Sunday, June 14, 2009

Tarheel 200A-HP Antenna: Installation & Initial Tests


Issue One: How do I mount this thing? My first thought was to hammer a 1" galvanized steel pipe into the ground. Then I remembered this heavy duty speaker stand that I scavenged out of the dumpster at the music store a few years ago. It's one of those stands that is designed to elevate a PA monitor; the handle normally used to raise and lower the cabinet to the desired height was broken off but it was otherwise in good shape. I had no idea what I was going to do with it but I figured someday I would adapt it to one of my hobbies -- maybe a telescope mount or a camera tripod. It sat around in my boiler room in NJ for years, then almost got put in the trash when we moved but at the last minute I threw it into the U-Haul. I'm glad I did.

It never occurred to me to use it as an antenna mount until the HamCom show where I saw a similar setup at the Buddipole booth. That rang a bell in my head, so I dug it out of the garage Saturday night. The upper mast is 1", exactly the size needed for the MT-1 mounting bracket and ground radial plate. And though the mast cannot be raised or lowered from it's broken position, it turned out to be the perfect height to mount the MT-1 with the ground plate just a few inches below. This puts the plate about 3.5" above the ground, which means the ten supplied 9' radials droop down before extending outward.

The antenna stands over 15' tall when fully extended for 75/80m. I'm a little concerned about the top-heaviness of the whole thing; wish the mounting bracket U-bolts would fit around the lower section so I could remove the gear assembly and top mast to get the center of gravity closer to the Earth's core. It's not going to tip over all by itself, it's fairly sturdy, but I fear a good blast of Texas-style sever weather might topple it. I'm considering some sandbags on the tripod legs to hold it in place. At least it's portable enough to move indoors or lay down if the wind picks up. While not an ideal permanent mounting solution, it works quite well as a temporary/emergency/portable/field day antenna mounting system. I may eventually move to the pipe-in-ground method; for now, however, this gets me on the air.

While mounting the antenna to the MT-1 bracket I had a bitch of a time fastening the upper half-bracket. Even threading the lock nut on one side just enough to grab the screw, I could not get the screw on the other side through the bracket enough to fasten the other nut. I tried numerous times until the nut got dropped and went bouncing off into the tall grass. I finally just fully tightened the one side and left it for another day -- it's not going to be going down a highway at 80MPH so it's good enough for now. I'd ultimately like to adapt some sort of cam-lever quick release for this upper bracket to facilitate quick break down.

Assembly completed, I did some initial testing with SWR analyzer and found the 200A to tune well on most of the bands I care about -- 1.2:1 or better on 40m through 17m, about 1.25:1 on 15m, and a hair under 2:1 on 75m. With the inductor fully retracted the antenna was resonant on 22.9 MHz, well under 1.2:1; I'm guessing I will need a shorter whip to tune 12m and 10m, which I'll worry about when the sunspot cycle perks up.

Getting the control cable and coax to shack is going to be my next mission. Right now I have it coming in through the window but despite stuffing bubble wrap into the gap I'm still getting bugs working their way into the house. The MFJ feed-through panel is pricey, so I might try to fabricate something similar as soon as I can get to home depot to buy a 2x4 and a circular saw (I left way too many tools in NJ...).

With the 200A hooked to the K3 I'm hearing many more stations that are weak or inaudible with the RX wire antenna; other stations that are strong with the RX antenna are 10dB or more stronger with the 200A. KH6IB was S9+20dB on 20m but I couldn't break through the small pileup he was working through before he started calling for mobile stations only -- even though others who were most definitely not mobile still called him, I followed his instructions and held off. Finally I shut it down since I'm exhausted and need to hit the sack. First QSO will have to wait...

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