Small 5-Element Yagi

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 and an electrical boom length of 64″. It is slightly smaller than an Antennacraft FM6. A European version with metric dimensions 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.

Modeling Results

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. These results are for the U.S. version.

Frequency  Impedance    SWR   Mismatch  Conductor   Forward    F/R
   MHz        ohms             Loss dB   Loss dB   Gain dBd     dB
    88   58.5 - j20.6   1.49     0.17      0.01      5.23     20.63
    89   69.4 - j17.4   1.29     0.07      0.01      5.26     21.94
    90   78.2 - j15.7   1.23     0.05      0.01      5.25     22.23
    91   84.9 - j15.2   1.25     0.06      0.01      5.22     22.12
    92   89.9 - j15.1   1.29     0.07      0.01      5.21     21.73
    93   93.4 - j15.3   1.33     0.09      0.01      5.23     21.38
    94   95.6 - j15.2   1.35     0.10      0.01      5.27     21.04
    95   96.7 - j15.0   1.36     0.10      0.01      5.35     20.43
    96   96.5 - j14.4   1.35     0.10      0.01      5.45     20.20
    97   95.7 - j13.2   1.34     0.09      0.01      5.59     20.12
    98   93.7 - j11.8   1.30     0.08      0.01      5.74     20.25
    99   90.8 - j9.7    1.25     0.05      0.01      5.92     20.48
   100   87.2 - j6.6    1.19     0.03      0.01      6.11     20.81
   101   82.7 - j2.7    1.11     0.01      0.01      6.32     20.88
   102   78.2 + j2.7    1.06     0.00      0.01      6.52     20.96
   103   74.0 + j9.5    1.14     0.02      0.02      6.71     20.98
   104   70.8 + j17.7   1.28     0.07      0.02      6.86     21.01
   105   70.0 + j26.5   1.45     0.15      0.02      6.96     21.01
   106   72.3 + j33.5   1.57     0.22      0.03      7.04     21.08
   107   78.5 + j31.7   1.51     0.18      0.04      7.15     20.61
   108   68.1 + j10.4   1.19     0.03      0.06      7.18     20.23

Antenna Comparison

When using a 75Ω feedline, the Antennacraft FM6 requires a 75:300Ω balun with long leads and a typical loss of 0.85 dB. In contrast, the small Yagi has a 75Ω feedpoint and needs only a 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 one longer and two shorter elements, shunt feedpoint inductor, and halfwave coaxial balun. The Triax FM 5 is a five-element European Yagi with an assumed halfwave PCB balun loss of 0.1 dB. The log-Yagi is described here.

Ground Effects

I optimized the design in free space. These curves show how ground proximity affects the pattern.

E-Plane Stacking

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. (Both sections 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.

H-Plane Stacking

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 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.

Antenna Files

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
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 - 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
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″ (Europe: 10 mm) elements mounted through a nonconductive boom or supported by insulated mounting brackets. Symbols r, de, d1, d2, and d3 are element half-lengths (center to tip), dep, d1p, d2p, and d3p are element positions relative to the reflector (center to center), and ang is the driven element angle. Split the driven element leaving a gap no greater than ¼″ (Europe: 6 mm) and angle each half 20.4° (Europe: 20.1°) so that the tip centers are 6Ľ″ (Europe: 154 mm) from the centerline of the reflector. Feed directly with 75Ω coax and use a 75Ω current balun at the feedpoint. Keep the stripped coax leads as short as possible.

Sensitivity Analysis

The following table shows the change in average performance over 88, 93, 98, 103, and 108 MHz in dB for the U.S. version when altering a single dimension by ⅛″ (116″ for symbols that represent element half-length and 3° for ang).

Symbol    Gain    F/R
   ang    0.01   0.18
     r    0.01   0.16
    de    0.01   0.00
    d1    0.01   0.03
    d2    0.01   0.11
    d3    0.01   0.01
   dep    0.00   0.05
   d1p    0.00   0.03
   d2p    0.00   0.02
   d3p    0.00   0.00

Gallery

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 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 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.

Petr Vozár erected this vertically polarized Yagi near Javornik, Czech Republic.

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.

Mike Fallon mounted a vertically polarized Yagi on the boom of his Körner 19.3 in Saltdean, East Sussex, England.


June 22, 201488–108 MHz