Small Five-Element Yagi

This small Yagi has 5.27.2 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 87.5108 MHz. It is 27.5 cm shorter than a Triax FM 5.

HS Publications sells a version of this antenna that uses ″ elements. The performance figures and graphs are valid only for ⅜″ or 10 mm elements.

I designed the antenna using 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. A ten-element version is here.

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.

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

Antenna Comparison

The Antennacraft FM6 requires a 75:300Ω balun with long leads and a typical loss of 0.85 dB. The small Yagi has a 75Ω feedpoint and uses a current balun. 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 log-Yagi is described here.

Ground Effects

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

Driven Element

Using the same performance trade-offs and boom length, I reoptimized the design with a straight driven element. These curves show the benefit of the bent element.

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 ferrite 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

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
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, dep, d1p, d2p, and d3p are element positions relative to the reflector, and ang is the driven element angle. Split the driven element leaving a gap no greater than ″ (Europe: 6 mm) and angle each half 20.4 (Europe: 20.1) so that the tip axis is 6″ (Europe: 154 mm) from the reflector axis. Feed directly with 75Ω coax and use a current balun. 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 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 250300 km away in northern Germany almost like locals.

Ivan Dias Jr. built this antenna in Sorocaba, So 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 m 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 m nonconductive mast to receive any polarization with this portable setup in Duisburg-Rheinhausen, Germany.

Petr Vozr 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 at his holiday address near Sareiser Joch, Liechtenstein.

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

David Bunyan used clip-on ferrite chokes for this Yagi in Sittingbourne, Kent, England.

Roland Nogell reports that the main beam narrowed greatly when he added a second Yagi.

Glenn Davis erected this stack in Hutto, Texas.


April 7, 201688108 MHz