Car Mounted Mast for Field Ops

A Vehicle Mounted Mast

For Field Operating

Geoff Haines, N1GY

This project actually came about because of a little visit to one of those charity operated thrift shops that are very common in most parts of the country. After dropping off our donations to the cause, I was just scanning the room when my eyes lit on a bright yellow object in a corner. It turned out to be one of those telescopic poles with a spring loaded light bulb changer basket on top. The price was irresistible (less than $5.00) so I paid the bill and took it home. The project was already taking shape in my head.

I have always liked to be able to operate from my car at a park or beach. The only problem with setting up my big telescopic mast (as shown in QST Dec. 2007 with the "Octopus" Antenna) was the need to set up the base, then drive the car onto the base, then mount the mast, extend it, guy it down, etc and only then be able to hook up the coax and begin to operate. While this can be a one-person job, it is much easier with two or more. The guying also takes up a fair amount of room around the car, which can be a problem.

The new mast would have to be totally contained within the footprint of the car. No guy ropes, no separate base, just erect the mast and antenna and get on the air. My SUV already has a Converta-ball hitch stud on the rear bumper, and a length of 1"ID diameter PVC pipe fit both the hitch stud and the lower section of the light bulb changer mast perfectly. The next job would be to convert the changer basket to something that could accommodate a VHF/UHF antenna like the one I wrote about in QST August 2006.

Amazingly, once I removed the light bulb changing basket's components, I was left with a screw-on base that with a little work with a narrow chisel was ready to be mated with the PVC Tee at the base of the antenna. Four short sheet metal screws were used to secure the screw-on base to the antenna. Now the antenna can be simply screwed onto the top of the mast and secured with a small set screw that was included with the base.

The next task was to ensure that the mast, which in its stock form extends about 11 feet, was rigidly mounted and stable. This is important because once the 1" ID PVC pipe is added to the bottom of the telescoping mast and the antenna is added to the top of the mast, the whole assembly is 21' from the ground to the top of the antenna. That ought to add some range compared to the mag mount antenna on top of the car's roof.

In looking around the SUV, I discovered that the adjustable crossbar at the rear of the rooftop rack has tie-down loops at each end. Some scrap 1/2" diameter aluminum tubing that was left over from some long dead antenna provided enough material for two braces that run from those tie-down loops to a pipe clamp attached to the 1" PVC pipe. The ends of the 1/2" aluminum tubing were flattened and bent to the appropriate angle. Holes were drilled in each end to accommodate stainless steel bolts with Nyloc type nuts at the ends that go into the tie-downs and a 3/8" stud attached to the pipe clamp. A 3/8" wing nut secures the braces to the pipe clamp and the bolts at the other end of the braces slip into the tie-down rings. These ends are secured with a bungee cord across the luggage rack and the braces, exerting downward pressure to keep the braces in position. Another bungee cord is run from the 3/8" wing nut and stud down under the rear bumper to again apply downward pressure to keep the mast assembly in position.

The length of the aluminum tubing braces was carefully measured after much experimentation to ensure that the mast assembly is truly vertical in all planes when erected. Adjustment fore and aft is achieved by sliding the pipe clamp up or down the PVC pipe and side to side adjustment by twisting the pipe clamp left or right before securing the clamp to the pipe. These adjustments are relatively narrow in scale, so careful measurement of the length of the braces is vital. Once all the adjustments are completed, the setup is done. Disassembly and removal of the mast and braces should not result in having to readjust again. In setting the system up once a suitable parking location is reached, one simply assembles the braces and mast (with the antenna mounted) and the mast will always be vertical relative to the vehicle. Only if the parking spot is not flat and level will more adjustment be required.

The entire system breaks down into 7 pieces: one 1" ID PVC pipe, one telescopic mast, two aluminum braces, two bungee cords and one dual band antenna. All of this fits easily into the back of my SUV along with my emergency go-kit and the other stuff I have accumulated, such as the 22' telescopic mast and "Octopus antenna, the drive-on mount for the larger mast, several runs of coax, a different VHF/UHF antenna, foul weather gear, first aid kits and the like. For Field Day, a practice SET, a trip to the beach, or an emergency call-out when a disaster strikes, the SUV is ready. Using the smaller mast means that the setup is completely contained within the space of the vehicle. This makes setting up and breaking down is much easier and takes less time.

There are a couple of caveats that come with this project. First, under no circumstances should one try to drive the vehicle with the mast erected. Contact with overhead power lines could be fatal. Second, because of the light-duty nature of the mast, only relatively small and light VHF/UHF antennas are suitable. Larger antenna arrays need larger and more rugged mounting systems to be safe.

My costs were minimal because I already had the PVC pipe and the aluminum tubing on hand. Besides the cost of the telescopic light bulb changer, which should be available at any home improvement store, I spent another $10 or $11 on stainless steel fasteners and the pipe clamp. The contents of your "Junque box" may vary but I doubt that one could spend more than $50 on the project even if everything had to be purchased new.

This project might make an good first project for a newer ham. The extra range that a 21 foot high antenna gives the operator over the regular mobile antenna will come in very handy, particularly if they get involved (as they should) with their local ARES group or public service event support.