Small Wideband Yagi

If you don't need high performance, the Antennacraft FM6 is hard to beat for value. But you may want a little more gain or a better pattern, or maybe you just want something to build for fun. The Yagi described here has 5.2–7.2 dBd forward gain. All backlobes are more than 20 dB down across the FM broadcast band. The antenna is about the same size as the FM6, with five elements on a 66″ boom. I include metric dimensions for a European version optimized for 87.5–108 MHz.

I designed the antenna using the global optimizer and response flattener of the AO 8.07 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 the Triax FM 5, a small, five-element European Yagi with an assumed balun loss of 0.75 dB.

Paul Logan in Lisnaskea, Fermanagh, Ireland, uses this commercial version of the antenna manufactured by VHF Teknik AB in Trelleborg, Sweden. They call it the FM5.1. It uses a ferrite choke balun.

Sven Jacobson of VHF Teknik AB installed a vertically polarized prototype antenna at his home in Ljunghusen, Sweden.

Ivan Dias Jr. built this antenna in Sorocaba, São Paulo, Brazil. The feedpoint box contains a coiled-coax balun.

Cedric Lamouche, F4EGZ, installed this antenna 7m up a tapered fiberglass mast in Domerat, France. He used a coiled-coax balun.

Hans-Peter Dohmen, DL9EBA, uses a hinged mount, rope, and 4.8m 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.

Modeling Results

Below are calculated performance figures for a segmentation density of 28 segments per halfwave. Mismatch loss is due to SWR. Wire loss is due to conductor resistance. Mismatched gain is forward gain including wire and mismatch losses. F/R is the ratio of forward power to that of the worst backlobe in the rear half-plane. The SWR reference impedance is 75Ω. These results are for the U.S. version.

88.000 MHz:   Impedance          59.1 - j17.3 ohms
              SWR                 1.42
              Mismatch Loss       0.13 dB
              Wire Loss           0.01 dB
              Mismatched Gain     5.24 dBd
              F/R                20.35 dB

93.000 MHz:   Impedance          92.7 - j11.3 ohms
              SWR                 1.29
              Mismatch Loss       0.07 dB
              Wire Loss           0.01 dB
              Mismatched Gain     5.22 dBd
              F/R                21.15 dB

98.000 MHz:   Impedance          93.1 - j4.8 ohms
              SWR                 1.25
              Mismatch Loss       0.05 dB
              Wire Loss           0.01 dB
              Mismatched Gain     5.72 dBd
              F/R                20.34 dB

103.000 MHz:  Impedance          77.5 + j18.2 ohms
              SWR                 1.27
              Mismatch Loss       0.06 dB
              Wire Loss           0.02 dB
              Mismatched Gain     6.62 dBd
              F/R                20.56 dB

108.000 MHz:  Impedance          65.8 + j16.8 ohms
              SWR                 1.31
              Mismatch Loss       0.08 dB
              Wire Loss           0.06 dB
              Mismatched Gain     7.19 dBd
              F/R                20.35 dB

Patterns

Stacked Antennas

The Yagi is small enough to make stacking practical in many situations. Stacking two antennas horizontally with the booms 90″ apart increases forward gain 2.6 dB (2.3 dB with power combiner loss) and greatly narrows the main beam. I chose this stacking distance to keep the first sidelobes 20 dB down at 98 MHz. Vertical stacking does not work well because high mutual coupling degrades the azimuth pattern.

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 = 21.40216
r = 32.2135
de = 29.64291
d1 = 26.09838
d2 = 25.13815
d3 = 22.64967
dep = 18.37341
d1p = 23.94593
d2p = 37.8247
d3p = 66.01822
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

28 segments/halfwave best matches NEC
Trade-offs: 20% gain, 80% F/B
Frequency weighting: average gain, worst-case F/B
Enable bent-wire correction


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 = 20.50024
r = 823.4559
de = 756.4987
d1 = 663.8234
d2 = 639.4972
d3 = 574.6371
dep = 470.0715
d1p = 613.0242
d2p = 953.0939
d3p = 1666.598
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

28 segments/halfwave best matches NEC
Trade-offs: 20% gain, 80% F/B
Frequency weighting: average for gain, worst-case for F/B
Enable bent-wire correction

Bracketed dimensions that follow are for the European version. Use ⅜″ [10mm] tubing supported by nonconductive mounting brackets. Symbols r, de, d1, d2, and d3 are element half-lengths (center to tip), dep, d1p, d2p, and d3p are element positions (center to center) relative to the reflector position of 0, and ang is the driven-element angle. Split the driven element leaving a gap no larger than ¼″ [6mm], angle each half 21.4° [20.5°] so that the tips are 7 916″ [205mm] center-to-center from the reflector, and feed with 75Ω coax. At the feedpoint install a current balun. Keep the stripped coax leads as short as possible.


July 11, 201088–108 MHz