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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?

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  • $\begingroup$ What do you mean, "not centering the antenna in the ground plane"? Is that related to the rest of your question at all? $\endgroup$
    – Phil Frost - W8II
    Sep 22, 2019 at 22:53
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    $\begingroup$ From the opening post, "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." $\endgroup$
    – Richard Fry
    Sep 24, 2019 at 7:21

2 Answers 2

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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.

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    $\begingroup$ Top loading may be the cheapest way to get more current flowing over the entirety of the vertical radiator, with the added benefit of reducing the capacitive reactance you have to match. Think of the combination as simply bending a longer radiator. This can be as simple as a wire from the top of the "radiator" to a nearby support, a la the "inverted L", but the wire doesn't have to be horizontal to provide the benefit. The top loading wire can even be trapped for multi-band operation. Multiple wires, looking more like a "hat," may make radiation more omnidirectional. $\endgroup$
    – Brian K1LI
    Sep 21, 2019 at 12:08
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    $\begingroup$ @BrianK1LI That comment is good enough for an answer! (Please?) I might add that many hams on the 160 and 80 successfully use part of the top guys as top-loading. No coil needed, and it boosts the antenna's performance. $\endgroup$
    – Mike Waters
    Sep 22, 2019 at 21:49
  • $\begingroup$ I agree with most of what you wrote, but Isn't the maximum current always going to be at the base? $\endgroup$
    – Phil Frost - W8II
    Sep 22, 2019 at 22:51
  • $\begingroup$ @PhilFrost-W8II For resonant 1/4 verticals, yes. However, it depends on the length of the antenna and how it's loaded. For example, I've modeled my 160m inverted-L in different configurations and seen how the point of maximum current moves up and away from the base. The feedpoint impedance rises as this occurs. $\endgroup$
    – Mike Waters
    Sep 23, 2019 at 13:43
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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.

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  • $\begingroup$ But the required L to bring it into resonance also goes up, so if Q doesn't change, Rcoil also goes up, diminishing the improvement... $\endgroup$
    – tomnexus
    Sep 21, 2019 at 19:57
  • $\begingroup$ @tomnexus And approximately how much larger would the inductor wire diameter have to be so as not to decrease the Q? $\endgroup$
    – Mike Waters
    Sep 21, 2019 at 23:33
  • $\begingroup$ @mikewaters I'm not sure, perhaps double? But if you make the wire thicker, that could give a lower loss base load too. $\endgroup$
    – tomnexus
    Sep 22, 2019 at 21:07
  • $\begingroup$ @tomnexus I'm not sure either, but I think you are not far off. However, that wouldn't raise the point of maximum current. $\endgroup$
    – Mike Waters
    Sep 22, 2019 at 21:41

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