Simple Wi-Fi Yagi

It's easy to make a small Yagi for a wireless router even if it lacks an antenna connector. The photo shows how I added two parasitic elements to the sleeve dipole of my Netgear WGR614.

With the AO 8.51 Antenna Optimizer I modeled a three-element Yagi at 2412, 2437, and 2462 MHz. These are center frequencies of the lowest, middle, and highest U.S. Wi-Fi channels. To model the sleeve dipole, I optimized the length of an isolated driven element for minimum SWR. Then I added the parasitics and optimized their lengths and positions for maximum forward gain, including mismatch loss for a 50Ω source impedance. All optimizations weighted the three frequencies equally. Finally, I rounded the dimensions. Below are calculated performance figures at the channel centers using 30 analysis segments per halfwave. Forward gain includes mismatch and conductor losses.

Channel  Frequency   Impedance      SWR   Mismatch  Conductor   Forward    F/B 
            MHz        ohms               Loss dB    Loss dB   Gain dBi    dB 
    1       2412    35.7 + j0.2     1.40     0.12      0.01      8.48     11.98
    2       2417    35.7 + j2.0     1.41     0.13      0.02      8.51     11.85
    3       2422    35.7 + j3.9     1.42     0.13      0.02      8.54     11.72
    4       2427    35.7 + j5.7     1.44     0.14      0.02      8.57     11.56
    5       2432    35.8 + j7.7     1.46     0.16      0.02      8.59     11.40
    6       2437    35.8 + j9.7     1.50     0.17      0.02      8.60     11.22
    7       2442    35.9 + j11.7    1.54     0.20      0.02      8.61     11.04
    8       2447    36.0 + j13.8    1.58     0.23      0.02      8.61     10.84
    9       2452    36.1 + j15.9    1.63     0.26      0.02      8.60     10.64
   10       2457    36.3 + j18.1    1.69     0.30      0.02      8.59     10.43
   11       2462    36.4 + j20.3    1.76     0.34      0.02      8.57     10.21

The broad forward lobe makes aiming noncritical. The antenna in the photo is aimed to the right.

Construction

I cut a piece of plastic from a yogurt container top to support the parasitic elements. I folded the plastic and cut slits for the sleeve dipole. Then I drilled holes for the elements, which are made of #14 copper wire. The reflector is 2¼″ long and spaced 1916 from the center of the sleeve dipole. The director is 2″ long and spaced 1516. Conductor diameter affects element tuning. If you use unfolded paperclips, lengthen the director to 2116. Metric dimensions for 2.5-mm wire: reflector length 28 mm, spacing 39 mm; director length 25 mm, spacing 24 mm.

The signal to my wireless adapter card dropped 6 dB when I removed the parasitics from the router dipole. The Yagi provides a fourfold increase in power, which will double the operating range. When signal strength is the limiting factor, it will quadruple throughput.

Antenna File

3-Element Wi-Fi Yagi
Free Space
2412 2437 2462 MHz
3 copper wires, inches
diam = #14
ref = 1.125
dir = 1
rp = -1.5625
dp = .9375
1   rp  0   -ref   rp  0   ref   diam
1    0  0 -1.085    0  0 1.085    #14
1   dp  0   -dir   dp  0   dir   diam
1 source
Wire 2, center

Sensitivity Analysis

The following table shows the change in average performance over 2412, 2437, and 2462 MHz in dB when altering a single dimension by 132″ (164″ for ref and dir, which represent element half-length, and 25% for diam).

Symbol    Gain    F/B
  diam    0.13   0.92
   ref    0.04   0.38
   dir    0.09   1.34
    rp    0.01   0.17
    dp    0.04   0.62

Larger Yagi

I optimized a five-element Yagi that maintains mechanical balance with two offset reflectors. Forward gain is 2.5 dB greater than for the three-element design. This should increase range 36% or throughput 78%. These improvements may not be worth the additional complexity.

Antenna file:

5-Element Wi-Fi Yagi
Free Space
2412 2437 2462 MHz
5 copper wires, inches
diam = #14
ref = 1.125
d1 = 1
d2 = .9875
s = 1.3125
rp = -2
d1p = 1.06125
d2p = 2.75
1    rp -s   -ref    rp -s    ref   diam
1    rp  s   -ref    rp  s    ref   diam
1     0  0 -1.085     0  0  1.085    #14
1   d1p  0    -d1   d1p  0     d1   diam
1   d2p  0    -d2   d2p  0     d2   diam
1 source
Wire 3, center

Sensitivity analysis:

Symbol    Gain    F/B
  diam    0.30   4.20
   ref    0.03   1.77
    d1    0.06   3.87
    d2    0.04   1.43
     s    0.01   0.67
    rp    0.00   0.57
   d1p    0.04   0.39
   d2p    0.03   1.19

April 27, 201488108 MHz