An electromagnetic wave illuminates both an antenna and its feedline. Signal current induced on the outer surface of a coaxial feedline can enter the cable at the antenna feedpoint. This makes the coax shield part of the antenna structure, which can degrade the directive pattern. A current balun, sometimes called a choke balun, can attenuate unwanted shield current.
You can make a simple current balun for the FM broadcast band by coiling the 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-59 coax with tape at two spots 26″ apart (27″ for RG-6). 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, easy to bend, 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 spec for 1855A is 1˝″. Loss is 3.33 dB/100′ at 100 MHz.
Times Microwave LMR-300-75 has a minimum bend radius spec of ⅞″. Loss is 1.75 dB/100′ at 100 MHz.
Hans Peter-Dohman, DL9EBA, uses this balun on a 75Ω Yagi. Position the coil away from anything conductive and orient it perpendicular to the elements. The balun is most effective when placed at the feedpoint where shield current is highest. If 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 coupling between the antenna and an asymmetrical feedline, as may occur for vertical polarization.
This shows shield attenuation from 88 to 108 MHz in a 50Ω system. This particular balun resonated at 97 MHz.
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 characteristic impedance. A Laird 28A0593-0A2 snap-on, split-ferrite choke, stocked by Mouser and Digi-Key, is simple to install. It provides a common-mode impedance of 407Ω at 100 MHz. It will accomodate 0.258″ coax, which includes RG-59. RG-6 may not quite fit. If you try it, make sure the core halves join without a gap. The Laird 28A0640-0A2 handles RG-6 and provides an impedance of 240Ω.
Split-ferrite chokes do not seem to degrade even with years of exposure outdoors. But a nonsplit core, installed before terminating the cable, is inherently more robust. A Fair-Rite 2643625202 choke, available at Mouser, has an impedance of 384Ω at 100 MHz and a hole diameter of 0.312″. This choke should have no trouble accomodating RG-6.
Ferrite choke impedances are much lower than those a coiled-coax balun provides. But coil resonance varies with jacket material and construction, and verification requires instrumentation. Ferrite chokes provide moderate, noncritical, broadband attenuation. To increase the impedance, use more than one choke.
With the AO 8.50 Antenna Optimizer I modeled a highly directive narrowband Yagi in free space. Due to its extremely small backlobes, this 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 6′ below the Yagi. The horizontal section is 20′ long. It is symmetrical with respect to the elements and does not couple to them. Note the discontinuity in the 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. 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 wire. 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 is the same view with the balun included in the model.
This overlaps azimuth patterns with and without the coax shield. Forward gain with the shield is 6.52 dBd. A linear-dB scale reveals low-level detail.
This model includes the shield. Patterns are for a coiled-coax balun and for two Fair-Rite 2643625202 chokes, modeled as a 768Ω resistor. Forward gain for the ferrite model is 6.72 dBd.
To try a resonant shield, I removed the termination and adjusted the horizontal wire length to 231″ to maximize its current. For a traveling wave, shield current at the feedpoint with no balun was 6.8% of maximum model current. For the resonant shield, it is 19%. Forward gain with no balun is 6.02 dBd.
Here I adjusted the horizontal wire length to 194″ to minimize its current, which was 2.9% of model maximum. Forward gain with no balun is 6.68 dBd. I'd expect an actual installation similar to the modeled geometry to perform somewhere between the two resonant models, as does the traveling wave model.
Yagi/Balun Model 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 Use 27 segments/halfwave
Most 75:300Ω VHF/UHF consumer baluns are voltage baluns. Some use an autotransformer, but most use two transmission-line transformers as shown above. Both kinds force equal voltages with respect to the coax shield at the 300Ω terminals. But when the surge impedances of the parallel lines differ, as they might when one line is closer to metal than the other, equal voltages cause unequal currents. The result is incomplete field cancellation and unwanted signal pickup. To raise the common-mode impedance and reduce current imbalance, follow a 300Ω voltage balun with a 75Ω current balun. I measured the transmission loss of various 300Ω baluns as 0.5–0.9 dB at 98 MHz.
This is what you'll find inside a typical balun. A two-hole ferrite core accomodates both transmission-line transformers without adverse coupling. With 75Ω and 300Ω loads, common-mode attenuation, or isolation, for the worst balanced conductor measured about 3 dB in a 50Ω system. This implies a common-mode impedance of about 40Ω.
To try to improve isolation, I rewound the core as an ordinary RF transformer with a turns ratio of 1:2. I used twisted trifilar windings, with two windings in series for the 300Ω side. Transmission loss was about 0.5 dB. Return loss with a 300Ω load was 19 dB. Common-mode isolation was 11.5 dB, which implies a common-mode impedance of about 275Ω. An input/output capacitance of 4.7 pF accounts for most of the residual coupling. I tried to reduce the capacitance by separating the windings, but the transmission loss greatly increased.
An easier way to turn a voltage balun into a current balun is to unground the leads that interconnect the two transformers. With this simple modification I obtained about 11 dB of isolation, an impedance of 255Ω. In addition, transmission loss dropped 0.15 dB and return loss improved 1 dB.
This indoor balun is already a current balun. A wire at the center of the core and two leads instead of four to the coax shield identify it. I measured 11.5 dB of isolation, an impedance of about 275Ω.
Although not evident, this outdoor balun is also a current balun. I measured 9 dB of isolation, or about 180Ω impedance. Transmission loss was 0.6 dB and return loss about 18 dB.
For lower loss, you can make a 300Ω voltage balun from an electrical half-wavelength of 75Ω coax. Peter Körner used small-diameter coax to save space and two ferrite chokes on the feedline to suppress common-mode current. 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.
This shows mismatch loss in dB due to nonideal impedance transformation for a halfwave balun made of 75Ω coax (it's even lower for 93Ω coax). Total loss for a compound voltage/current balun using RG-6 for the halfwave line and for a coiled-coax choke is less than 0.25 dB anywhere in the FM band.
For a compact, low-loss alternative, check out Ken Wetzel's 300Ω current balun.
88–108 MHz