A Narrowband Low-Backlobe Yagi

This small Yagi suppresses co-channel interference from the rear for a single vertically polarized FM station. It has four elements on a six-foot boom. I optimized the design with the YO 7.66 Yagi Optimizer program.

This image shows the H-plane pattern at three frequencies. The numbers within the patterns are frequency, mismatched gain (forward gain including mismatch loss, with mismatch loss alone in gray), F/R (here, worst-case F/B from 120 to 240 degrees to the rear), unmatched impedance, matched SWR, and the difference between the gain figure and the maximum gain practical on the boom length. The red trace on the Yagi sketch depicts element current. The orange trace is the propagation velocity of the wave traveling along the structure.

This shows performance from 87.3 to 89.3 MHz.

This is the radiation pattern in polar coordinates at 88.3 MHz. The yellow trace shows YO results, while the red trace is an accuracy check using the Numerical Electromagnetics Code. Unlike simpler antennas with a single sharp null to the rear, this design has very high signal rejection over a wide rear angle.

These are the YO and NEC patterns in rectangular coordinates.

Mast Effects

A conductive mast can profoundly alter the radiation pattern of a vertically polarized Yagi. The following images generated by the AO 7.02 Antenna Optimizer program show the effect of a ten-foot, 1.25"-diameter mast at several positions.

Yagi File

KSDS Yagi
87.300 88.300 89.300 MHz
4 elements, inches
           0.3125
 0.0000   33.4375
13.5000   32.3125
30.4375   30.6875
67.8125   26.5625

Match: 4 0.0641 0.5000 7.0000 0.0000 43.0 75.0 0.0000

Optimized in the H-plane at 87.3, 88.3, and 89.3 MHz.
Trade-offs: 20% gain, 80% F/R.
F/R region: 120-180 deg.
Element lengths and positions rounded to 1/16".

The first column is element position and the second is element half-length. The antenna uses 0.3125"-diameter isolated elements. The dimensions are valid only for isolated elements. Passing them through a conductive boom, even if insulated, will degrade the pattern, as will use of conductive mounting brackets. If the feedline can't be brought away from the antenna at right angles to the elements, route it a couple of feet past the reflector on a boom extension and then let it drop.

The T match for 75-ohm feed consists of #14 wire spaced 0.5" center-to-center from the driven element, extending 7" on each side of the boom. Use a 43-pF capacitor in series with each T lead (a single 22-pF capacitor in series with the coax center conductor also will work). Coil the coax feedline into a current balun at the feedpoint. For even better feedline isolation, form another balun 33" away.

More is here.

Updated July 9, 2007