This four-element Yagi has extremely small backlobes over a single FM broadcast channel. The boom length is 63″ to 77″ depending on frequency. I optimized the design with the AO 8.50 Antenna Optimizer.
This overhead view shows the antenna geometry. The red dot marks the feedpoint.
This is the azimuth pattern at the design frequency of 97.5 MHz, the geometric center of the FM band.
Rectangular coordinates reveal low-level detail.
I optimized the design at 97.4 and 97.6 MHz to obtain uniform backlobe rejection over the 200-kHz channel passband. The high-rejection region is quite narrow so accurate construction is important. Rear rejection is at least 20 dB over 3 MHz or so.
This is the response after scaling the design to 88.1 MHz. The scaler compensates for the unscaled ⅜″ element diameter and rounds dimensions to the nearest 1⁄16″. It also compensates for 10 mm elements and rounds metric dimensions to the nearest millimeter.
The scaled response at 107.9 MHz is similar.
The azimuth pattern isn't quite as good when the Yagi is oriented for vertical polarization. The side nulls due to the off-end element geometry that benefits horizontal polarization are gone. But far more serious is that the mast is now in the element plane. The following patterns show the effect of a 10′-long 1¼″-diameter mast at several locations.
Be cautious generalizing these results to other antennas. A mast 1′ behind the reflector of a similar design degraded the pattern more seriously than shown above. In addition, resonance of an actual mast depends on its length, diameter, what it connects to, and what it couples to. Use the patterns to compare the relative degradation for various mast positions, but not to predict the actual pattern for a particular installation.
When using vertical polarization, install a current balun every 30″ on the feedline in the vicinity of the antenna, starting at the feedpoint. This will inhibit signal current on the coax shield, which can disrupt the pattern in the same way a mast can. Extending the boom a couple feet past the reflector and dropping the feedline there will minimize pattern degradation.
Tiny-Backlobe Narrowband Yagi Free Space Symmetric 97.5 MHz 4 6063-T832 wires, inches x1 = 0 x2 = 19.771 x3 = 33.63879 x4 = 69.30097 y1 = 30.39627 y2 = 30.31443 y3 = 27.81846 y4 = 24.20959 1 x1 -y1 0 x1 y1 0 0.375 1 x2 -y2 0 x2 y2 0 0.375 1 x3 -y3 0 x3 y3 0 0.375 1 x4 -y4 0 x4 y4 0 0.375 1 source c = 35.13497 Wire 2, center 1 0 c pF 27 segments/halfwave best matches NEC Optimized at 97.4 & 97.6 MHz Trade-offs: G 25%, F/B 75% 100% worst-freq weighting
The x symbols are element positions and the y symbols are element half-lengths. Element diameter is ⅜″. The dimensions are valid only for isolated elements (insulated mounts above the boom). Use the dimensions provided by the scaler. Split the driven element leaving a gap no larger than ¼″, solder the specified capacitor across the feedpoint, and feed with 75Ω coax. Install a current balun at the feedpoint.
88–108 MHz