The 30 meter band is very narrow and therefore a shortened antenna isn't a great disadvantage. Computer modeling by W6SR, confirmed that an effective 2 element beam on an 11.5 foot boom could be constructed from 20 meter antenna parts. Coils made from aluminum tubing could be placed approximately half way along each side of the elements to achieve resonance.
After the first antenna was built and tuned in place at W6SR's QTH, a second antenna made from commonly available parts was started.
The boom to element clamps were made from .188 thick, 6061-T6, 1.50 X 1.50 inch aluminum angle and .25 thick, 6061-T6, aluminum plate. The elements are insulated from the boom with schedule 40, outdoor, PVC conduit. The 11 inch lengths are split with a .25 inch gap to allow tighting around the angle. The angle is 24 inches long and the plate is 4.50 X 6.00. The plate just needs to be wide enough to allow the two .25-20, stainless steel, clinch studs to clear the 2 inch boom.
The driven element measures approximately 38.5 feet across and 39.5 feet for the reflector.
The ends of the 88" lengths of 1" tubing that are in the center of the driven element are re-inforced with 18" lengths of .875 tubing. The 14" length of .75 fiberglass is placed into each side of the re-inforced tubing so that 2" of fiberglass is showing at the center. Two holes are drilled .5" from the exposed fiberglass on each side to pin the tubing and fiberglass together with a pair of #10-32 stainless steel screws. The screws are long enough to leave enough exposed thread to serve as attachment points for the coax feed. The two 11" PVC pipes from the clamp assembly can now be slipped over each side of the 1" tubing and secured with the hose clamps.
The two 24.5 lengths of .875 tubing are inserted 4" into the exposed ends of the 1" tubing and are secured with three .125 X .25 aluminum pop rivets. Before inserting any of the tubing, the contact surfaces are coated with NOALOX to stop corrosion. The two 20.5" lengths of .875 tubing are joined to the tubing just installed with the 8" lengths of .75 fiberglass. The fiberglass is centered so that 2 inches of fiberglass is exposed. One inch from this exposed fiberglass, a hole for #8 hardware is drilled in the same plane on each side. This will result in exposed threads that will provide a 4" mounting pattern for the 3 inch OD, 10 turn coils. Each coil should be located 110.5 inches from the center of the element and has an inductive reactance of 165 ohms.
The 43.375" lengths of .75" tubing are inserted 4" into each end of the .875 tubing and are secured with three .125 X .25 aluminum pop rivets. The two 33" lenghts of .625" tubing is inserted 3" into each end of the .75 tubing and secured with three .125 X .25 aluminum pop rivets. The two 30" lengths of .50 tubing are inserted into the .625 tubing to achieve the desired overall length of the element. The .625 tubing needs to be slotted to alow a hose clamp to tighten the end sections.
The process is repeated for the reflector except for the center, where the 12" length of .75 tubing replaces the fiberglass and thus electrically connecting the two ends.
TUNING
On a shortened beam, the tuning is very critical. With the driven element three feet off the ground, it will resonate about 9.9 MHz. When the element is elevated to about 25 feet, it resonates about 10.05 MHz. At 50 feet it will be within the band. The addition of a reflector will also raise the resonant frequency so it should be adjusted at its full hight. A very close approximation can be made by standing the beam up vertically on the reflector and testing the resonant frequency. Once the overall driven element length is known, the reflector is made 1 foot longer. This is a close starting point. As the reflector length is reduced in small increments, the SWR will approach 50 ohms. The length is too short when there is a rapid increase in SWR. The reflector should be set to a length just before this increase starts. This characteristic was predicted when the proposed antenna was modeled on the computer.
The first beam was installed and tested on the W-51 tower of W6SR. The elements are about the same length as the 20 meter elements of the C3 above it. In comparisons with a rotary dipole at 85 feet on the adjacent tower, the beam at 51 feet outperformed the dipole and had much less noise as well as good front to back. This is especially important on 30 meters because of the common multipath conditions. This antenna is now shown on N6JV's W-51 tower at 52 feet.
Element inventory:
(All element aluminum is 6061-T6)
2 X 1.00" OD, 88" long
2 X .875" OD, 18" long
2 X .875" OD, 24.5" long
2 X .875" OD, 20.5" long
2 X .75" OD, 43.375" long
1 X .75" OD, 12" long (Reflector only)
2 X .625" OD, 33" long
2 X .50" OD, 30" long
1 X .75" OD, 14" long (fiberglass rod) (Driven element only)
2 X .75" OD, 8" long (fiberglass rod)
2 X .25" OD, 10 turn coils, 4" long, 3" OD made from 3003-0 tubing
2 X .50" OD, hose clamps
12 X .125 X .25 all auminum pop rivets
2 X #10-32 X 1.75" SS screws
4 X #10-32 SS nuts
4 X #10-32 SS washers
4 X #10-32 SS lock washers
4 X #8-32 X 1.75" SS screws
8 X #8-32 SS nuts
8 X #8-32 SS washers
8 X #8-32 SS lock washers
SOURCES OF SUPPLY:
These are approximate costs for materials in Sacramento
TUBING - - -Blue Collar Supply = BC
1.00 OD, .050 wall, 6061-T6 $8.69/8 feet
.875 OD, .058 wall, 6061-T6 $7.53/8 feet
.75 OD, .065 wall, 6061-T6 $6.79/8 feet
.625 OD, .058 wall, 6061-T6 $5.25/8 feet
.50 OD, .055 wall, 6061-T6 $4.52/8 feet
.188 thick, 6061-T6, 1.50 X 1.50 inch aluminum angle, $23.50/8 feet from BC
.25-20 X .875 clinch studs $.50 each from Olander Fasteners
.25 aluminum plate, 6061-T6, $2.50 per pound from BC
.25 OD, 3003-0 soft tubing, $33.73/50 foot coil from McMaster-Carr (5177K25) or OSH Hardware
Hose clamps (large) $.50 each from BC
Hose clamps (small) $.25 each from BC (better quality at Kragans)
.75 OD fiberglass solid rod, $15.55/6 feet from Tap Plastic
1 inch ID Schedule 40 sunlight resistant plastic conduit, $2.67/10 feet from Lowe's or Home Depot
NOALOX Antioxidant joint compound available at Home Depot
.125 X .25 all auminum pop rivets available from ACE Hardware or Olander Fasteners