Mobile Grounds

I keep seeing the same misconceptions over and over on these pages about grounding mobile antenna mounts, and the chassis of the mobile transceiver in question. It is touted as an absolute cure to every mobile malady from high SWR to ignition noise reduction. It isn't! So lets compare the mobile set up with a fixed station one, in an effort to clear up this misconception.

It is easy to set up an transceiver, and feed it a nominal 12 volts DC from a battery. A chunk of coax run up to a resonant dipole, and we have near instant communications capabilities. Does this set up need to be DC grounded to work? Absolutely not! Would you have to ground the chassis of the radio to make it work? Absolutely not! Doing so might afford some level of lightening protection, but as far as communications goes, it is not necessary.

If we were to remove the side of the dipole that is connected to coax shield, and string the remaining wire straight up, we have a vertical antenna without a ground plane under it. It obviously wouldn't work too well as the coax cable itself would be the only return path the RF would have. This is not an ideal situation to say the least.

We could pound in a ground rod, connect the shield to it, and our vertical would work a little better perhaps, but what it really needs is a radial field. Not just a few radials, but a lot of them if we want a good level of efficiency and minimal problems with RFI. Neither the radial field nor the transceiver need be DC grounded to work efficiently.

When mobile, instead of a radial field we have the body of the vehicle, and what ever amount of capacitive reactance there is between it and the surface under it. The net result is, there is ground loss. Depending on the vehicle, the surface under it, how the antenna is mounted, and what frequency we're operating on; this loss is from 2 to about 20 ohms, but can be more. This induced ground loss reduces efficiency in a similar manner to our aforementioned vertical wire without a proper radial field. Thus, it is not uncommon to have efficiencies of less than 5% on the lower HF bands.

Improper mounting will cause additional losses. For example, mounting the antenna too close to the body. Although doing so is not really ground loss per sé (it is stray capacitance loss), the net result is the same; less efficiency. Since mobile antennas are already a big compromise, it behooves us to keep the losses we have control over as low as we can. One way to reduce the losses is by properly mounting our antennas, and adequately bonding the various bolted on pieces of the vehicle.

Lots of amateurs are unwilling to drill holes in their vehicles, and the reasons are quite varied (but this is not the subject at hand). These folks make do with trunk lip mounts, license plate mounts, and all manner of trailer hitch mounting schemes. All well and good. But somewhere along the line, the idea that grounding these types of mounts with a wide braid to a chassis hard point will magically make them as efficient as center-of-the-roof mounting. It won't!

In all fairness, the coax shield should be connected to the body of the vehicle, and close to the base of the antenna. A ground strap might improve this connection, but it won't do anything to improve the ground losses encountered! And it won't magically cure RFI egress from the various electronic devices in the vehicle! And it won't magically cure RFI ingress to those same electronic devices! If you installed a ground, and it solved a problem (RFI or other wise), then something else in your installation is (was) amiss.

DC Wiring & Grounds

There are misconceptions about DC wiring too. In a mobile installation, any requisite DC ground is supplied adequately by the power cable that came with the transceiver. Adding a ground to the chassis isn't all bad, but if it solved a problem (RFI or other wise), then something else in your installation is (was) amiss.

In fact, there are instances where hard grounding the chassis can cause you grief. For example, if there is a differential in current flow between the ground side of the power cable and the chassis of the radio, you have what is commonly referred to as a ground loop. This problem occurs most often when body-on-frame vehicles are not properly bonded, and/or the DC power wiring is incorrectly installed.

Part of the problem is the surfeit of anecdotal information across the web. For example, I recently read an article where the author suggested twisting the DC power cord in as many twists as possible using an electric drill! This technique is supposedly the cure for alternator whine, and a host of other mobile maladies. This is an absurd notion without any merit. I might add, using RG8 coax as power cabling is as inane. The truth is, induced and radiated RFI are best cured at the source.

Related Problem

If my e-mail inbox is any indication, the most prevalent problems are those related to coax, and coax connectors. I have information on my web site about properly installing PL259s. There's a ton of information other places on the net about them too, including these very pages. The ARRL Handbook also explains the procedure in detail. So does information from the manufacturers themselves (primarily Amphenol). Why then is this still a problem?

Adding insult are screw on and crimp on connectors. While they might be okay in some installations, vehicle vibration makes their use suspect.

A typical HF mobile installation has a transceiver, wattmeter, antenna controller, perhaps an amplifier, and an antenna. That's about eight coax connections. If even one of them becomes intermittent, you're going to have problems.

Do yourself a couple of big favors. Buy silver plated connectors, not the "japanned" or Astrofinish ones because they are nearly impossible to solder. Don't buy connectors with white, Teflon-look-alike insulators. They're not Teflon, they're polystyrene (or polyester), and they melt! Silver plated, TFE insulated, PL259 connectors cost about $6 each. If you paid a lot less, you got a lot less.

The best advice I can offer about PL259 connectors is this; take whatever time it takes to install them correctly. Using the proper tools, it takes me about 3 minutes to complete a PL259 without a reducing sleeve (RG8), and about 5 minutes with one (RG58 or RG8X). One of my good friends can't do one any faster than 15 minutes. The point is, it doesn't make any difference how fast you do it, but how good you do it that counts.

Here's a piece of trivia. Circa 1930, an Amphenol engineer named E. Clark Quackenbush, designed the UHF coaxial connector. We commonly call it by its military designator, PL259. I bet he'd turn over in his grave if he knew the grief he has caused us all. If you want a little more history about UHF and other types of connectors, go to the Marvac web site.

Conclusion

We're in the age of Instant Gratification, where anything will do as long as some communication is possible. If this is your situation, and you're willing to live with it, fine. I'm not. I prefer to plan well, execute precisely, and operate moderately. While this takes more time, the rewards are much greater.

Alan, KØBG

www.k0bg.com