Practical Antenna Theory

(Part 6)
by AD5XJ, Ken Standard
ARRL Technical Specialist
ad5xj@arrl.net

This series of articles deal with the more technical aspects of antennas and the practical theory behind their use. You will find this guide to be a pragmatic approach to antenna theory. The views expressed here are the author’s and are not meant to be the definitive text on the subject. You should also be armed with the knowledge that there may be changes from time to time where technology advances and the author deems it approprate to update the text.

You should do your won research and fill in where there are “learning gaps”.

Much of the material in this series is derived from publications by the ARRL. Namely,

The ARRL Handbook ®

and

The ARRL Antenna Book ®.

You should consider those publications reference material in this cursory study of a very complex subject. The most recent editions are available from the ARRL Store online and can be seen at many hamfests, but may vary in quality from used to mint conditon, over many past versions. Any should be considered a valuable addition to your reference library on antennas.

I hope you enjoy this. Please email me if you have questions or would like to see a particular subject enhanced.

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.

© Arrl Handbook 1996

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.


© Arrl Handbook 1996 Chap. 9.8
© Arrl Antenna Book 20th Edition 1-10

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 half–inch 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 above ground. 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.

Lightning protection

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:

http://www.polyphaser.com/ppc_ptd_home.aspx

Very safe rules of thumb for lightning protection grounding are:

  1. Make the route to ground with as large a conductor as possible in as straight a line as possible and as short as possible.
  2. Avoid sharp bends or loops in the grounding conductors as well as pointed or sharp edges on connections.
  3. If in doubt about whether there is adequate conduction for lightning protection, there isn’t!
  4. Never take a chance with your expensive equipment – disconnect completely all outdoor antenna from station equipment during severe weather that contains dangerous lightning.

The station electrical, and to a lesser extent, the RF grounding system provide 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 in the case of vertical ground mounted antennas (reference our discussion earlier).

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.

© Arrl Handbook 1996

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.

 

Mobile Installation and Grounding

©2006 ARRL August QST

 

A vehicle is not a ground plane, but rather acts like a capacitor between the antenna and the surface under the vehicle which acts as the ground plane. Since the surface in question is a poor conductor of RF, ground losses occur. If we wish to maximize the system efficiency (the mobile station as a whole), we need to maximize the RF continuity of the vehicle, hence proper bonding. Obviously, proper antenna mounting, and placement are important too. Remember, it is the metal mass directly under the antenna, not what’s along side, that counts. And a ground strap is not a replacement for proper mounting!
— many thanks to KØBG k0bg.com/bonding.html

Image courtesy KØBG k0bg.com

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.).

Rule one for making connections to the bodies and frames of vehicles is simply, do not sand the finish to bare metal! The reason is, bodies and frames of modern vehicles are dipped in a zinc compound. This zinc compound oxidizes in the presence of air (oxygen) and seals scratches in its surface.

Rule two is, be very cognizant of existing wiring! Modern vehicles have dozens of wire looms placed throughout the superstructure. This includes the A, B, and C pillars, under both door sills, inside the doors, under the carpet, and behind almost every piece of trim. If in doubt, find out what’s behind the panel before you drill or screw. This is why it is wise to purchase a repair manual for your vehicle.

When bonding the tail pipe for example, there are usually strengthening members, or existing mounting bolts which can be utilized. The bottom edge seam of the rocker panels is also a safe bet. Some vehicles have predrilled and tapped holes for accessories and/or for assembly purposes. They're usually metric which will necessitate a trip to the hardware store. What's more, they're safer than drilling into panels unless you know for certain, nothing is behind them.

Image courtesy
KØBG k0bg.com

While you’re at the hardware store, pick up a supply of star (serrated) lock washers, as standard lock washers aren’t worth the effort. The preferred type have both internal and external teeth, but are not always available in the smaller sizes. When properly installed, they bite through the various finish layers, and into the base metal below. Once exposed, the zinc compound seals the connection. The use on NoOx® and similar compounds really isn’t necessary, but they do retard rust and oxidation of the mounting hardware. In any case, do not use these compounds under star washers, and between the mount and the vehicle’s structure. In other words, let the zinc oxide do its job of preventing rust.

Speaking of rust. No matter what you do, sooner or later any fastening device, washer, bolt, screw, etc., will rust. The photo on the right is a typical example of the long-term (five years in a dry climate!) issues they have. It pays to do regular maintenance of all electrical connections, bonding or otherwise.

Image courtesy KØBG k0bg.com

If you can find them, use lugs with built in star washers. Some versions have an embossed serration rather than an actual star washer design, and are easier to find. I purchased mine from Fastenal, but there are other sources such as Mouser Electronics and Ace hardware stores. Uninsulated ones are usually cheaper, but harder to find than the insulated ones.

I typically use two sizes of lugs; ones with a ¼ inch bolt hole, and ones with a #10 screw hole. Wire size depends on the braid size. Lugs for #12 are adequate for RG–8 shield, and lugs for #4 work best for 1 inch braid material. I use the lugs with a ¼ hole when I’m attaching to an existing bolt, and the #10 ones for everything else.

The actual screw type is up to you. I use several different types. Number 10 self-tapping sheet metal screws work well if the material is 12 gauge or less. Self–drilling ones work better for thicker material. Phillips head and hex head both work well. In any case, I wouldn’t buy any longer that ½ inch, and 3/8 is a better choice. Longer isn’t going to hold better, and just might run into something you cannot see. I usually make up several 6” long straps for the doors, 10” long ones for both sides of the engine, both hinges of the hood and trunk, and at least three for the exhaust and tail pipes. Remember to crimp and solder them as crimping alone allows moisture to seep into the connection with predictable results. If the strap is subject to abrasion, cover it with heat shrink tubing.

Heat GunHeat shrink tubing requires a heat gun. Besides the tubing, all of the aforementioned companies carry heat guns with prices varying between $40 and $250 depending on both quality and duty cycle. Since we don’t use one all day long, we don't need a high priced one, so here’s a suggestion. Hobby Lobby sells (in store and on line) a heat embossing gun for ≈$30. It works perfectly as a light duty heat shrink gun, and its small size and light weight making it easy to store.
— from KØBG k0bg.com/bonding.html

Do not depend on the coax shield alone to provide the ground connection for the antenna. If the base ground connection of the antenna is not a solid 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 plus the star washer mentioned above, will insure good electrical and RF conduction to the metal it will attach to.

Also, 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 braid to chassis when a mobile antenna is mounted on a towing bar or other removable metal structure.

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.

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).

73 AD5XJ

 

 

 

 

Rev. 1.10 2018-03-14 AD5XJ