Practical Antenna Theory - Part 6

by AD5XJ ARRL Technical Specialist

One of the most often discussed topics in Ham Radio (and the most misunderstood) is SWR. In this series of articles we will learn some basics, some theory, and dispel a few myths that are generally perpetuated in the Ham Radio community.

The discussion last time involved some pretty heavy theory. We covered the most common type of simple antenna – the vertical mobile antenna. We also mentioned the close variation of the mobile – the base station vertical.

It is important that there is a thorough understanding of the material presented so far. Granted, the material can be overwhelming on first read. However, with patience and perseverance, you may find this knowledge to be invaluable to you in even the most simple of tasks where antennae of all type are concerned.

Our discussion this time is not specifically about SWR, but about one of the contributing factors to unacceptable SWR – improper grounding.

There are a number of lessons to be learned on this subject, not the least of which is the difference between grounding for proper conduction of RF and maximum protection against lightning damage and electrical hazard. Quite often there is ignorance to the fact that the two are not always synonymous. They are, in point of fact, quite often at odds with each other. Good electrical ground techniques seek to protect the user against power line AC hazards and intrusion by lightning. In ham radio we must also consider our signal path to ground. Good electrical grounding is mandatory. So, if we construct our station to comply with NEMA, National Electrical Code, and local electrical codes, is this grounding method sufficient to provide a good RF signal path for our station? The answer is not a straightforward one.

Consider the following diagram of two grounding methods. The method employed in (b) of the diagram may appear to have the best protection since there is a direct path to ground from each appliance. The fact is that it is not only inadequate, it is not compliant with any accepted electrical code, and does not follow accepted guidelines for electrical shock hazard and lightning protection.

What about the station gear, and tower, rotor, and computer? Do we use method (a) in the illustration. Again the answer is not simple as you think.

There is yet another consideration where a vertical ground mounted antenna is part of the station equipment.

As you evaluate the next illustration, notice the difference between electrical connections for AC power, and RF connections for control of transmitted signals.

An effective station electrical ground bonds the chassis of all equipment together with low-impedance conductors and ties into a good earth ground where the electric service panel has its origin. In multi-level or large sprawling structures, care must be taken to bond to the closest earth ground source (this is not to say a cold water pipe is a good ground source). If one is not available, a separate heavy gage bonding ground wire should be run to the nearest earth ground. In most cases the best approach is to drive one or more ground rods into the earth near a window or access point to the station. Bonding to this ground rod will provide needed protection against electrical hazards and provide some lightning protection. If your soil is soft and contains few rocks, an acceptable alternative to the 8 ft copper clad rods from an electrical supply house is ½” copper water pipe. Since this material is relatively soft, care should be taken when driving it into the ground as it is quite malleable material and will bend easily. Bonding to a conventional ground rod or a copper pipe alternative should be of good electrical quality and weatherproofed as much as possible. Unexplained noise can creep into station systems where ground systems develop high resistance or noisy connections to ground due to corrosion and oxidation at the ground connection.

Operators who enjoy the upper frequencies should pay particular attention to the length of the connections from equipment to ground rod. The length should be as short as practicable so as to avoid resonant lengths to ground that may cause ground-looping noise and RF high voltage on the station equipment chassis. Possible causes of this type of noise include (but are not limited to) insufficient ground conductor size, loose or corroded ground connections, and ground-looping. Ground-looping is an unusual phenomenon that occurs when the ground system does not drain RF current away, but becomes part of the RF system and begins to oscillate undesirably or radiate with RF. To illustrate this, consider a length of grounding cable (regardless of size/gage) measuring about 33 feet. If the RF equipment is operating on 20 or 40 meters, this is very near a fractional wavelength of these frequencies. Sympathetic oscillations will occur in the grounding because of the near resonant length of the grounding conductor.

Protection against lightning on a comprehensive level is a complicated and controversial subject that has been published at length. Some of the best information is provided by Polyphaser Corp. in their quarterly newsletter and on their web site: www.polyphaser.com/services/media-library

The system electrical and RF grounding system provides protection against hazards from equipment and lightning in the shack.

However, the use of artificial grounding methods also has a place where antenna efficiency is concerned (at least for vertical ground mounted antennas). As we discussed in a previous article, inadequate ground currents in a vertical antenna can cause losses contributing to radiation inefficiencies. To lower the power losses in ground system absorption, an artificial ground plane may be constructed to improve ground currents. This artificial ground plane is primarily multiple wires of fractional wavelength radiating in all directions from the antenna axis and connected to the ground or shield of the antenna system. Wires of this type should be distributed in regular equally spaced fashion in all directions. Often, limited space applications do not allow ground plane wires to be placed in a straight line in all directions. When this is the case, radial wires may be curved or bent at an angle about half way the length of each. This provides a smaller umbrella, but a proper RF ground. The number of radials needed for maximum efficiency ranges from 8 to 128 depending on frequency, soil type, terrain, antenna height, and cost of construction. The next illustration shows the two common methods described.

In this way the impedance of the ground structure is lowered and overall losses to power dissipation in the ground (often called I²R loss) are reduced to negligible values. The reduction of ground absorption loss improves overall efficiency and as a result – more available power radiated.

Variations and permutations of this altered pattern are not only possible but very workable in practice. The object is to provide an efficient ground plane for the vertical element in the limited space available.

In the case of a mobile installation, the ground plane is the auto body or frame. As more and more body parts are becoming non-metallic, the use of the body/frame is more important than ever. In the case of automotive electrical systems, care must be taken so as not to cause RF currents in and around the on-board electronic devices (the engine computer, GPS, DVD Player, etc.). Proper methodology for mobile operations would be to make all ground connections for RF as near the same point as possible. Otherwise insure that the antenna, radio, and battery negative share the same electrical conductor and it is as low a resistance as possible. External connections should be protected with auto body undercoat or some other spray on protective coating to minimize corrosion. Do not depend on the coax to provide the ground connection for the antenna. If the base ground connection of the antenna is not a solid bare metal connection to the same conductor as the battery and radio, run a separate low-impedance ground conductor to it from the antenna. A good source for this kind of conductor is the shield from a length of coax. Strip the vinyl outer cover and push back the shield to make it expand enough to pull out the center conductor. Carefully flatten the braid to form a ribbon. Applying a terminal to each end will insure good electrical and RF conduction to the metal it will attach to. Don't be fooled into thinking that a removable metal towing bar will serve as a good ground. Vibration can, and will eventually cause noise in the receiver as the metal bar moves in the receiver frame, even if it is bolted tightly in place. Provide a separate antenna ground to chassis when a mobile antenna is mounted on the towing bar.

Do not use the metal dash superstructure as ground for the transmitter or battery negative to the radio. If necessary, run a shielded plus and minus battery lead directly from the battery to the radio to avoid engine noise being picked up inside the engine compartment or RF traveling from the radio to the engine. It should be obvious to you by now that the shield connects to the auto chassis and the negative lead to battery negative. An adequate shield can be created using the method described above. If engine ignition noise is a problem in receiving, a flexible grounding lead may be bolted to a metal hood from the frame to provide greater shielding effect.

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Next time we will kind of switch gears and talk about antennas that are not ground mounted or mobile – i.e. beam antennas and dipoles.