Holes

I'm convinced that latter day amateurs have an unwritten code when it comes to not drilling holes in their vehicles, new used, or otherwise, to mount antennas. This unwritten code is supported by a number of excuses, some of which are as lame as a one-legged marathoner! No matter which side of the debate you're on, there will no doubt be an ensuing argument. That said, let's look at a few of the more popular excuses, and see if they hold up to scrutiny.

Excuse 1: Mounts leak.

Truth 1: That depends on the mount, and I might add nomenclature isn't necessarily an indication. The NMO mount is a very good example. NMO stands for New Motorola, and is a patented mounting method, typically employed to mount VHF and UHF antennas. The main design advantage of the NMO is the fact the mount can be installed without access to the under surface, as typically required in roof mounting locations. You just have to be careful not drill through a lateral roof brace.

There are several versions of the NMO ostensibly designed to circumvent the original patent. Unfortunately, they're still with us. The one above is a Larsen NMO, and an exact copy of the original design. The one below is knock-off, and even a casual glance will tell you they're not compatible.

In all fairness, both of these mounts are waterproof even when the antenna is removed. This is not always the case.

This is an SO239 type mount. Note the weather cap in the bottom foreground. If you have to remove the antenna for whatever reason, you have to screw on the cap to prevent water from entering the vehicle. By the way, this mount requires underneath access even though the instructions say you don't. There are other mounts that leak, and probably the worst of the bunch was the Antenna Specialists 3/8 inch snap-in mount. Fortunately, I couldn't find a photo of it.

So the bottom line answer is, if you choose the correct antenna and mount, and you follow the installation instructions properly, mounts don't leak!

If you're really into esoteric data, here's a white paper on the NMO's loss characteristics.

Oh! One last item. The NMO wasn't designed to hold an antenna heavier than a few ounces. Nonetheless, at least one pacific rim company sells an HF antenna (it's really a dummy load) which utilizes their version of an NMO. My advice? Save your money!

Excuse 2: Holes depreciate the value of a vehicle.

Truth 2: That depends on a lot of factors not actually related to the mounting hole itself. Contrary to popular belief, the overall condition of the vehicle, and the odometer reading are much more important to a new car dealership than an NMO mount in the center of the roof. If you doubt this premise, then read this. Pay particular attention to the last paragraph on page one.

If the dealer just happens to ask about the left-over mount, tell them it's for a cellphone antenna, and don't elaborate! Too much information (TMI) is never a good idea.

Here's another car fact. Unless the vehicle in question is exceptionally clean appearing, vehicles with odometer readings over 50,000 miles or so, are dispatched to the nearest auto auction, or sold wholesale. In this case, the dealer couldn't care less about dings, dents, and antenna mounts. The same goes for vehicles over 6 model years old.

Excuse 3: I drive a lease car, and the contract says I can't drill holes.

Truth 3: Here is the lease agreement from Ford Motor Company (GM and Chrysler agreements are essentially duplicates). Show me where it says you can't drill holes! I suppose one could read something into paragraph 19 or 23 that doesn't exist, but even that requires a stretch.

Personally, I don't know why anyone would lease a vehicle for personal use (non-business amortization). If the appearance of keeping up with the Jones' is important to you, and you drive less than 15,000 miles per year, a lease might make sense. However, the truth remains, it is a very expensive way to buy a car. If you think otherwise, read this.

Excuse 4: My wife won't let me (it's a family vehicle).

Truth 4: This is the lamest excuse of all, and one that really doesn't warrant an answer. If this is really the case in your household, I'm truly sorry for your plight.

Excuse 5: I use a clip mount/mag mount, so I don't have to worry about body damage.

Truth 5: Au contraire! Here is a photo of a clip mount with obvious damage to the lip of the trunk it is mounted on. Here's the full-res image. You may disagree, but this type of damage is all to prevalent; the norm really.

Unfortunately, I couldn't find a mag mount photo showing what the surface under them looks like after a few months of use. Road debris, primarily metallic brake dust, builds up on the magnet and eventually finds its way under the mount with predictable results.

Mag mounts also increase the ground plane losses, especially at HF frequencies, no matter how many magnets they sport. Installing ground strap to a vehicle hard point is not a cure or substitute.

There is another aspect of mag mounts you seldom read about; they're dangerous. They do come off at highway speeds, and as a result of a vehicle crash. Whether they cause bodily injury is moot; the potential to do so should be enough. Unfortunately it isn't, as there has been two reported deaths caused by them dislodging. And don't be like one, recent, hapless thread poster about the number of deaths being good odds!

There are many more excuses to be sure. Nonetheless, there are a few things you should know before you use one of these excuses.

First, any mounting scheme you use, which doesn't securely mount your antenna permanently to sheet metal, will have a measurable reduction in efficiency, even UHF ones.

Any mount which attaches via a clamp or set screw, will eventually come loose.

Any antenna mounted to a door, hatch, or trunk will have extra G forces applied to it. This fact will exacerbate wear and tear on any mount (and the antenna), especially those held on by clamps and set screws.

Ground straps are not a replacement or substitute for an inadequate amount of metal mass directly under the antenna. To assume otherwise is folly!

RF chokes are an integral part of mobile operation; motor lead chokes are a ubiquitous example. What they aren't, is a substitute for poor mounting. If you need more information on this subject, read this.

One of the biggest RFI factors encountered by mobile operators is common mode currents flowing on the coax and/or motor leads of improperly-mounted antennas, even when correctly choked! Again, ground straps (ad nauseam) are not a cure!&

Mobile operating is just one facet of amateur radio. It can be enjoyable, safe, and trouble free. How enjoyable, safe, and trouble free, is directly proportional to the effort put into your installation. So, stop using excuses, and drill the hole!

Alan, KØBG

www.k0bg.com

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