Maximum-Gain Narrowband Yagi

If you have a favorite FM station you'd like to better receive and there's no interference from the rear to contend with, consider a dedicated, gain-optimized Yagi. This six-element design has high gain over a 1-MHz passband and decent gain over 4 MHz. The boom length is 203″ when designed for 97.5 MHz.

I optimized the design with the YO 7.70 Yagi Optimizer. This image shows the E-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 (ratio of forward power to that of the worst backlobe in the rear half-plane), unmatched impedance, matched SWR, and the difference between the gain figure and the maximum gain practical on the boom length (an estimate, here 0.2 dB low). The red trace on the Yagi sketch depicts element current, while the orange trace is the relative propagation velocity of the wave traveling along the structure.

Yagi File

Max-Gain Narrowband Yagi
6063-T832
97.500 MHz
6 elements, inches
            0.3750
  0.0000   29.2005
 31.7715   28.6026
 62.9851   27.0360
106.9888   26.4881
157.5921   26.1425
202.8073   26.5579

Match: 1 0.0000 0.0000 0.0000 0.0000 47.0 75.0 0.0000

The first column is element position and the second is element half-length. Element diameter is ⅜″. The dimensions are valid only for isolated elements (insulated mounts above the boom). The matching network is the lowpass version of a hairpin match. Split the driven element leaving a gap no larger than ¼″, solder a 47-pF capacitor across the feedpoint, and feed with 75Ω coax. Coil the feedline into a current balun at the feedpoint.

To scale the design to another frequency, change all element lengths and spacings proportionally. The diameter can remain ⅜″. For example, to scale the design to 88.1 MHz, multiply the dimensions and capacitance by 97.5 ⁄ 88.1.


December 18, 200988–108 MHz