A Simple 75Ω Coax Balun for 88-108 MHz
An incoming signal illuminates both an antenna and its feedline. Signal current induced on the outside of a coaxial feedline can enter the cable at the antenna feedpoint, making the coax shield part of the antenna structure. This 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 a 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 places 26" apart (27" for RG-6). Add 2" to favor the low end of the FM band. 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. Make sure adjacent turns touch everywhere within the coil.
Hang the coil in the clear away from anything conductive. The balun is most effective when placed at the feedpoint where shield current is highest. For additional current suppression, form the feedline into a second balun a quarter wavelength away from the first (30" midband, 32.5" low end).
This shows shield attenuation from 88 to 108 MHz in a 50Ω system. This particular balun resonated at 97 MHz. Common-mode shield attenuation can't be directly related to differential shield-signal pickup. But computer simulation of a dipole, balun, and coaxial feedline with a traveling wave on the shield showed unwanted pickup to be at least 19 dB below the antenna signal over the entire FM band. Balun loss is negligible.
Test setup. HP 8443A tracking generator driving the balun shield to an HP 141T/8553B/8552B spectrum analyzer. Patented low-inductance ground strap.
300Ω Baluns
Common 75Ω:300Ω VHF/UHF consumer baluns are voltage baluns. These devices use a small ferrite transformer to force equal voltages with respect to the coax shield on the balanced line. But when the surge impedances of the two lines differ (due to asymmetrical feedpoint construction or unequal coupling to nearby conductors, for example), the line currents will differ. This causes incomplete field cancellation and unwanted signal pickup. To create a high impedance and reduce unwanted signal pickup, follow a 75Ω:300Ω voltage balun with a 75Ω current balun. Typical loss for a 75Ω:300Ω consumer balun in the FM band is 0.75 dB.
For lower loss use a 75Ω:300Ω voltage balun made of a half wavelength of 75Ω coax (45" of RG-6, 40" of RG-59, 48" of RG-59 foam). Form the coax into a compact coil of a few turns. Connect the feedline shield to the shield at both ends of the halfwave line. Connect the feedline center conductor to one antenna terminal. Connect the center conductors of the halfwave line to both antenna terminals. Use the shortest possible leads for all connections. Finally, coil the feedline into a current balun at the feedpoint. Loss for the compound voltage/current balun when made of RG-6 is less than 0.2 dB anywhere in the FM band.
For a compact, low-loss alternative, see Ken Wetzel's 75Ω:300Ω current balun.
More is here.
Updated May 11, 2008
