This Yagi has 5.2–7.2 dBd forward gain with all backlobes more than 20 dB down across the FM broadcast band. The antenna has five elements on a 64″ boom and is slightly smaller than an Antennacraft FM6. A European version uses metric dimensions and is optimized for 87.5–108 MHz.
I designed the antenna using the AO 9.00 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 a modified FM6, with three shorter elements, shunt feedpoint inductor, and halfwave coaxial balun. The Triax FM 5 is a small, five-element, European Yagi with an assumed halfwave PCB balun loss of 0.1 dB.
Below are calculated performance figures for a segmentation density of 28 segments per halfwave. Forward gain includes mismatch and conductor losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane. These results are for the U.S. version.
88.000 MHz: Impedance 58.2 - j20.7 Ω
SWR 1.49
Mismatch Loss 0.17 dB
Conductor Loss 0.01 dB
Forward Gain 5.23 dBd
F/R 20.50 dB
93.000 MHz: Impedance 93.3 - j15.3 Ω
SWR 1.33
Mismatch Loss 0.09 dB
Conductor Loss 0.01 dB
Forward Gain 5.23 dBd
F/R 21.39 dB
98.000 MHz: Impedance 93.8 - j11.9 Ω
SWR 1.30
Mismatch Loss 0.08 dB
Conductor Loss 0.01 dB
Forward Gain 5.74 dBd
F/R 20.25 dB
103.000 MHz: Impedance 74.1 + j9.4 Ω
SWR 1.13
Mismatch Loss 0.02 dB
Conductor Loss 0.02 dB
Forward Gain 6.71 dBd
F/R 20.98 dB
108.000 MHz: Impedance 68.9 + j10.9 Ω
SWR 1.19
Mismatch Loss 0.03 dB
Conductor Loss 0.06 dB
Forward Gain 7.19 dBd
F/R 20.30 dB
I optimized the design in free space. These curves show how ground proximity affects the pattern.
Paul Logan in Lisnaskea, Fermanagh, Ireland, uses this commercial version of the antenna once manufactured by VHF Teknik AB in Trelleborg, Sweden. It uses a ferrite choke balun.
Sven Jacobson installed a vertically polarized antenna in Ljunghusen, Sweden.
Ivan Dias Jr. built this antenna in Sorocaba, São Paulo, Brazil. The feedpoint box contains a coiled-coax balun.
George Martins, PU7MAN, used a PVC boom for this antenna in Iguatu, Ceara, Brazil.
Cedric Lamouche, F4EGZ, installed this antenna 7 meters up a tapered fiberglass mast in Domerat, France. He used a coiled-coax balun.
Hans-Peter Dohmen, DL9EBA, uses a hinged mount, rope, and 4.8-meter 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.
The Yagi is small enough to make stacking practical in many situations. Stacking two horizontal antennas side by side with the booms 90″ apart keeps the first sidelobes 20 dB down at 98 MHz. It yields the following results in free space. Subtract 0.3 dB from the gain figures in this section and the next to account for the loss of a 75Ω power combiner. (This section and the next use an older design very similar to the current design.)
88.000 MHz: Impedance 58.2 - j19.2 Ω
SWR 1.47
Mismatch Loss 0.16 dB
Conductor Loss 0.01 dB
Forward Gain 7.79 dBd +2.55 dB
F/R 18.39 dB -1.96 dB
93.000 MHz: Impedance 91.8 - j13.3 Ω
SWR 1.29
Mismatch Loss 0.07 dB
Conductor Loss 0.01 dB
Forward Gain 7.88 dBd +2.66 dB
F/R 23.30 dB +2.15 dB
98.000 MHz: Impedance 90.2 - j5.0 Ω
SWR 1.21
Mismatch Loss 0.04 dB
Conductor Loss 0.01 dB
Forward Gain 8.36 dBd +2.64 dB
F/R 21.04 dB +0.70 dB
103.000 MHz: Impedance 76.6 + j21.3 Ω
SWR 1.32
Mismatch Loss 0.09 dB
Conductor Loss 0.02 dB
Forward Gain 9.15 dBd +2.53 dB
F/R 22.37 dB +1.81 dB
108.000 MHz: Impedance 66.7 + j16.0 Ω
SWR 1.29
Mismatch Loss 0.07 dB
Conductor Loss 0.06 dB
Forward Gain 9.82 dBd +2.63 dB
F/R 21.44 dB +1.09 dB
Roland Nogell reports that the main beam narrowed greatly when he added a second Yagi. Feedlines perpendicular to the elements and a nonconductive crossboom minimize pattern disruption.
Stacking horizontal Yagis in the vertical plane is better mechanically, but it doesn't work well unless the antennas are high and well separated. Elevation patterns for the two antennas differ, and at low heights the fields tend not to combine coherently. For example, with one Yagi at 30′ and the other 116″ below, the spacing that maximizes stacking gain in free space at 3.1 dB, gain at 1° elevation over the upper antenna alone is only 1.6 dB. For the same spacing, gain increases to 2.1 dB with the upper antenna at 40′, and to 2.3 dB at 50′. Closer spacing improves the gain, but the azimuth pattern then degrades due to increased mutual coupling, as shown above.
Adverse mutual coupling similarly compromises vertically polarized small Yagis stacked horizontally, seriously degrading the backlobe suppression.
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 = 20.38894 r = 32.24072 de = 29.5168 d1 = 26.17286 d2 = 25.25078 d3 = 23.04833 dep = 17.95156 d1p = 23.16246 d2p = 36.11559 d3p = 63.69878 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 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 = 20.13522 r = 823.0353 de = 752.8616 d1 = 665.5374 d2 = 641.4514 d3 = 582.9676 dep = 448.32 d1p = 578.4351 d2p = 897.7509 d3p = 1597.723 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
Use
The following table shows the performance degradation of the U.S. version when changing a single dimension by 1⁄32″ (1⁄64″ for symbols that represent element half-length and 1° for ang). AO calculated the performance drop when increasing and decreasing a symbol value, keeping the worst results. Listed are degradations for average and worst performance over 88, 93, 98, 103, and 108 MHz. Gain includes mismatch loss (listed separately as MML). Values are in dB.
----- Average ---- ------ Worst -----
Symbol Gain F/R MML Gain F/R MML
ang 0.01 0.04 0.01 0.02 0.51 0.04
r 0.00 0.04 0.00 0.00 0.06 0.00
de 0.00 0.00 0.00 0.01 0.00 0.01
d1 0.00 0.01 0.00 0.00 0.03 0.00
d2 0.00 0.03 0.00 0.00 0.15 0.00
d3 0.00 0.01 0.00 0.00 0.00 0.00
dep 0.00 0.01 0.00 0.00 0.01 0.00
d1p 0.00 0.00 0.00 0.00 0.05 0.00
d2p 0.00 0.00 0.00 0.00 0.05 0.00
d3p 0.00 0.00 0.00 0.00 0.01 0.00
88–108 MHz