If you don't need high performance, the Antennacraft FM6 is hard to beat for value. But you may want a little more gain or a better pattern, or maybe you just want something to build for fun. The Yagi described here has 5.2–7.2 dBd forward gain. All backlobes are more than 20 dB down across the FM broadcast band. The antenna is about the same size as the FM6, with five elements on a 66″ boom. I include metric dimensions for a European version optimized for 87.5–108 MHz.
I designed the antenna using the global optimizer and response flattener of the AO 8.07 Antenna Optimizer program. This image shows the antenna geometry. The red dot marks the feedpoint. The bent driven element couples to the reflector in a way that improves the pattern at the low end of the band and the gain everywhere.
When using a 75Ω feedline, the FM6 requires a 75:300Ω balun with long leads that typically adds 0.85 dB of loss. In contrast, the small Yagi has a 75Ω feedpoint and needs only a simple, lossless current balun. Including balun loss, the small Yagi has more gain than the FM6, as well as a better pattern, across the entire FM band. The graphs include the Triax FM 5, a small, five-element European Yagi with an assumed balun loss of 0.75 dB.
Paul Logan in Lisnaskea, Fermanagh, Ireland, uses this commercial version of the antenna manufactured by VHF Teknik AB in Trelleborg, Sweden. They call it the FM5.1. It uses a ferrite choke balun.
Sven Jacobson of VHF Teknik AB installed a vertically polarized prototype antenna at his home in Ljunghusen, Sweden.
Ivan Dias Jr. built this antenna in Sorocaba, São Paulo, Brazil. The feedpoint box contains a coiled-coax balun.
Cedric Lamouche, F4EGZ, installed this antenna 7m up a tapered fiberglass mast in Domerat, France. He used a coiled-coax balun.
Hans-Peter Dohmen, DL9EBA, uses a hinged mount, rope, and 4.8m nonconductive mast to receive any polarization with this portable setup in Duisburg-Rheinhausen, Germany.
Roland Nogell uses this antenna at his summer house near Lysekil, Sweden.
Mark van Wijk, PA5MW, erected this antenna of unusual construction at his holiday address near Sareiser Joch, Liechtenstein.
Below are calculated performance figures for a segmentation density of 28 segments per halfwave. Mismatch loss is due to SWR. Wire loss is due to conductor resistance. Mismatched gain is forward gain including wire and mismatch losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane. The SWR reference impedance is 75Ω. These results are for the U.S. version.
88.000 MHz: Impedance 59.1 - j17.3 ohms
SWR 1.42
Mismatch Loss 0.13 dB
Wire Loss 0.01 dB
Mismatched Gain 5.24 dBd
F/R 20.35 dB
93.000 MHz: Impedance 92.7 - j11.3 ohms
SWR 1.29
Mismatch Loss 0.07 dB
Wire Loss 0.01 dB
Mismatched Gain 5.22 dBd
F/R 21.15 dB
98.000 MHz: Impedance 93.1 - j4.8 ohms
SWR 1.25
Mismatch Loss 0.05 dB
Wire Loss 0.01 dB
Mismatched Gain 5.72 dBd
F/R 20.34 dB
103.000 MHz: Impedance 77.5 + j18.2 ohms
SWR 1.27
Mismatch Loss 0.06 dB
Wire Loss 0.02 dB
Mismatched Gain 6.62 dBd
F/R 20.56 dB
108.000 MHz: Impedance 65.8 + j16.8 ohms
SWR 1.31
Mismatch Loss 0.08 dB
Wire Loss 0.06 dB
Mismatched Gain 7.19 dBd
F/R 20.35 dB
The Yagi is small enough to make stacking practical in many situations. Stacking two antennas horizontally with the booms 90″ apart increases forward gain 2.6 dB (2.3 dB with power combiner loss) and greatly narrows the main beam. I chose this stacking distance to keep the first sidelobes 20 dB down at 98 MHz. Vertical stacking does not work well because high mutual coupling degrades the azimuth pattern.
Small Yagi - U.S. Version Free Space Symmetric 88 90 92 94 96 99 102 105 107 108 MHz 5 6063-T832 wires, inches ang = 21.40216 r = 32.2135 de = 29.64291 d1 = 26.09838 d2 = 25.13815 d3 = 22.64967 dep = 18.37341 d1p = 23.94593 d2p = 37.8247 d3p = 66.01822 1 0 0 0 0 r 0 .375 shift x dep rotate z -ang 1 0 0 0 0 de 0 .375 rotate end shift end 1 d1p 0 0 d1p d1 0 .375 1 d2p 0 0 d2p d2 0 .375 1 d3p 0 0 d3p d3 0 .375 1 source Wire 2, end1 28 segments/halfwave best matches NEC Trade-offs: 20% gain, 80% F/B Frequency weighting: average gain, worst-case F/B Enable bent-wire correction Small Yagi - European Version Free Space Symmetric 87.5 90 92 94 96 99 102 105 107 108 MHz 5 6063-T832 wires, mm ang = 20.50024 r = 823.4559 de = 756.4987 d1 = 663.8234 d2 = 639.4972 d3 = 574.6371 dep = 470.0715 d1p = 613.0242 d2p = 953.0939 d3p = 1666.598 1 0 0 0 0 r 0 10 shift x dep rotate z -ang 1 0 0 0 0 de 0 10 rotate end shift end 1 d1p 0 0 d1p d1 0 10 1 d2p 0 0 d2p d2 0 10 1 d3p 0 0 d3p d3 0 10 1 source Wire 2, end1 28 segments/halfwave best matches NEC Trade-offs: 20% gain, 80% F/B Frequency weighting: average for gain, worst-case for F/B Enable bent-wire correction
Bracketed dimensions that follow are for the European version. Use ⅜″ [10mm] tubing supported by nonconductive mounting brackets. Symbols r, de, d1, d2, and d3 are element half-lengths (center to tip), dep, d1p, d2p, and d3p are element positions (center to center) relative to the reflector position of 0, and ang is the driven-element angle. Split the driven element leaving a gap no larger than ¼″ [6mm], angle each half 21.4° [20.5°] so that the tips are 7 9⁄16″ [205mm] center-to-center from the reflector, and feed with 75Ω coax. At the feedpoint install a current balun. Keep the stripped coax leads as short as possible.
88–108 MHz