ANTENNAS FROM THE GROUND UP
Unwanted or misplaced currents within the antenna system represent wasted power. In addition, they can often disrupt some of the more sensitive circuits in a transceiver, such as the keying or VOX control circuits. Sometimes, RF can show up as a hum on audio lines. With higher than QRP power, the final indignity is a tingle or shock hazard on cabinet corners and case screw heads. Often, these phenomena occur even when the operator is satisfied that there is a good and effective common ground for all of the equipment.
The only way to remove these currents is to block them from the shack. There are tried and true means of accomplishing this preventive measure. In fact, there are enough means that sometimes we use the wrong one in the wrong place. Therefore, it seems useful to briefly look at the sources of unwanted currents from the antenna system and the means to rid ourselves of them.
Unwanted currents have two general sources, both of which can be analyzed as disruptions to the standard transmission line currents. Transmission line currents, whether on the outer surface of the center conductor and the inner surface of the braid of coax or on the two wires of a parallel transmission line, have under ideal conditions equal magnitude and opposite polarity at any position along the line. Anything that changes this condition may be trouble.
Fig. 1 shows the ideal condition at the top. Remember that we are dealing with alternating currents, so the arrows represent an instant in time. They would all reverse at some other instant, and they will continuously change size as a representation of magnitude.
Common mode currents are shown at the bottom. In general, they travel at any instant in the same direction on both conductors of the transmission line. (However, on coaxial cables, skin effect forces all common mode currents to the surface, that is, to the outside of the braid.) The termination for these currents is an earth ground (or a reasonable substitute). If one side of the line is connected to the same ground line as the equipment case(s), then common mode currents travel on the case(s). From the case, the currents can affect the circuit common of any section of the equipment, and what affects the circuit common can be just as disruptive as introducing a current into the "hot" side of the circuit. Our strategy should not just be to eliminate such currents, but to eliminate their possibility.
Fig. 2 shows two common sources of common mode currents. When we feed a balanced antenna, like a dipole, loop, or beam, directly with coaxial cable, we are inviting trouble. Although the currents inside the coax are presumed to be balanced, the current on the braid has a potential double path: on the inner and on the outer surfaces. The current on the outer surface becomes a common mode current.
Alternatively, if the feedline--either coax or parallel--is long enough, radiation can couple directly to it and also produce common mode currents. In fact, such currents do not have to be from one's own antenna, but might come from a nearby high power source, such as a broadcast transmitter or a noise source. These external sources often create problems in receivers ranging from front-end overload to mixer products and resulting spurious signals.
Before we look at cures for common mode currents, let's look at a second source of unwanted RF currents in the station equipment. These currents arise from unbalanced conditions in the feedline. Sometimes, the unbalanced condition is natural to the type of antenna that we may be using.
End fed and off-center fed doublets are the most common examples of antennas that produce unbalanced currents in the transmission line. However, there are many more possible cases. We can feed verticals off center--sometimes without knowing that the feed is not at a true voltage-minimum point. Some stations use non-symmetrical triangles, with a resulting imbalance in currents on the line. Poor parallel line installation is another source.
Fig. 3 simply illustrates the condition. One of the difficulties that current imbalance presents is that we often cannot distinguish it from a common mode current condition. Both situations can result in RF in the shack, with consequences for equipment operation. Both can result in power radiated from the transmission line instead of from the antenna. The amount of power does not have to be great to create interference with home devices or with station equipment. A few millivolts is often enough to create interference or circuit disruption.
The are some antenna vendors who imply or even boldly state that feedline radiation can make a positive difference in the operation of some antennas. Most of these claims are more hype than fact. Getting significant radiation from a feedline requires the use of certain line lengths for any frequency and lines that are free and clear of potential coupling into unwanted wires or metallic surfaces. Good design practices strongly suggest that we let the antenna itself do the radiating in a predictable manner and let the feedline do its work of conveying power from the source to the load without radiation.
A problem facing the QRP operator is knowing when there are unwanted currents from the antenna system. QRO operators can often detect them by touching the equipment cases and getting a small shock. Another symptom for the QRO operator is keying circuit lock-up or VOX lock-up, as the currents are rectified and change the bias level on the trigger circuits. At QRP levels, the current level may be insufficient to create these problems.
One good indicator for coax systems is to have a good external SWR-power meter designed for QRP power levels. The first test is to make up two coax cables, a very short one and a longer one--perhaps at least 3 times longer. First, place the meter close to the gear and then farther from the gear. The forward and reverse power levels should not change at all in a system with no unwanted currents. If there is a difference, then unwanted currents exist and beg for treatment.
Actually, we should always think in terms of preventing such currents from occurring in the shack as a basic part of antenna system design. In short, a few ounces of prevention can save a lot of work later on trying to diagnose and treat problems. The prevention and the treatment measures are the same.
The name of the cure is choking. An RF choke is any device that presents to an unwanted current a sufficiently high impedance that the current is reduced to an acceptable level. Some choking occurs by virtue of neutralizing the unwanted current. Let's look at the three most common means of choking or neutralizing unwanted currents in coaxial cable systems.
1. The coil of coax: By making a tight coil of several turns of coaxial cable, we can create a considerable inductive reactance on the outer surface of the braid without disturbing the transmission line currents between the center conductor and the inner braid surface. This form of choke is perhaps the cheapest and most convenient of all, but it does suffer some limitations. First, such chokes are heavy, consisting of between 8 to 20 feet of cable wound into 8 or more turns. From the current ARRL Antenna Book (page 26-21), here are some recommended single and multi-band sizes for RG-58:
Single-Band (very effective)
80/75 20'; 6-8 turns
40 15'; 6 turns
30 10'; 7 turns
20 8'; 8 turns
15 6'; 8 turns
10 4'; 6-8 turns
Multi-Band (less effective)
80-10 10'; 7 turns
80-30 18'; 8-10 turns
20-10 8'; 6-7 turns
The chokes are considered most effective when placed closest to the antenna, which can be a weight problem at the center of a dipole or other wire antenna.
2. W2DU-type ferrite bead "baluns": A series of ferrite beads placed over a piece of coaxial cable can provide a high impedance over a broad frequency range. For 80-10 meters, about 50 cores will provide the desired impedance of about 1000 Ohms, while about a dozen adequate for VHF. Type 73, 77, and 43 ferrite materials are most used at HF, with type 43 and 61 considered the best choices for VHF. Type 77-2401 cores fit well over RG-58A, but may not slip over RG-8X. Walt Maxwell used a foot of RG-142 and smaller beads for his original: the coax has high power capability despite small size due to the use of a silver coated center conductor and teflon insulation.
You can build your own ferrite bead balun, but be certain to make it water proof. Inexpensive commercial bead baluns (in kit or complete versions) are available from such sources as the Wireman of South Carolina. Incidentally, the device is a balun because eliminating the unwanted currents from the coax effectively transforms the balanced feedpoint of a dipole into the condition needed for single ended coax to function properly.
3. A 1:1 current balun: A current balun can also effectively suppress common mode currents. The most common types of current baluns on the market are transmission line transformers. Jerry Sevick has two books available on the design and construction of these transformer types. The transmission line transformer can be used for both common mode current suppression and for impedance transformation. Most of the designs from commercial vendors are wound over toroidal or linear ferrite cores. Hence, the units tend to be somewhat heavy, although some types are specifically designed as dipole center devices.
A true transmission line transformer operates best with minimal SWR on the line. Hence, the best use of such baluns is with antenna terminal impedances having a very low reactive component. Mono-band dipoles, beams, trap antennas, and others having the a low reactance source impedance are the best candidates for most types of current baluns.
The transmission line transformer effectively suppresses common mode currents by neutralizing them within the turns of the device. We shall look at this action in a moment. First, let's see where we should place our common mode current suppressors. For this purpose, Fig. 4 can be useful to our planning.
The sketch shows two chokes instead of the usual one-choke system. Most folks place a choke at the antenna terminals, and in many cases that is enough. It solves to a large degree the problem of the balanced-to-unbalanced condition at the junction of the antenna and feedline. However, it does not always resolve the problem of radiation pick-up by the feedline itself.
At the shack entry, a ground lead to a good earth ground can go part of the way in suppressing common mode currents. However, a second choke, with the shack common (ground) lead brought to the choke and then to the earth ground, can often suppress remnant and random common mode currents that a line might pick up. A bead balun for the second choke is a good choice. The use of bulkhead connectors at both ends can provide the necessary connection points for the earth ground leads.
So far, we have noted suppression means for coax systems, but not for parallel line systems. Fig. 5 may help us out here. For parallel lines, the most effective suppression means is a transformer of some sort. Note that transmission line currents create a proper set of currents in the turns for normal transformer action. However, common mode currents create equal magnitude but opposite polarity currents in the transformer turns, thus canceling themselves out relative to transformer action. In effect, this neutralization of common mode currents is what occurs in a transmission line transformer and in transformers of standard design.
A transformer can also be used to convert a balanced output side to an unbalanced input side--or we can let the input side also be balanced. Therefore, although not common on the amateur market, there are broadband RF transformers available that are designed to do the same work as most baluns. The cost is a percent or 2 less efficiency than a properly matched transmission line transformer. The benefit is that a standard transformer handles complex impedances as a matter of course, so long as we do not let the core of a toroidal version reach saturation.
The transformer does not have to be toroidal or use a core of any kind. Air-core transformers work fine, although they tend to be frequency limited. However, the most promising place to find such a transformer is in a link-coupled antenna tuner. However, for both older and younger tuners, we have to make a revision to common tuner building practice. The secondary of the tuner that connects directly to the feedline terminals must "float" relative to the system ground or common. The common would allow a path for unbalanced or common mode currents through the equipment cases. A floating secondary isolates the offending currents.
Our little exercise has shown that we can overcome unbalanced currents and suppress common mode currents, if we plan them into our antenna systems.
Updated 11-05-2003. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.
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