The Cushcraft 40m yagi has 43

DESIGN CONSTRAINTS

The VE6WZ QTH is a small city lot about 100’ X 60’ and this put limits on boom and element length. The tower is offset only 14’ from the eastern neighbor, so the maximum boom length possible was 28’. Using 66’ long elements it was possible to keep the antenna within the VE6WZ "air-space". (when not QRV the crank-up tower is kept retracted at the 30' level). Modeling shows that this close element spacing still achieves acceptable gain and F/B figures. The feedpoint impedance and bandwidth are also manageable.

At left is shown the very approximate half-element dimensions. The 40m elements are spaced 22', and the 80m elements have 28' spacing.

MININEC MODELING

NEC4WIN was used to model the yagi. A full size 2 el model was first created to confirm that the short boom design would yield an acceptable pattern. The 40m elements were added to the model, and the 80m elements were shortened to 66’ and loaded with the necessary inductance of about 30 uH to tune them to the required resonance points. A series 1-ohm resistance was included at each coil to approximate the coil loss. Element diameters where selected to closely reflect the final mechanical design of the aluminum tubing used. The reflector element was "tuned" for maximum F/B close to the desired minimum SWR frequency (3795 kHz). Plots of Gain, F/B and SWR vs. frequency were created to analyze the bandwidth of the model. Azimuth and elevation plots were made to analyze the far-field pattern of the antenna at a height of 100’. These plots and patterns are all shown below.

SWR VS. FREQUENCY AND IMPEDANCE CHART

At 3.786 MHz   Z = 13.40 - j 11.57 SWR = 2.210
At 3.787 MHz   Z = 14.14 - j 10.16 SWR = 1.982
At 3.788 MHz   Z = 14.86 - j 8.88 SWR = 1.798
At 3.789 MHz   Z = 15.64 - j 7.58 SWR = 1.631
At 3.790 MHz   Z = 16.45 - j 6.35 SWR = 1.490
At 3.791 MHz   Z = 17.33 - j 5.10 SWR = 1.361
At 3.792 MHz   Z = 18.20 - j 3.94 SWR = 1.254
At 3.793 MHz   Z = 19.08 - j 2.80 SWR = 1.163
At 3.794 MHz   Z = 19.95 - j 1.74 SWR = 1.091 Resonance
At 3.795 MHz   Z = 20.87 - j 0.69 SWR = 1.056 Resonance
At 3.796 MHz   Z = 21.79 + j 0.28 SWR = 1.091 Resonance
At 3.797 MHz   Z = 22.73 + j 1.23 SWR = 1.151 Resonance
At 3.798 MHz   Z = 23.68 + j 2.12 SWR = 1.215
At 3.799 MHz   Z = 24.59 + j 2.99 SWR = 1.279
At 3.800 MHz   Z = 25.53 + j 3.78 SWR = 1.344
At 3.801 MHz   Z = 26.47 + j 4.56 SWR = 1.409
At 3.802 MHz   Z = 27.40 + j 5.27 SWR = 1.470
At 3.803 MHz   Z = 28.29 + j 5.96 SWR = 1.531
At 3.804 MHz   Z = 29.14 + j 6.57 SWR = 1.588
At 3.805 MHz   Z = 29.99 + j 7.20 SWR = 1.645
At 3.806 MHz   Z = 30.85 + j 7.75 SWR = 1.700
At 3.807 MHz   Z = 31.66 + j 8.27 SWR = 1.752
At 3.808 MHz   Z = 32.42 + j 8.78 SWR = 1.801
At 3.809 MHz   Z = 33.18 + j 9.27 SWR = 1.851
At 3.810 MHz   Z = 33.90 + j 9.72 SWR = 1.897
At 3.811 MHz   Z = 34.62 + j 10.15 SWR = 1.943
At 3.812 MHz   Z = 35.29 + j 10.55 SWR = 1.985
At 3.813 MHz   Z = 35.93 + j 10.96 SWR = 2.027

Azimuth Total Field at 3800	Zenith Total Field at 3800

Azimuth Total Field at 3790	Zenith Total Field at 3790

Azimuth Total Field at 3785	Zenith Total Field at 3785