Cushcraft A-3S 3 element 3 Band Yagi

The Cushcraft A-3S triband beam with the A-743 kit covers 20, 15, and 10 meters as a 3 element Yagi and 40 meters as a rotatable dipole (short loaded dipole with capacitive hats). It was installed in July 2019 to replace the Hy Gain TH#-JRS.

A Cushcraft A-3S with A-743 is used on 40, 20, 15, and 10 meters. A Comtek COM-BAL-11150TYB balun is mounted on the boom. The balun is driven by 75 feet of RG-8U from an MFJ 4712 antenna switch after the Dentron Clipperton-L linear amplifier. The Cushcraft A-3S is supported by a Rohn TRT-60 tripod, a 10 foot mast, an old Channel Master TV antenna rotator, and another two foot mast section. The mast sections have an outside diameter of 1.25 inches and a wall thickness of about .085 inches. Guy ropes are run from the rotator down to the tripod base to provide additional support with wind.
The tripod sits on 2x10 inch x 4 foot long pressure treated wood planks. The tripd supports masts up to 1.75 inches in diameter. Duplicate pieces are under the rafters in the attic crawl space. 3/8 inch by 10 inch carriage bolts go through the planks beneath the rafters, through the roof, through the planks on the roof, then through the tripod leg mounting plates. Downward force by the tripod is spread between the rafters instead of being applied to the roof sheets. Upward force by the tripod similarly is spread between the rafters instead of pulling on the roofing sheet. The planks on the roof are liberally coated with roofing tar to prevent leaks down the carriage bolts. The crawl space was not a fun place to work! The planks in the crawl space were attached to the rafters using wood screws, then holes drilled through the outside planks and the roof. A 1/4 inch diameter 12 inch long drill was then used to drill from the roof through the inside planks. 3/8 inch holes were then drilled up through the inside planks using the 1/4 inch holes as pilot holes.
SWR sweeps were run on the antenna on each band. All SWR sweeps are done with 75 feet of RG-8U between the antenna analyzer and the antenna. Antenna element lengths were set for minimum SWR in the mid portion of each band as defined in the manual. For each band, the plot shows the frequency and SWR at the frequency with the minimum SWR (mSWR). The second marker is pretty much randomly set near the center of the band. The first image for each band is from August 2019. The second image for the band is from November 13, 2019, after replacing a burned out 20 meter trap (Cushcraft TK).


Effect of ice on A-3S
A pretty heavy coat of ice has shifted the resonant frequency from 7.125 MHz to 6.962 MHz. Ideally this will shift back when it warms up! However, you can see the end of the 20 meter trap near the capacitive hat is now dark black! New trap on order!
Burned Out Trap
Here are some photos of the burned out Cushcraft TK (20 meter) trap. The trap consists of a coil with a metal shell over it. The shell is connected to the element on the "boom side" of the trap (the arrow on the cover points towards the boom) with a sheet metal screw. The shell is centered over the inductor with a couple white plastic circular spacers. The shell and the inductor form a capacitor. This, along with the inductance, forms a parallel resonant circuit. There is high voltage between the shell and the end of the inductor away from the boom. With the antenna coated with ice, it appears this voltage was higher than normal and arced over the white plastic circular spacer.






Antenna Inspection 2/15/20

On 2/15/20, the A3S was disassembled, all traps checked, and the antenna reassembled and tuned to mid-band. Notes are here.

Antenna Tuning of 3/14/20

On 3/14/20 (pi day!), the A3S was adjusted for minimum VSWR. Notes are here.

Current SWR Sweeps

A3S Repair of 1/16/21

On 1/14/21, the VSWR of the antenna went very high. The repair is documented here.

Building Permits

The city of Arvada CO Code of Ordinances section 5.2.38 regulates "Wireless Communications Facilities". Part B1 exempts amateur radio antennae if the height is no more than the distance from the base of the antenna to the property line. Clarification from the city indicates that the "height" includes the height of the roof on roof mount antennae. Initially, the city said a building permit would be required and provided information on wind and snow load for my structural engineer. I explained that hiring a structural engineer would probably cost many times the cost of the antenna. Further, a review of building permits issued in the past 20 years showed only one amateur radio permit application going all the way through inspection and only 3 permits issued. On the one going through inspection, only the height was checked in the inspection. The city asked for more information on the antenna. I sent the antenna specification sheet along with the tripod specifications and a description of how this was to be assembled (10 foot mast, etc.). The city answered back that this could be classified as a TV antenna and no permit was required.