A Narrowband Maximum-Gain 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 narrowband Yagi. This five-element design has high gain over a one-MHz passband and decent gain over three or four MHz. At the design frequency it comes within 0.1 dB of the maximum gain possible for five elements and for the boom length, which is 151" at 98 MHz.

I optimized the design with the YO 7.69 Yagi Optimizer program. 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 (worst-case F/B 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 0.1 dB low). The red trace on the Yagi sketch depicts element current, while the orange trace is the propagation velocity of the wave traveling along the structure. Gain and F/B curves are compared with those of other antennas here.

Yagi File

Max-Gain Narrowband Yagi
6063-T832
98.000 MHz
5 elements, inches
            0.3750
  0.0000   28.9865
 31.0631   28.3000
 62.9345   26.8505
106.5925   26.4055
150.6059   26.5666

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. All elements are 3/8" diameter. For the dimensions given, the elements must be completely isolated (insulated from the boom, not pass through it, and not use conductive mounting brackets). To scale the design to another frequency, change all element lengths and spacings proportionally. The diameter can remain unscaled at 3/8".

Impedance matching uses the lowpass version of a hairpin match. Split the driven element, solder a 47-pF capacitor across the feedpoint, and feed with 75-ohm coax. Coil the feedline into a current balun at the feedpoint.

More is here.

Updated October 3, 2007