TH-3JRS 3 lement 3 Band Yagi

hy-gain TH-3JRS is used for 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 SEA 1630 antenna tuner. The hy-gain TH-3JRS 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. 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. 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. The sweeps to the right are with the antenna just above the tripod. 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 phone portion of each band to optimize performance for SSB. Optimization for SSB was chosen over CW since CW can be copied with considerably less SNR and the antenna tuner will load into almost anything, so SWR is not that critical.


The SWR sweeps at the right were done with the antenna at full height (about 13 feet above peak of roof). On 20 meters, the minimum SWR frequency moved from a bit above the band to 14.160 MHz, close to the CW/phone boundary. On 15m, the minimum SWR frequency moved from 21.4 MHz to 21.35 MHz, about the middle of the 15m phone sub-band. On 10m, the minimum SWR frequency moved from 28.534 MHz to 28.5 MHz, towards the bottom of the 10m phone sub-band. The SWR over the entirety of each band is well within the capabilities of the antenna tuner.


Performance

Preliminary tests using the Reverse Beacon Network show a gain of about 6 dB over the existing inverted V antenna on 20 meters. The height above ground and height above the roof are clearly not optimum, but are about the best that can be done at this location. There is a fairly large evaporative cooler on the roof (visible in the second photo) that is probably having the most effect on the radiation pattern.

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.

TH-3JRS Assembly Notes

I recommend doing a complete inventory of parts before starting assembly. I received two of the director boom pieces and no reflector boom piece. Also, the driven element to boom bracket was the wrong one (stamped #4 instead of #1). hy-gain quickly shipped replacement parts.

The phone number listed in the manual is out of date. The correct number is +1 662 323 9538.

Dimensions in figure 4 are quite unclear. A better copy of figure 4 is available here. Even this is not extremely clear. You can get US measurements by dividing the millimeter distances (right side of page) by 25.4 . The millimeter measurements are often more clear.

When assembling the antenna, make sure the trap adjustment holes are on the bottom of the trap so no water can get in (and any that does can drain). This IS included as the very last instruction, but it would be helpful to know this as traps are placed so they do not need to be rotated later.