2365
Director 9 102.1657 643.5047
Director 10 102.8873 738.8214
Director 11 103.8887 820