Rooftop Antenna Tower w/Rotator!

If you’ve been watching my site, then you’re aware that I’ve been slowly building a VHF contest Rover. I’m documenting my process on my new “Rover” page. My coverage there is brief; so, this entry will share more details about the project. “What is a rotator?,” you ask. Most are familiar with vertical whip antennas, which have omnidirectional radiation patterns. Directional beams allow operators to focus their emissions in a specific direction. Some Rovers use a temporary mast that’s set up at each operating position and then pointed by hand; others mount a permanent mast and then steer their vehicle to point the beam. A more sophisticated approach that’s adapted from home equipment includes the use of a rotator, which is a motorized device which points the antenna array in the direction that’s chosen by the operator. They can even be used while the vehicle is in motion.

The photo above shows the rotator itself with a PVC test pipe. I chose a G-450-ADC medium-duty unit by Yaesu and took heavy inspiration for its mounting from K5ND’s setup. My base and physical attachment to the car is different, but the “micro-tower” is very similar. My choice of tower shelf is the foundation of the micro-tower’s design. Once capped with a thrust bearing to provide a second support point for the mast, I then chose 22-inch tall “tower” legs made from 1-3/8″ galvanized steel fence pipe. Each leg has a welded flange. I’m using a 6-foot galvanized steel 2-inch fence post for a mast.

The micro-tower and rotator are fastened to pressure treated decking boards. I glued and screwed 1×4″ cross members into place to provide additional rigidity and to aid in positioning the platform on my roof rack. The photos shared so far show the wood unfinished so that you can see the fastening hardware better. I will stain the wood with Olympic Maximum Exterior Wood Stain in Mystic Black once the pressure-treatment has dried. I had spray painted my M2 Loop Module platform, but chose stain this time since it soaks into the wood better, including the bolt holes, and can be applied in January temperatures.

The 1/2″ galvanized fasteners that secure the tower to the base are overkill, but I wanted to eliminate all slop from the flanges. I had experimented with using T-bolts to fasten cargo to my Yakima JetStream crossbars. However, I’ve found Yakima’s Mighty Mounts to be easier and much more convenient, especially since my various antenna modules are installed and removed frequently. The hardware that I’ve chosen should be far more than enough to hold this platform to the rack, even at ~75 lbs! Ironically, the 165-lb rated crossbars are held to each Yakima Baseline tower by just one M4 bolt. The tensile strength of an M4 bolt is over 3,500 lbs; so, I’m probably safe enough. Still, I’ve added retention guys, just in case!

I currently have three horizontal beam antennas on the rotating mast with plans to add at least one more. For 50 MHz, I’ve chosen an SM-50 Stressed Moxon by Par Electronics, referred to as a cheese slicer by my wife. It is a folded two element Yagi-Uda antenna with 3.7 dBd of gain and a front-to-back ratio of 17 dB, which is excellent for rejecting off-axis noise. My other antennas are “Rover Specials” by Directive Systems & Engineering. They feature 8-ft booms to keep them road-legal. The DSE144-6RS is a 6-element Yagi with 10.1 dBd of gain and the DSEFO432-15RS is a 15-element Yagi with 13.5 dBd of gain. I will add a DSEFO222-10RS 10-element Yagi when I’m ready to add a 222 MHz transverter. All of these antennas will accept 1000 watts or more, which is far more than I will ever use in a mobile application.

Some physical aspects of the “Contest Module:” The base and tower combined weigh ~50 lbs! The 6-ft galvanized steel mast adds 9 lbs and each antenna weighs ~3.5 lbs. That puts the entire piece at ~75 lbs. To mount on my car, I lift the base and tower to the roof, tilt them on their side, add the mast, antennas, and feed lines, then tip everything upright to slide the module into its final position, which is slightly off-center so that it can ride with the “Loop Module.” The Contest Module is held to the crossbars with twelve 5/16-inch carriage bolts. I mentioned that the “Rover Special” antennas keep the car road-legal. That’s because the maximum allowable width of a vehicle on the highway is 8 feet. I can maintain that width even if I have the antennas turned sideways while driving down the road. The height at the top of the mast is 11’7″. So, no drive-thrus with this module mounted!

My feed lines are more complicated than one would think. Switching between loops and beams is accomplished with an array of Type-N coaxial relays. The relays default to the M2 loops. The beams are selected when I energize the relays via remote switch. I made a pass-thru to route up to ten feed lines through my driver’s side rear window. I use LMR-240UF feed lines below 432 MHz and LMR-400 at 432 MHz and higher. Some Rovers insist on LMR-400 on all bands. I find the compromise of LMR-240UF to be very small below 432 MHz. Even at 5700 MHz, the difference is less than 1/2 of an S-unit on the other end of the conversation. If you’re curious, my car has over 226 feet (69 meters) of feed line when all antennas are mounted and 186 feet (58 meters) with the Yagis mounted.

The obvious benefit of these antennas is that they greatly increase my effective radiated power (ERP), yielding ~1200 watts ERP on my most effective band, which is awesome from my small mobile platform! However, their greater benefit may be the amplification of received on-axis signals and rejection of off-axis signals. My relative lack of success during my first contest efforts may have been because I simply could not hear distant stations. I look forward to seeing how this setup performs during future ARRL VHF contests!



About Scott

I grew up near Houston, TX and served in the U. S. Coast Guard ordnance and electronics communities for over 35 years. I became involved with ham radio and computers in 1995. The explosion of technology made my jobs and hobbies quite interesting. My hobbies include Volkswagens, bicycling, photography/videography, electronics, ham radio, and web management.
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3 Responses to Rooftop Antenna Tower w/Rotator!

  1. Pingback: Guying the Tower | KE4WMF

  2. Pingback: 2023 CQ Worldwide VHF Contest | KE4WMF

  3. Pingback: Improving the Micro-Tower | KE4WMF

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