I Have A Thing About Grounds

As the title suggests, this article is about grounds. Not just any old ground mind you, but the one(s) that relate to amateur mobile operation. It appears that far too many amateurs don’t know the difference between DC, RF, ground as in Earth, and Ground Plane. It is this specific misunderstanding that has prompted me to write this article.

Although this article could delve into all aspects of the subject matter, in this case there is just one misunderstanding I want to dispel. It is simply this; a ground strap IS NOT a replacement for a proper ground plane! This very misunderstanding comes up almost daily in the Mobile forum on this web site.

A vehicle is a very poor ground plane especially under 30 MHz. In reality, it isn’t much more than a capacitor to what ever surface is under the vehicle. A surface with a conductivity factor we have no control over. Fact is, as we motor around, the surface makeup and conductivity change continuously. Normally, we don’t see this changing conductivity reflected in any of the meters commonly used to monitor transceiver output or SWR. There are exceptions to this, and I’ll touch on them further down.

There are some very important facts that need to be mentioned at this point. Ground loss is the largest single factor we have to deal with when we’re trying to achieve the best HF antenna efficiency we can. It isn’t the only loss we deal with, however.

Since an HF mobile antenna can seldom be a full 1/4 wave in length, it is necessary to place an inductor in the antenna. This inductor (loading coil in common vernacular) cancels out the capacitive reactance caused by the requisite shortening. The word “caused” isn’t really correct in this instance, but will do in place of a lengthy explanation.

Any HF mobile antenna has several resistive factors, that when combined make up its input impedance. One of those is radiation resistance (which isn’t real per se), but the big one is coil loss. In very simple terms, coil loss is the resistive component which does nothing more than produce heat. Again, in simple terms, the more inductance a coil has the more loss it has. As we go down in frequency, the coil must get larger in inductance, thus losses go up, and efficiency goes down. Radiation resistance goes down as well, adding insult to injury. We can increase the coil’s physical size which increases the coil’s Q and reduces its losses, but there is a point of diminishing return. In most cases (not all), coil loss pales in comparison to ground loss.

Definitive numbers for these losses are readily available on the net, and in publications from a variety of sources especially the ARRL. I won’t repeat them here because it is out of context of the main point of the article. Suffice to say, we want to keep all of the losses low (except radiation loss) if efficiency is our goal.

Assuming it is our goal, the most efficient place to mount an HF mobile antenna in in the center of the roof. This reduces the ground losses to a minimum, and it also minimizes antenna coupling losses. Antenna coupling losses are caused in part by the capacitance between the antenna and the surface under the vehicle. Keep in mind, the lower the mounting position the higher ground and coupling losses will be. Although I use the term ground loss as inclusive of all the encountered losses except coil loss and radiation resistance, it is in fact much more complex than most amateurs realize.

Since ground loss is the largest loss factor, it behooves us to keep it as low as we can. Because the body of the vehicle (and its capacitance to the surface under it) is the other half of our antenna, any thing we place between the two, adds additional loss. I’ve picked a couple of examples to illustrate the point.

Keeping coil losses low is an important goal in maintaining efficiency, so the coil must be located as far away from the body of the vehicle as possible. This fact causes some amateurs to assume that by placing their antenna atop a long stalk will reduce coil losses. That’s true, but the ground losses skyrocket!

Some amateurs don’t like to drill holes to mount their antennas (at least ones that can be seen), so they mount them on various kinds of brackets. One popular place is the rear hatch or trunk lid as the case may be.

Because these locations and mounting methods are not ideal, some amateurs assume a simple ground strap between the mount and the body of the vehicle will replace the lack of an adequate ground plane under the antenna, and the associated losses therein. This assumption is incorrect! Unless of course the ground straps are long enough to act as radials, which is a ridiculous notion. In fact, the mount doesn’t have to grounded to the vehicle at all! It could even be made of plastic! If grounding the mount cured an RFI problem, or lowered your SWR, then something else was (is) amiss.

There is a lot of popular discussion with respect to how much importance all of this has. After all, some antenna is better than no antenna. The other justification goes something like this. “I’ll have you know I worked Peter Island on 40 meters with my hatch-mounted mini bug catcher and got a 59 report.” This only proves one thing; just how little radiated power it really takes to communicate.

My father liked to use the phrase “high, wide, and handsome” in describing an ideal. It is very apropos when it comes to the ideal HF mobile antenna, and its location on the vehicle. Higher, bigger, and better are the main adjectives if the prime goal is to get heard. Anything less is exactly that, less.

I mentioned above that you don’t normally see the changing conductivity of the surface you’re driving over reflected in any of the meters commonly used to monitor transceiver output or SWR. If you do, it means your antenna is mounted to close to the pavement. In a similar fashion, if the SWR changes as the antenna moves around in the slipstream, it's mounted too close to the body. Remember my father’s admonition, “High Wide and Handsome”.

Achieving an efficient HF mobile set up is almost an oxymoronic statement. Some very popular commercial HF mobile antennas are less than 1% efficient on 75 meters, and perhaps approach 10% on 20 meters (compared to a well designed and properly ground planed 1/4 wave base station vertical). Yet, their owners rant and rave about how many DX stations they have worked. Assume for a moment you could increase the efficiency to 5% on 75 meters, and to 50% on 20 meters; how many DX stations do you think you could work then? It is, in fact, possible to achieve these higher efficiencies, but I’ll guarantee you that it takes more than a chunk of RG 8 braid between the antenna mount and the body of the vehicle to do it.

Alan, KØBG

http://www.k0bg.com