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Author Topic: What's wrong with end-loading an antenna?  (Read 2583 times)
AF8F
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Posts: 32




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« on: December 04, 2011, 07:47:30 PM »

I've been playing with 4nec2 and it doesn't like placing loading coils at the ends of wires.

There are not any antennas sold with inductance coils at their ends.

To me it seems that placing loading coils at the ends is the most efficient place, because more current flows at the at the feedpoint or base of an antenna.

Of course this would not be practical for a mobile antenna.  But how about for a dipole?

What's wrong with placing coils at the end?  Does anyone have insight into this matter?

Thanks for your help.
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WB6BYU
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Posts: 13573




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« Reply #1 on: December 04, 2011, 09:01:34 PM »

The effect of the loading inductance varies with the current through it, or (if you prefer) with
the amount of wire beyond it.  The end of an antenna has very little current flowing.
Theoretically it would require an infinite inductance.  In practice there is some capacitance
between the end of the coil and the rest of the universe, so the current isn't zero and the
coil has some effect.  For the same length antenna, the closer the coil is to the feedpoint
the smaller it has to be for the same resonant frequency.

However, there ARE many commercial antennas sold that have coils at the end.  Most of the
fiberglass helical mobile antennas are just that if they don't have an adjustable whip on the
end.  I have a pair of Heli-Whips for 40m and 80m that are basically a coil at the top of the
rod, and many CB whips are built similarly.

Oh, and the ARRL had a design for a helical 160m antenna that was just a continuous coil.
They recommended putting an aluminum pie plate or some other capacity hat on the top
to keep it from turning into a large Tesla coil.

But if 4nec2 doesn't model parasitic capacitances, there is no way for it to calculate
any current through a coil at the end of an antenna.  You should, however, be able to put
it in the middle of the end segment.  Try it - see how much loading inductance it requires,
then move the coil one segment closer to the feedpoint and try again.
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W5DXP
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« Reply #2 on: December 05, 2011, 06:06:34 AM »

I've been playing with 4nec2 and it doesn't like placing loading coils at the ends of wires.

Try two feet of wire feeding a large top (end) hat on the end of the coil. That works great. I won a CA 75m mobile shootout using a CB whip with the loading coil at the end followed by a top hat.
--
73, Cecil, w5dxp.com
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W8JI
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« Reply #3 on: December 05, 2011, 11:44:43 AM »

What's wrong with placing coils at the end? 

1.) Bandwidth

2.) Efficiency

What you really want is a lot of capacitance at the open end. Then the coil can go anywhere, because current can be almost made uniform.
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W4OP
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« Reply #4 on: December 05, 2011, 05:18:50 PM »

Coils at the ends of a dipole tend to behave  like capacity hats.
As you move the coil out from the center, more and more turns are required.
Use a capacity hat to draw the current out.

Dale W4OP
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