Portable masts attached to cars

I wanted to come up with some sort of quickly deployed antenna system for rover/field day/portable use. As a sort of design guideline, I wanted the total deployment time to be around 5-10 minutes from parking the car, to having the antenna up in the air. I didn't want to have to deploy guy lines either (there's not always a place to put them.

One scheme that occurred to me was an antenna mast that attached to the car (using the car as ballast and a mounting point). You've seen this sort of thing, where you put one of the car wheels on a plate which holds the base of the antenna, and the antenna is bolted to the vehicle. The ARRL books have had pictures of this sort of thing attached to RVs, for instance.

One issue is that the car's suspension system will flex under the loads from the antenna, causing the mast to sway, but I figured that's not a real big deal, as long as the mast is rigid enough that it won't buckle. I really wanted to know whether the car would be tipped over by wind loads, so here's some quick calculations. I assumed a 50 foot high mast, 4" in diameter, with a 10 square foot antenna on top (yes, that's pretty big, but we want design margin). The car is assumed to weigh 3000 pounds with the strut attached 6 feet off the ground on the mast, which is 8 feet from the center of the car. I also assumed 80 mi/hr winds. Here's a drawing:

drawing showing mast next to car

 

Will it tip the car over?

Now the calculations. At 80 mi/hr, the wind load is about 16.4 lb/square foot, therefore we have:

Antenna wind load: 10 sq ft * 16.4 lb/sq ft = 164 lb

Mast wind load: (assume mast is rectangle 4" wide by 50' high) 16.7 sqft * 16.4 lb/sqft = 274 lb

Note: wind speeds are not constant from surface to 50', and the drag of a cylindrical mast might be different, but we're just ballparking here.

Calculate the moments.

Moment due to antenna wind load: 164 lb * 50 ft = 8200 ft lb.

Moment due to mast wind load: 274 lb * 25 ft = 6850 ft lb.

Total moment = 15050 ft lb.

Now.. compare that to the moment available from the car to keep things upright.

Moment from car: 3000 lb * 8 ft = 24000 ft lb

Oops... that's pretty close. And even if we go with a lower wind speed (like 50 mi/hr, which would cut the wind load moment to about 6000 ft lb) it's still pretty dicey. And what sort of loads are we going to be putting on those struts (and, by extension, the roof rack of the car)?

Horizontal load on bracing strut: 15050 ft lb/6 ft = 2508 lb!

Hmm.. that will probably rip the rack right off the roof, even if we go to the lower windspeed (where the load would be 1000 lbs), or even a shorter mast (30 ft vs 50 ft would reduce the loads to 600 lbs). The rack on my car is rated to carry something like 150 lb, but probably has a failure load around 4 or 5 times that. After all, it's not really designed to take huge side to side, or fore/aft loads (it's hard to stop a car at more than 1g, and most cars can't corner anywhere near that tightly)

Conclusions

All in all, then, a "bad plan". Actually, though, for short term rover/portable operation, something like this might work, but you'd have to be willing to accept catastrophic failures of the system. You'd probably want to go to a real lightweight/inexpensive mast and only work 30 or 40 feet up, and accept that it's going to occasionally fold over.

By the way, these calculations also would apply to things like tower trailers, and you can see why they go to outriggers, rather than depending on the weight of the trailer to keep it upright. So, the next design idea is a giant, quickly erectable tripod of some sort. If you can get the outriggers/tripod legs out far enough and attached high enough up, the loads are quite reasonable. The idea of attaching it to the roof rack is still appealing, because it could make the transportation and setup easier. Anyone who's stood on top of a van putting up a push-up mast will appreciate how this works!

(Force 12 sells something like this: http://.force12inc.com/. I note that their LPT1242 is a 42 foot mast (extended) and rated at 9sqft 60 mi/hr, but doesn't mention much about whether the rigid guys will take the load.)


radio/antenna/mastcar.htm - 18 August 2004 - Jim Lux
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