An electromagnetic wave illuminates both an antenna and its feedline. Current induced on the outer surface of a coax shield may enter the feedline at the feedpoint. Common-mode signals induced on parallel transmission line may become differential at the receiver. Either makes the transmission line an unintended part of the antenna structure, which can degrade the directive pattern and forward gain. A current balun can attenuate unwanted coax shield current. A current or voltage balun can attenuate common-mode signals on parallel line.
You can make a simple current balun by coiling coaxial feedline in a particular way. The coil inductance and distributed capacitance resonate as a parallel trap whose high impedance inhibits unwanted shield current.
To construct the balun, mark RG-6 coax with tape at two spots 27″ apart (26″ for RG-59). Coil the coax into three turns with the marks aligned. At the marks secure the coil with dark, UV-resistant tie wraps overlapped as shown. Tie-wrap the coil across all three turns at two other places so that adjacent turns everywhere touch.
The coil may not meet the cable's minimum bend radius specification. Bending coax too sharply may cause an impedance change, or with certain dielectrics and enough time, an internal short. The spec varies among cables and manufacturers. It is 2˝″ for Belden 1505A RG-59. For 1530A RG-6, it is 3″.
Ken Wetzel uses Belden 1855A coax because it is small, flexible, and has a very accurate characteristic impedance. He uses three turns with an outside diameter of 223⁄32″. He bonds the turns together with superglue and uses two tie-wraps to secure the exit leads. The minimum bend radius for 1855A is 1˝″. Loss is 3.33 dB/100′ at 100 MHz.
Times Microwave LMR-300-75 should make an excellent coiled-coax balun. Its minimum bend radius is ⅞″ and loss is just 1.75 dB/100′ at 100 MHz.
Hans-Peter Dohman, DL9EBA, uses this balun on a 75Ω Yagi. Mounting it perpendicular to the elements minimizes adverse coupling. A current balun is most effective when placed at the feedpoint where shield current is highest. When that's not possible, form the feedline into a second balun a quarter wavelength (30″) away from the first. Spaced baluns are particularly effective at reducing shield current induced by asymmetrical coupling to the antenna, as may occur when the antenna is vertically polarized.
This shows shield attenuation from 88 to 108 MHz in a 50Ω system. This particular balun resonated at 97 MHz. The attenuation range corresponds to a common-mode impedance of 1.9kΩ to 63kΩ.
Test setup. HP 8443A tracking generator driving the balun shield to an HP 141T/8553B/8552B spectrum analyzer. When making a measurement, I move the balun out and away from the conductive surfaces with an insulated tool.
Passing coax through ferrite material increases its common-mode impedance without affecting its differential impedance. A Laird 28A0807-0A2 snap-on, split-ferrite choke, stocked by Mouser and Digi-Key, is simple to install. Rated impedance at 100 MHz is 348Ω. It will accomodate quad-shield RG-6.
The plastic closures of a split-ferrite choke may become brittle and fail when flexed after long outdoor exposure. Tape or tie-wrap a broken housing to ensure that the ferrite halves remain firmly joined. The ferrite itself does not seem to degrade outdoors.
A nonsplit ferrite sleeve, installed before terminating the cable, is inherently more robust than a split core. A Fair-Rite 2643625202 choke, available at Mouser, has a rated impedance at 100 MHz of 384Ω (Ken Wetzel measured 330Ω). The minimum hole diameter is 0.3″, which will accomodate quad-shield RG-6.
The impedance of a ferrite choke is much lower than that of a coiled-coax balun. But coil resonance varies with jacket material and geometric detail, and performance verification requires instrumentation. Ferrite chokes provide reliable, moderate, broadband attenuation in a lightweight, compact package. To increase their impedance and unwanted-signal attenuation, use more than one choke.
To evaluate the effectiveness of coiled-coax and ferrite choke baluns, I modeled a highly directive narrowband Yagi in free space with AO 9.52. Due to its extremely small backlobes, the antenna is very sensitive to stray signal pickup. I added a conductor to one side of the feedpoint to represent the coax shield. In practice, the shield surge impedance and resulting current depend on the length of the coax, what it couples to, and what it connects to. Since these parameters are unknown, I modeled a traveling wave on the shield as a general, nonresonant example. As you lengthen any conductor, it develops a traveling wave as the incident power gradually radiates away. I created a traveling wave on a relatively short wire by placing a 350Ω load a quarter wavelength from the far end. I adjusted the load impedance and position for the most uniform wire current.
This is the model geometry. The red dot is the feedpoint and the green dot is the traveling-wave termination. The yellow traces represent current magnitude. A traveling wave on a vertical wire radiates mostly downward. To examine a worse case, I bent the shield wire horizontal six feet below the Yagi. The horizontal section is 20 feet long. It is orthogonal to the elements and does not couple to them. Note the discontinuous driven-element current and the nonsinusoidal shield current.
With a circuit analysis program I modeled a coiled-coax balun as a parallel trap in a 50Ω system. I adjusted the component values to obtain the response I had measured for the test balun, centered at 98 MHz. Then I used the values for an RLC load in AO. Analysis at 88.1 MHz tests the worst-case shield-current suppression of a coiled-coax balun resonated at midband.
This magnified view sights down the horizontal section of the shield conductor. Here the current traces are phasors. The distance from the wire to the trace is magnitude, while the angle with respect to the wire is phase. A slowly decaying spiral is characteristic of a traveling wave.
This overlaps azimuth patterns with and without the coax shield. Shield current at the feedpoint was 6.6% of maximum model current. A linear-dB scale reveals low-level detail. The center of the plot is −50 dB.
These patterns are for a coiled-coax balun and for two Fair-Rite 2643625202 chokes, modeled as a 660Ω resistance.
To try a resonant shield, I removed the termination and adjusted the horizontal wire length to maximize its current, which occurred at 231″ and was 19% of model maximum.
Here I adjusted the horizontal wire length to 194″ to minimize its current, which was 2.9% of model maximum. The higher common-mode circuit impedance reduces balun effectiveness since its series impedance becomes a smaller percentage of the total.
Yagi/Balun Model ; use 27 segments/halfwave Free Space 88.1 MHz 9 6063-T832 wires, inches x1 = 0 ; element positions x2 = 21.9375 x3 = 37.25 x4 = 76.8125 y1 = 67.375/2 ; element half-lengths y2 = 67.1875/2 y3 = 61.6875/2 y4 = 53.6875/2 a = -2 ; position of balun below antenna b = -72 ; position of shield bend c = x2 - 209.5 ; position of traveling-wave termination d = c - 30.5 ; position of shield endpoint 1 x1 -y1 0 x1 y1 0 0.375 ; reflector 1 x2 -y2 0 x2 0 0 0.375 ; one side of driven element 1 x2 y2 0 x2 0 0 0.375 ; other side 1 x3 -y3 0 x3 y3 0 0.375 ; first director 1 x4 -y4 0 x4 y4 0 0.375 ; second director 1 x2 0 0 x2 0 a 0.15 ; dipole to balun 1 x2 0 a x2 0 b 0.15 ; balun to bend 1 x2 0 b c 0 b 0.15 ; horizontal run to termination 1 c 0 b d 0 b 0.15 ; beyond termination 1 source Wire 2, end2 1 0 39 pF ; shunt capacitor for 75-ohm match 2 loads Wire 7, end1 .75 uH 3.52 pF 3 ohms ; RLC balun model resonant at 98 MHz Wire 9, end1 350 ohms ; termination to create traveling wave
I modeled a shorted folded dipole fed against a vertical conductor to determine the common-mode impedance and resulting mismatch loss of a typical Yagi driven element and coax shield. For shield lengths between 15 and 30 feet, 75Ω mismatch loss is at most 6.3 dB. This demonstrates in another way that a nonresonant feedline length provides little protection against unwanted common-mode signals.
Most 300Ω ferrite baluns use two transmission-line transformers interconnected as shown. These baluns produce equal voltages of opposite phase at the 300Ω terminals when driven at the 75Ω terminals. Conversely, voltages common to the 300Ω terminals cancel at the 75Ω terminals. With both ports shorted the common-mode impedance typically is less than 20Ω.
Inside a typical balun you'll find a two-hole ferrite core that accomodates both transformers without adverse coupling.
Some baluns unground the transformer interconnecting lead. These baluns tend to equalize current rather than voltage at the 300Ω terminals. This promotes signal cancellation when the parallel-line surge impedances differ, which might occur when one line is closer to a conductor than the other. The common-mode impedance typically is 250Ω.
A wire at the center of the core and two leads instead of four to the coax shield identify this as a current balun.
Some voltage baluns use a single toroidal transformer as a phase inverter. What look like series capacitors actually are integrated RC networks. Not all single-transformer baluns have these networks, and they can be found in other kinds of baluns as well. The capacitance improves common-mode rejection below the transformer passband and the resistance drains any static antenna charge.
To compare the common-mode rejection of ferrite baluns, I cut a 31″ piece of 300Ω twin-lead. This is a quarter wavelength at the low end of the FM broadcast band. Fed against a surface, a conductor of this length makes an effective antenna. I terminated the twin-lead with a 300Ω resistor at one end and spade lugs at the other. I connected it to several baluns that I plugged directly into my Wavetek SAM RF-level meter, which I tuned to a local FM signal. Keeping the orientation of the twin-lead constant, I swapped baluns and compared signal levels. Except for a tiny residual, any signal was due to unwanted common-mode balun response.
After connecting the twin-lead directly to the SAM as a reference, I measured the following relative signal levels in dB for several baluns:
Voltage baluns, two transformers <-40 <-40 -35 -29 -27 -26 one transformer -18 -18 Current baluns -20 -19
The single-transformer baluns had the worst common-mode rejection. The current baluns were almost as bad even though their common-mode impedance was much higher. I had created one of the current baluns by ungrounding the transformer interconnecting lead in a voltage balun. When I grounded it with an insulated tool while watching the meter, the unwanted signal dropped about 10 dB.
These are the two <-40 baluns. I assume they have two transformers inside. Mismatch loss due to the extra lead length should have increased apparent rejection only 0.5 dB. In addition to having the best common-mode rejection, these baluns had the lowest transmission loss, 0.5 dB. The other baluns were the push-on kind with screw terminals, with a typical loss of 0.75 dB.
Even with perfect common-mode balun rejection, nonzero transmission line spacing causes a residual differential signal. Simulation of the terminated twin-lead oriented vertically above an infinite ground plane gave a peak differential signal 44 dB below the peak common-mode signal.
You can increase the common-mode rejection of a 300Ω voltage balun by following it with a 75Ω current balun. Since the common-mode impedance of a voltage balun is low, the cascaded common-mode rejection should be that of the voltage balun plus the common-mode attenuation of the current balun.
A halfwave coaxial line that inverts the signal makes a low-loss 300Ω voltage balun. Here Peter Körner used small-diameter coax to save space. Two ferrite sleeves provide further common-mode attenuation. All shields connect. The halfwave-line center conductors go to the two antenna terminals, while the feedline center conductor goes to one of them. Keep all leads as short as possible, as Peter did here.
An electrical half-wavelength at 98 MHz is 60.1″ multiplied by the velocity factor of the coax. Use the manufacturer's value for your specific cable.
This circuit models the loss of a halfwave coaxial balun. It uses a lossless transmission line with a characteristic impedance of 75Ω and a delay of 5.1 ns. The T-pad models the 0.083-dB loss of a 50″ length of RG-6 (assumed loss 2 dB/100′ and velocity factor 83%).
This circuit models the common-mode rejection.
This Triax FM 5 balun uses serpentine traces on a printed circuit board to form a halfwave transmission line.
The PCB bolts directly to a folded dipole. A plastic enclosure weatherproofs the balun and connections.
RF radiation from lightning is about 80 dB stronger at 10 Hz than 100 MHz. Well below their design frequency, halfwave coaxial baluns, coiled-coax baluns, and ferrite chokes provide no rejection of common-mode signals. A nearby lightning strike that induces common-mode current on a long feedline may generate a high-voltage differential pulse at the receiver. 300Ω ferrite baluns have a winding that shunts low frequencies, and those with series capacitors provide further reduction. The low-frequency attenuation of these baluns may or may not be enough to protect a receiver.