Re: Is this a loading coil?

James Barrett wrote:

Hi, I'm not really sure what all of that means. I'm learning about
antennas, and I've been doing a lot of reading on the web until I can
get a copy of the ARRL Antenna book. I'm learning the terminology and
the other day read a thread about loading coils. I had no idea what that
was, so I went in search of an explanation. So, when I saw the radio
shop selling antennas with funny looking coils I wondered if that's what
a loading coil looked like. But I had expected a loading coil to be at
the bottom of the antenna, these looked like they might be in the middle
of the antenna. Does that even work?
. . .

The ARRL Antenna Book is a very good choice.

The function of a loading coil, as you've probably learned, is to add inductive reactance at the feedpoint to cancel the capacitive reactance normally present when the antenna is short. This results in a purely resistive feedpoint impedance, which is the definition of resonance. There's nothing magic about resonance insofar as antenna radiation characteristics are concerned, but it often makes the antenna easier to match to the transmitter.

You can put the loading coil anywhere along the length of the antenna. As you move it higher, two things happen. The first is that the current it's carrying decreases. This decreases the coil loss, which is proportional to I^2. The other thing is that it has less effect on the feedpoint reactance, so you need more inductance in the coil. This increases the coil loss, which is approximately proportional to the length of wire making up the coil (all else being equal). The net result is that the overall coil loss is usually minimum at somewhere around the middle of the whip, which is why center loading is often used. Another benefit of center loading is that it might move the coil away from objects which it can couple to in a way as to increase its loss.

That being said, W8JI has pointed out that at least on the lower frequency bands, ground loss is so much greater than coil loss, that there's no point in going to much trouble to minimize coil loss unless it's outrageously high to begin with. This would have to be determined on a case-by-case basis.

Have fun!

Roy Lewallen, W7EL
.

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