Improving the Micro-Tower

I ran my original micro-tower on and off for about seven months. It had been to Orlando, FL and Raleigh, NC for exhibitions, roved through seven Maidenhead grid squares for June’s ARRL VHF contest, and been through hundreds of miles of road testing. The setup is effective and roadworthy, but I’m always open to learning ways to improve it or make it safer. Andrea, K2EZ, a seasoned contest rover and engineer, interviewed me for a YouTube video. We had a good discussion afterwards and she recommended improvements such as adding triangular supports to the tower legs and raising the thrust bearing a bit to improve support for the mast.

It’s hard to disagree with her proposal, especially since Yaesu’s rotator literature calls for “more than 1 m” of mast below the thrust bearing and “less than 1 m” of mast above it, which could imply a minimum mast height of 74 inches (1.9 m), given my antenna population goals. The photo above shows that I have it backwards. Yes, plenty have broken that rule without consequence. But what if I can implement Andrea’s ideas and (almost) comply with Yaesu’s installation criteria at the same time? If you read about my original micro-tower, then you know that it was custom cut and had welded mounting flanges. Making it taller and adding triangular supports would require a total rebuild and have costs, both in parts and labor. Instead, I sought to find the shortest section of manufactured tower that might work on my rooftop rack.

I discovered an inexpensive tower base section by Amerite, the AME25-B3. It’s intended to be buried in the ground and encased in concrete. Still, there’s hardly a reason why I couldn’t modify it for use in my light application. Okay, there IS a reason: Amerite’s warning label specifically states not to cut or weld it; nor do they recommend using it anywhere other than in the ground! But that still leaves a bit of leeway, provided I ignore their legal butt-covering that warns me to NOT do what I’m about to do! 😀

Hey! Everyone loves a good radio tower on their car, right? 😉

My existing tower shelf will not work with the AME25-B3. You see, I bought my DXE-AS455G tower shelf simply as a place to mount a thrust bearing and then built my stanchions to fit it. I had no idea that it was not compatible with my 25-series tower part when I shopped for a better tower. Sure, I could have made it work, just as I had with my home-brewed micro-tower, but I wanted a solid product. I called DX Engineering for suggestions and they quickly recommended using Rohn’s BPL25G Tower Bearing Plate, which is essentially a cap for a 25-series tower and a place to mount my DXE-TB300 thrust bearing.

There’s a catch with the parts as delivered: The 40-inch tower base and 16-inch tower top assembled into a unit that stands ~52 inches (1.32 m) tall, which is about 16 inches (41 cm) taller than what I want. Remember, my mast is only 72 inches (1.83 m) tall and going much taller will result in me either exceeding legal road height restrictions or not having enough mast to hold four Yagis. I need at least 39 inches (1 m) of mast above the thrust bearing to hold my current setup. Squeezing four Yagis in the same space will be tight, but functional. I can keep the mast “less than 1 m” above the tower, just barely. However, I still have “less than 1 m” below the tower bearing plate since I want to maintain my current overall height. It’s not ideal, but it’s still much better than where I started.

The execution required some cutting and welding, activities that would make DX Engineering, Amerite, and Rohn very happy to support me! Okay, not really! 😉 A friend in my local amateur radio club who has a welder and an appreciation for my ham radio mischief did the cutting and welding for me. He managed to cut the tower pieces to a combined 38.5 inches (97 cm) and then welded them together, along with a steel base plate. Mounted to my roof rack and with 40.5 inches (1.03 m) of mast above the bearing plate, my new road-going clearance is ~11’8″, or 11’9″ (3.58 m) as a safety margin, which is ~3 inches (7.6 cm) taller than before. The cutting and welding voids the warranty and leaves the tower good for nothing other than a car, truck, or perhaps a flat roof with low expectations someday.

Another shortcoming with my original micro-tower was with its mounting base, which was comprised of decking boards. Decking boards are intended to be supported every 16 inches (40.6 cm), which is a standard distance between floor joists. My crossbar spacing is 32 inches (81 cm). As a result, my base tended to sag in the middle, especially once I tensioned the guy lines. My improved micro-tower uses two lengths of 12-gauge Superstrut. Each 1⅝” Superstrut bar can support ~1400 lbs (635 kg) at my crossbar spacing, or around 675 lbs (306 kg) when the car is in motion. I use a third strut bar to serve as a hinge point.

The combined weight of the Superstrut, tower, mast, antennas, coax, and other hardware is over 90 lbs (41 kg). Adding hinges to my platform allows me to tip the tower after removing six bolts. I use a pair of bungee cords to slow the tower’s descent and to aid with tipping the tower upright. I remove one section at a time while the tower is tipped. Loosening four nuts on the rotator’s mast clamp and three bolts on the thrust bearing, as well as clipping some wire tires, allows me to remove the mast and antennas as one piece. That removes 34 lbs (15 kg)! Next, I stand the tower upright to unbolt it from the platform. The 38-lb (17 kg) tower is much easier to remove at that point. After that, I can either remove each of the 6-lb (3 kg) Superstut bars or leave them on the car for more kooky ideas. 😉

Some have disputed my decision to guy such a small tower on my car. I understand the head-scratching, but I’ll ask this simple question: If you were to put a 6-foot (1.8 m) tower with three Yagis on the roof of your house and KNEW that it would be subjected to routine earthquakes and strong gale force winds every day, that is winds that exceed 47 mph (75 kph), would YOU guy that small tower? I think most would answer YES. This is a highway-driven vehicle. So, the tower is subjected to lateral forces and gale force winds every time it leaves the house and hurricane force winds every time I drive in excess of 74 mph (119 kph) on the highway. So, YES, guying this tower is prudent. Also, the 25G-series tower is not designed to free-stand; it requires guying or bracketing. I’ve chosen to guy the tower with 3/16″ Ultra Cord to avoid detuning the adjacent antennas. The platform does not flex and there does not appear to be much of a downside to using the Ultra Cord once the advertised “3% stretch” has been removed.

I refinished the tower and the new welds with satin black paint, an appropriate color for an “automotive look.” I’m pleased to report that the new tower is very stable and has no noticeable flex. I’ve shared photos of the work and finished product in a photo album below. Feel free to ask questions or visit the car in person at upcoming hamfests in Virginia, North Carolina, and elsewhere near the southeast coast.