40 METER DIPOLE



The 30 meter beam worked so well that a new 40 meter shortened dipole was built. The center clamp was the same as on the 30 meter beam. Coils made from aluminum tubing could be placed approximately half way along each side of the elements to achieve resonance.



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 12 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 dipole is approximately 42 feet across.

The ends of the 4' lengths of 1" tubing that are in the center of each side of the dipole are re-inforced with 8' lengths of .875 tubing. Three .125 pop rivets secure the 1" and .875" lengths together at the end of the 1" material. The 10" 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 12" PVC pipes from the clamp assembly can now be slipped over each side of the 1" tubing and secured with the hose clamps.

Before inserting any of the tubing, the contact surfaces are coated with NOALOX to stop corrosion. The two 1 foot 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. 1.50 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 5" mounting pattern for the 15 turn coils. Each coil should be located 98 inches from the center of the element and has an inductive reactance of 400 ohms.

The 40" 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 40" lengths of .625" tubing is inserted 4" 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. The .625 tubing needs to be slotted to allow a hose clamp to tighten the end sections. The 48" lengths of .375 tubing are inserted 6" into each end of the .50 tubing and are secured with two .125 X .25 aluminum pop rivets.


TUNING


At about 25 feet and in the clear, the dipole will be approximately 6.990 MHz at resonance. At 65 feet the resonance should be about 7.050. The adjustable section with the hose clamp can make small adjustments. Once the antenna is adjusted to the desired frequency, it is fed with a quarter wave matching line made from paralleled runs of RG/59 coax. This results in a SWR of about 1.5:1. A quarter wave length of coax is equal to (246 X VF)/freq (MHz).


The dipole was placed at 65 feet just below the 30 meter beam. All of the other antennas were checked for interference and none was found. The old KLM linear loaded dipole caused the old tribander to change SWR especially on 15 meters.


Element inventory:

(All element aluminum is 6061-T6)

2 X 1.00" OD, 48" long

2 X .875" OD, 96" long

2 X .875" OD, 12" long

2 X .75" OD, 40" long

2 X .625" OD, 40" long

2 X .50" OD, 30" long

2 X .375" OD, 48" long

3 X .75" OD, 8" long (fiberglass rod)

2 X .25" OD, 15 turn coils, 5" long, 3.5" OD made from 3003-0 tubing

2 X .50" OD, hose clamps

12 X .125 X .25 all aluminum 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 aluminum pop rivets available from ACE Hardware or Olander Fasteners




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