This small 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. It is 4″ shorter than an Antennacraft FM6. I also optimized a European version with metric dimensions for 87.5–108 MHz. It is 27.5 cm shorter than a Triax FM 5.
I designed the antenna with the AO 9.00 Antenna Optimizer. This image shows the antenna geometry. The red dot is the 75Ω feedpoint. The bent driven element greatly improves the pattern. Results below are for the U.S version.
Calculated performance is for 28 analysis segments per conductor 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.
Frequency Impedance SWR Mismatch Conductor Forward F/R
MHz ohms Loss dB Loss dB Gain dBd dB
88 57.4-j23.1 1.55 0.21 0.02 5.21 20.18
89 68.4-j19.8 1.34 0.09 0.01 5.26 21.91
90 77.3-j17.8 1.27 0.06 0.01 5.25 22.20
91 84.1-j17.3 1.28 0.07 0.01 5.23 22.15
92 89.1-j17.3 1.31 0.08 0.01 5.22 21.77
93 92.7-j17.4 1.34 0.09 0.01 5.24 21.41
94 94.9-j17.3 1.37 0.10 0.01 5.29 21.09
95 96.0-j17.1 1.37 0.11 0.01 5.36 20.46
96 95.9-j16.5 1.37 0.11 0.01 5.47 20.20
97 95.1-j15.2 1.35 0.10 0.01 5.60 20.11
98 93.2-j13.8 1.31 0.08 0.01 5.76 20.23
99 90.4-j11.7 1.26 0.06 0.01 5.93 20.45
100 86.7-j8.7 1.20 0.04 0.01 6.13 20.77
101 82.3-j4.8 1.12 0.01 0.01 6.34 20.86
102 77.8+j0.5 1.04 0.00 0.01 6.55 20.94
103 73.6+j7.2 1.10 0.01 0.02 6.74 20.98
104 70.3+j15.5 1.25 0.05 0.02 6.90 21.00
105 69.2+j24.3 1.41 0.13 0.02 7.01 21.01
106 71.1+j31.6 1.54 0.20 0.03 7.10 21.06
107 77.1+j31.5 1.51 0.18 0.04 7.20 20.59
108 70.7+j11.9 1.19 0.03 0.06 7.26 20.52
The Antennacraft FM6 requires a 300Ω balun with long leads and a typical loss of 0.85 dB. The small Yagi has a 75Ω feedpoint and uses a current choke. The modified FM6 has one longer and two shorter elements, shunt feedpoint inductor, and halfwave coaxial balun. The Triax FM 5 is a five-element European Yagi with a halfwave PCB balun (0.1 dB assumed loss). The 2-meter Yagi is described here.
I optimized the design in free space. These curves show how ground proximity affects the pattern.
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 ferrite power combiner. (Both sections use an older antenna design that differs little from 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
Stacking horizontal Yagis in the vertical plane 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 feet 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 feet, and to 2.3 dB at 50 feet. 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.
Instead of ⅜″ tubing, you can use 6063-T5 aluminum angle from Home Depot. The 0.5″ × 0.5″ right-angle shape is electrically equivalent to a 0.4″ round conductor. Use 1˝″ ABS pipe (1.9″ OD) for the boom. Mount the elements with the vertical face to the rear using PVC conduit straps. Attach each parasitic element to the strap with two sheet metal screws. Drill a hole in the strap and secure it to the boom with another screw. Use two straps to hold a flat nonconductive plate and attach the driven element halves with sheet metal screws. I reoptimized the design so that the shortest two directors can be cut from one 96″ length of aluminum angle. The antenna requires four lengths total.
Small Yagi Free Space Symmetric 88 90 92 94 96 99 102 105 107 108 MHz 5 6063-T832 wires, inches ang = 20.38894 ; driven element angle r = 32.24072 ; element half-lengths de = 29.5168 d1 = 26.17286 d2 = 25.25078 d3 = 23.04833 rp = 0 ; element positions dep = 17.95156 d1p = 23.16246 d2p = 36.11559 d3p = 63.69878 1 rp 0 0 rp r 0 .375 rotate end1 z -ang 1 dep 0 0 dep de 0 .375 rotate 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 - Home Depot Version Free Space Symmetric 88 90 92 94 96 99 102 105 107 108 MHz 5 6063-T832 wires, inches ; actually 6063-T5 ang = 20.93938 ; driven element angle r = 32.22576 ; element half-lengths de = 29.67043 d1 = 26.11403 d2 = 25.14271 d3 = 48 - d2 ; d3 = 22.85729 rp = 0 ; element positions dep = 18.30026 d1p = 23.71732 d2p = 37.29313 d3p = 65.11169 1 rp 0 0 rp r 0 .4 rotate end1 z -ang 1 dep 0 0 dep de 0 .4 rotate end 1 d1p 0 0 d1p d1 0 .4 1 d2p 0 0 d2p d2 0 .4 1 d3p 0 0 d3p d3 0 .4 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 ; driven element angle r = 823.0353 ; element half-lengths de = 752.8616 d1 = 665.5374 d2 = 641.4514 d3 = 582.9676 rp = 0 ; element positions dep = 448.32 d1p = 578.4351 d2p = 897.7509 d3p = 1597.723 1 rp 0 0 rp r 0 10 rotate end1 z -ang 1 dep 0 0 dep de 0 10 rotate 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 ⅜″ (Home Depot: ˝″ × ˝″ L, Europe: 10 mm) elements mounted through a nonconductive boom or supported by insulated mounting brackets. Split the driven element leaving a gap no greater than Ľ″ (Europe: 6 mm) and angle each half so that the tip axis is 711⁄16″ (Europe: 189 mm) from the reflector axis. Use 75Ω coax with a current choke at the feedpoint. Keep the stripped coax leads as short as possible. Read these notes before building anything.
The following table shows the largest performance degradation over 88, 93, 98, 103, and 108 MHz in dB for the U.S. version when altering a symbol value by Tol.
Symbol Tol Gain F/R
ang 1.0000 0.02 0.70
r 0.0197 0.01 0.32
de 0.0197 0.01 0.00
d1 0.0197 0.02 0.05
d2 0.0197 0.01 0.17
d3 0.0197 0.01 0.03
rp 0.0394 0.00 0.04
dep 0.0394 0.01 0.06
d1p 0.0394 0.01 0.05
d2p 0.0394 0.00 0.06
d3p 0.0394 0.00 0.01
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.
Sven Jacobson installed a vertically polarized Yagi in Ljunghusen, Sweden.
Sven says this horizontal antenna receives stations 250–300 km away in northern Germany almost like locals.
Ivan Dias Jr. built this antenna in Sorocaba, Săo Paulo, Brazil. The feedpoint box contains a coiled-coax choke. The plastic end caps may detune the elements and add loss.
George S. Martins, PU7MAN, used a PVC boom for this antenna in Iguatu, Ceará, Brazil.
Cedric Lamouche, F4EGZ, installed this antenna 7 m up a tapered fiberglass mast in Domerat, France. He used a coiled-coax choke.
Hans-Peter Dohmen, DL9EBA, uses a hinged mount, rope, and 4.8 m nonconductive mast to receive any polarization with this portable setup in Duisburg-Rheinhausen, Germany.
Petr Vozár erected this vertically polarized Yagi near Javornik, Czech Republic.
Roland Nogell uses this antenna at his summer house near Lysekil, Sweden.
Jeff Mein, KP3FT, used aluminum angle elements to build this antenna in South New Berlin, New York.
Mark van Wijk, PA5MW, erected this antenna at his holiday address near Sareiser Joch, Liechtenstein.
David Bunyan used clip-on ferrite chokes for this Yagi in Sittingbourne, Kent, England. The plastic end caps may detune the elements and add loss.
Rajesh Kumar, VU2ORQ, used PVC sheet as element insulation for this Yagi in Mannarkkad, Kerala, India.
Mike Fallon mounted a vertically polarized Yagi on the boom of his Körner 19.3 in Saltdean, East Sussex, England.
Roland Nogell reports that the main beam narrowed greatly when he added a second Yagi.
Glenn Davis erected this stack in Hutto, Texas.
Konrad Kosmatka built this stack in Plock, Poland. The crossboom and guy are nonconductive. It rotates here.
Konrad's antenna with ice.
SWR at the tuner end of the feedline. Konrad said the iced antenna still worked fine.
88–108 MHz