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I currently use a homebrew 40/60/80m vertical consisting of a 5M aluminium tube with a multi tapped coil at the base. The ground plane is the metal roof at my house, and the antenna is not centered in it, but it's next to one of the edges.

I read that for center loading, the coil needs to be twice as big as for base loading. So I was thinking of adding another 5M length of aluminium tube and raising the loading coil to the center of the antenna. Since the required coil will be twice as big, but the antenna is now twice as long, the coil will be the same.

What I'd like to know is if there will be significant benefit of center loading the antenna, considering the hassle of raising a 10m tall radiator with a coil in the middle.

Also, how is the radiation pattern affected by not centering the antenna in the ground plane?

hjf
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2 Answers2

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In this context, loading is used to make an electrically short antenna resonant.

In an electrically short vertical, moving the loading higher is almost always an improvement because it raises the point of maximum current, where the majority of the radiation takes place.

Center is better than base loading and top loading is superior to center loading.

Top loading is usually accomplished by adding capacitance to the top, rather than inductance.

If you move the loading, don't expect the taps to be in the right place. You'll need some other way of tuning the antenna at the base, and not the tapped coil.

In your case, with the vertical all the way to the end of your metal roof peak, the gain will be strongest towards the roof and weakest away from the roof.

Mike Waters
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As a rough rule of thumb, moving inductive loading from near the feed point to near halfway up an antenna of the same dimension might improve radiation resistance by somewhere around 2X, minus added resistive losses.

But for antenna’s much shorter than lambda/4, radiation resistance goes up with roughly somewhere proportional to the square of the length. So your biggest gain will be from the 2X taller vertical, not the repositioning of the loading inductor.

hotpaw2
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