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I'm looking at dual band (2 m & 70 cm) antenna designs for my next project. A very interesting one to me is the Mighty Woof, which is essentially a fan dipole made of copper pipe.
Would the extra elements in this antenna affect the radiation pattern at all, or should it have essentially the same pattern as a regular dipole?
The Mighty Woof article seems to indicate that those are 1/2WL fan dipoles. Two fan dipoles will interfere if one of the dipoles is resonant on both bands. Unfortunately, 144-148 times three is 432-444 so a 1/2WL dipole on 2m will be a resonant 3/2WL "dipole" in parallel with a 1/2WL dipole on 70cm. It would be akin to having a fan dipole for 40m/15m on HF which I just modeled and the radiation pattern is cloverleaf. Most hams don't attempt a 40m/15m fan dipole - they just use the 40m dipole on 15m and live with the cloverleaf pattern. Seems a 1/2WL horizontal dipole used on 2m would work on 70cm the same way. If it is rotatable, you would have to remember to aim it 45 degrees from broadside on 70cm. However, if this antenna is used as a vertical, it will suffer the same fate as using a 2m 1/4WL vertical on 70cm, i.e. the take off angle will be high, something that considerably reduces the effectiveness on VHF/UHF.
Basically, fan dipoles should have similar patterns, as only one dipole is resonant at a time.
However, individual dipoles in any arrangement where they all share a common feedpoint should be spaced far enough apart from each other so as to minimize the interaction between them. Whether the design that you mention has them far enough apart would have to be determined by modeling.
Note that in this case, they are nearly harmonically related (3*146.6=440).
