On 31 May 2004 15:28:40 GMT, (Fractenna) wrote:

|I'm just about getting around to making my 5 or 6 element marine band yagi,
|covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole or
|can I use just a simple hertz dipole as the "driven" element? I'm trying to
|get away from using a folded dipole if I can. Any wideband designs out there
|that use a simple hertz dipole that I can scale? I'm looking, but if you
|have some links aready. TIA.
|
|
|A folded dipole as the driven element will have the best match over your
|frequency range..

Unless of course it doesn't.
|
|What you do have to worry about is the feedpoint resistance. Some Yagi-Uda
|designs are expressly designed for (a driven) folded dipole, which in isolation
|have a much higher feed resistance than 50 ohms. When you put parasitics in the
|near field, in certain gain-optimized Yagi-Uda designs, that feed resistance
|falls closer to 50 ohms.
|
|Typically, a non-matched regular dipole as driven has a feedpoint resistance of
|much less than 50 ohms in a high gain Yagi-Uda app.

Unless it doesn't. For the example I offered earlier:


Freq R X VSWR Gain F/B dB

156 51.50 1.97 1.050 9.95 14.34
157 50.12 2.77 1.057 9.99 16.43
158 49.34 5.22 1.112 10.04 19.20
159 50.38 8.31 1.180 10.08 23.41
160 54.16 9.95 1.230 10.10 30.84
161 59.78 5.68 1.229 10.07 28.01
162 57.99 -8.24 1.237 9.96 21.35