Small Five-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 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 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.

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.

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 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

H-Plane Stacking

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.

Home Depot Version

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.

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		; 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 71116″ (Europe: 189 mm) from the reflector axis. Use 75Ω coax with a current balun at the feedpoint. Keep the stripped coax leads as short as possible.

Sensitivity Analysis

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

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 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 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 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.

David Bunyan used clip-on ferrite chokes for this Yagi in Sittingbourne, Kent, England. Plastic end caps dielectrically load the elements and should not be used.

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 vertically polarized stack in Plock, Poland.


June 7, 201888–108 MHz