Related articles at
Balun Test contains model of "perfect" dipole currents.
Sleeve Balun shows how a sleeve adds impedance, useful for VHF and higher baluns
Receiving Common Mode Noise shows how lack of a balun can contribute to system noise (it applies to transmitting antennas as well)
Longwires, Verticals, and Baluns shows how unbalanced antennas can have similar problems
Balun and Core selection for transformers and baluns
Transmitting baluns on testing transmitting baluns
Feedline Common Mode Isolation
Common-mode currents can be detrimental to antenna system efficiency, noise or unwanted signal ingress, and/or pattern. Common mode currents:
Transmitting antennas, especially when using low power transmitters, are generally less critical for feedline isolation. Unless impedance at the insertion point is high, modest values of choking impedance are generally acceptable. Transmitting antennas, when used for receiving, are less worrisome for common mode. This is because antenna-mode signal levels at the feedpoint are generally very strong. Efficient transmitting antennas produce high signal and noise levels at the feedpoint. The strong signals often swamp out or override feedline conducted common mode, except in antenna pattern nulls. Unless common mode noise or signals are exceptionally strong, the main problems with transmitting antennas appear in pattern null areas, or appear as unnecessary RFI or loss of efficiency.
By including feedlines in models, we can see how common mode currents skew or alter radiation patterns. We can also get an idea of system common mode current levels.
If common mode currents are significant enough to alter patterns, common mode currents can also transfer unwanted signals and noise into our transmitting antennas when receiving. This is especially true with unfixable strong local noise sources. (If noise is strong but removable at the source, it should be removed at the source.)
Some antennas are inherently troublesome designs. Some small antennas typically perform beyond expectation for very short "antenna" length. This is because strong electric fields near the antenna excite the feedline with significant common mode currents, causing the feeder to radiate. Four compact antennas highly susceptible to feedline radiation are Isotron, TAK-tenna, CFA, and EH antennas. If those names are searched and theoretical descriptions read, we find the "inventors" or proponents attribute unexpected performance to some fictitious electromagnetic theory, such as increased surface area or a specially phased mixing of induction (energy storage) fields, rather than the true reason for radiation.
Lessons are learned by studying compact transmitting antennas, which have misled inventors and theorists into thinking they have discovered some physical or electrical magic. Feedline radiation can be a major issue in electrically short antennas. Electrically short antennas unavoidably produce significant energy storage fields, both magnetic and electric. The very strong localized electric field near the feedline has the ability to couple significant energy into the feedline in the form of common mode excitation. Another recent example appearing in Antennex's compact antenna articles was a thick stub "vertical" with no counterpoise. You can find an example on the baluns and verticals page of how poor or ineffective some counterpoise or Marconi transmitting antenna ground systems can be. If feedline common-mode currents are suppressed or eliminated, many compact radiators become significantly less effective as radiators.
Some transmitting antennas actually function because of intentionally created common mode currents. Examples are found in textbooks, such as the "Antenna Engineering Handbook" by Jasik on and around page 22-6.
The antennas below, copied from Jasik's textbook, outline the derivation of a skirt collinear antenna from a simple feedline with the open end terminated by a "stinger".
The center conductor termination in these drawings could easily be a ground rod (in the case of a Snake) or an antenna like a Beverage or loop. The termination does not have to be an "open circuit" 1/4 wl stinger that intentionally radiates!
Looking at (a), we find by hanging any low impedance on the end of a coaxial cable the shield is excited by common-mode current.
The electrical equivalent is just as if the transmitter or receiver (generator symbol in the drawings) is located at the end of the shield. This causes the outside of the shield to act like a longwire antenna.
Unless the coaxial shield connects to a zero impedance ground, current with flow on the shield. Looking at (c), we find even multiple sleeves appearing as parallel tuned high-impedance circuits do not fully decouple a shield! It takes grounding and series impedance to do a good job.
Analyzing our antennas, we often forget grounds are not perfect. We make assumptions that four radials, or worse yet two radials, form a perfect groundplane. Even a groundplane antenna many wavelengths from earth with four radials has considerable common-mode currents on the feedline. Consider the following model of a "perfect" Ten Meter groundplane using four perfectly horizontal 1/4 wl radials spaced every 90-degrees with a 1/4 wl feedline hanging vertically and attached to the radials. The main element current was set at 100.
EZNEC ver. 3.0
A glance at radial current shows the bulk of ampere-feet (ampere-feet, or current over spatial distance, determines E-M radiation levels) is on the feedline shield, not the antenna! Radiation from the feedline would be severe, yet most amateur antenna designers claim with only four radials, or worse yet two radials, no balun is needed! The claim that four radials makes a "perfect ground" is false.
Why do we depend on a simple ground rod with 50 or more ohms RF resistance to clamp a coaxial cable shield to ground?
Admittedly the above antenna is a worse-case example of feedline length and grounding, but even better cases can cause problems. A better-case system might be "nearly perfect" when transmitting (so far as efficiency and pattern are concerned), but the system could be a disaster receiving when significant amounts of conducted noise are present on the station ground. With significant in-shack wiring noise, only the shunting impedance of ground connections and feedline shield's series impedance the prevent excessive unwanted noise ingress at the antenna feedpoint.
Common Mode Currents and Receiving Antennas
A receiving example of an antenna that works because of common-mode excitation is the "snake" antenna. The "snake antenna", in order to receive signals, intentionally induces common-mode on the coaxial cable shield. In coaxial cables, current on the inside of the shield always flows opposite current on the center conductor. With the center conductor grounded and the shield floated, inner shield current makes the turn over the open shield end and the inner shield current flows back over the cable's shield outside. The entire shield picks up signal, the snake is simply a reverse-fed random wire lying on the ground.
Very small levels of conducted unwanted noise often go unnoticed in large high-level transmitting antennas. This is because a large, efficient, transmitting antenna has so much signal and noise level at the feedpoint that the antenna's signals and noise completely overwhelms any noise coupled in from the antenna feedline or support. Noise ingress is a non-issue if local noise levels on power lines are reasonably low, especially if the antenna has significant common-mode feedline rejection.
If a feedline is very long and lies directly on or is buried in the earth, ground losses aid in attenuating conducted noise and unwanted common-mode signals. Unfortunately, we almost never know if the feedline shield is contributing noise. We almost never test or evaluate feedline common-mode signal contribution!
Measuring Common-Mode Noise
We sometimes hear we can test or evaluate a system for unwanted noise or signal ingress by disconnecting and replacing the antenna with a dummy load. This idea actually has no theoretical foundation at all. Replacing an antenna with a small load significantly alters common mode impedance of the system, and removes the ingress point (the antenna's feedpoint) entirely. Dummy load substitution significantly changes system common-mode impedance.
The only real test would come from a dummy load with the same connections and impedances (both differential and common mode) as the actual antenna. In other words the test load has to be the actual antenna to keep feedline common mode ingress the same. Obviously, casual dummy load substitution is a useless test!
The best approach is to use preventative measures in initial system design and installation. Quite often the cost of being safe is less than a few percent of the initial system expense.
This circuit is simplification of typical common-mode paths in Beverage, EWE, and other similar antenna systems. In this simplified case, since we only want to develop a feel for series and shunt effect and how common mode gets into the system, standing waves and reactances are ignored. The system below assumes a compact system with pure resistances:
R_Source and V1 represent the source creating voltage across R_Station_Gnd, the station's ground impedance.
Feedline_R is the equivalent series-impedance of the feedline shield.
Current through the feedline shield path develops a voltage across R_Ant_gnd, which represents the earth connection ground impedance at the antenna.
V2 is a voltage source representing desired signals, while R_ant is an impedance representing the sum of the coaxial differential input impedance presented to the antenna (from the desired signal path into the coax) and the actual antenna impedance.
Using the circuit below, we can find the attenuation. Assume:
R_source is 90-ohms
R_station_ gnd is 10 ohms
R3 (the coax shield) is 500 ohms
R5 is the combined series resistance of antenna impedance and impedance presented by the feedline matching system, is 1000 ohms
In a typical system where a single six-foot or deeper rod (the earth's skin depth prevents deeper ground rods from decreasing resistance substantially) is driven into typical soil, R_ANT_GND will typically be between 40 and 120 ohms, assume 100 ohms.
We have the following results:
Using the model above, only ~1 volt of common-mode voltage across the station ground results in .152 volts driving the feedline exactly as a signal from the antenna would. Path attenuation from station ground to the feedline's differential input at the antenna is 20log 151.5/985 or 16.26dB.
Changing the ground resistance to 10-ohms results in:
19.1/982.7 or ~34dB attenuation of common-mode noise. Increasing R3 by adding beads has a similar effect. If R3 is effectively made ten-times larger, attenuation is in the 30dB range.
Obviously it takes a combination of reducing ground resistance and/or adding series impedance on the cable shield to significantly isolate any low-noise receiving antenna from conducted ground noise over the feedline's shield.
We sometimes observe much less noise on transmitting verticals after installing a large effective ground system. Decreasing ground impedance at the antenna reduces common-mode excitation of the antenna feedpoint and reduces noise ingress, although adding a feedline choke would sometimes help. There is no reason to go to extremes in choke value, because a simple ground or two, or even a buried cable, can multiply effects of any series impedance. Also, once the suppression system takes common mode significantly below antenna signal levels, any additional choke impedance is immeasurable and totally unobservable.
A typical isolation scheme would be to use an isolated primary and secondary in the matching transformer, and ground the feedline shield some distance away from the antenna's signal ground. This will introduce several thousand ohms of reactance in the common-mode signal path, as well as provide another path to earth for common-mode noise.
Another method, in cases where the feedline can not be isolated through a floating primary in a matching transformer, is the use of multiple independent ground rods with a series of choke baluns between each. This forms a multi-section pi attenuator, making even modest choke impedances effective. As an additional benefit, lightning paths are disrupted by this method.
Noise contribution can vary with time. A receiving antenna's ground connection resistance varies with soil moisture, and sources of noise come and go. As noise levels and grounding changes noise contribution as a ratio to antenna noise will change. The fact we can not readily measure noise contribution by substituting dummy loads further complicates the issue. Real systems are vastly more complex than the simple analysis above.
Since we can't easily measure noise contribution, we shouldn't take chances. It makes no sense to gamble that unwanted signals (from wrong directions) or noise are so low that they will never contribute to noise in a special antenna installed to reduce noise and interference.
While isolating feedline common mode effects from the antenna and antenna's ground may not reduce noise, isolation can generally be achieved at virtually zero time and material cost. With the low cost of prevention in mind, it is shortsighted at best and foolish at worse to not isolate a feedline shield from any low-noise antenna's signal ground path.
Follow these rules for receiving antennas: