Saturday, January 24, 2015

Simple Ham Radio Antennas: Let's Do Ground Planes Again. Post #365.

Let's Do Ground Planes Again: "Let's Do Ground Planes Again

from Alan Applegate, K0BG on January 24, 2015."Editor's Note: Due to the popularity of some of eHam's older articles, many of which you may not have read, the eHam.net team has decided to rerun some of the best articles that we have received since eHam's inception. These articles will be reprinted to add to the quality of eHam's content and in a show of appreciation to the authors of these articles." This article was originally published on: 07/02/2007






Let's Do Ground Planes Again

The Spanish language has a whole bunch of words which mean hot, while English gropes around with about two or three. Spanish gets a little fuzzier with the word Ground, but not nearly as much as English. We have the ground we walk on, hollowed ground, DC grounds, AC grounds, RF grounds, and grounds for divorce! We also have polo grounds, capital grounds, football grounds, common grounds, proving grounds, battle grounds, and even ground zero. Any one of these is certainly not any other. So it's no darn wonder neophyte amateurs (and a fair amount of old timers) get confused about which ground is which.

There is one type of ground that needs a different name applied to it because everyone seems to have a different opinion of what it is or isn't, and that is Ground Plane! In fact, there is an alternate name which seemingly has disappeared from the amateur lexicon, and that is Image Plane.

A quarter wave vertical antenna is nothing more than half a dipole. This is why it is sometimes referred to as a monopole. We wouldn't erect a dipole with just one leg, so why is it amateurs will erect a quarter wave monopole (vertical) without an adequate Image Plane (ground plane) under it? In part, it's because they're confused by all of the different references to ground (excuse the pun).

Exploring the Image Plane a little deeper; in very simple terms, an Image Plane is the missing half of a dipole. Again, in simple terms, it transforms a monopole into the functional equivalent of a dipole. In a ground mounted monopole, radials provide the Image Plane. On a vehicle, it is the metal mass, plus the capacitive coupling to the surface under it which provide what little Image Plane there is.

If there isn't an adequate Image Plane, losses increase drastically. A full-size, quarter wave monopole, will have an input impedance of roughly 35 ohms. If yours measures 50 ohms, the difference is most likely caused by an inadequate Image Plane. Contrary to popular belief, a ground rod is not an Image Plane. In other words, it isn't a replacement for radials.

Mobile wise, even the largest of vehicles are inadequate in terms of an HF Image Plane. Improper mounting (using mirror brackets for example) just adds insult to injury. Here too, a ground strap is NOT a replacement for the requisite Image Plane.

In a recent post here on eham.net, this very point was exemplified; an HF antenna was mounted on a semi's mirror bracket. One responder suggested adding another ground strap, assumedly to increase the Image Plane, and lower the SWR (one of the reasons for the original post). A link to photos of the responder's own installation was provided as a how-to example. The photos depicted a very good quality HF mobile antenna mounted in the same mirror bracket location, replete with a ground strap. There was no matching device in evidence in any of the photos.

This fact, graphically illustrates the additional losses incurred by this form of mounting. The reason is simply this; in an average mobile installation, the antenna in question will have an input impedance of about 25 ohms. Since the reported SWR was less than 1.2:1, the input impedance was either 41 ohms, or (most likely) 60 ohms. The difference is commonly referred to as ground loss, but it really relates to an inadequate Image Plane under the antenna. In any case, the efficiency stinks, the number of DX contacts notwithstanding!

So, just in case you missed the point, a ground strap (or a ground rod) is not a substitute for an Image Plane/Ground Plane, or what ever you wish to call the missing half of your monopole (vertical). If you think it is, then it is you that's on shaky ground!

Alan, KØBG
www.k0bg.com"
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Comment:

I ran across this excellent article from Alan Applegate (K0BG) in the 24 January 2015 edition of http://www.eham.com.  Alan does an excellent job of explaining the role of a ground plane (or image plane) in the proper functioning of a vertical antenna.  Without  the missing "half" of a vertical antenna, be it a ground radial system or the metal mass of your vehicle, a vertical antenna becomes a dummy load with most of its radiated power lost to the soil.  Alan adds that a ground strap or a ground rod is not an effective substitute for a ground radial system or a ground plane in your vertical antenna system.  Be careful of antenna claims that violate the laws of physics.  Alan maintains a website that focuses on the installation of mobile antennas.  His articles are well-written and worth every minute you spend reading them.

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