“Magnetic Loop Antenna”

Chapter 1: Introduction

You heard so much about this mysterious small loop antenna that works on all HF bands. A small size. Like invisible to the HoA. Is that for real? How does it work?  Does it really actually work?  How can it be so small?
Several products claim to be magnetic loops are available in the market, without really answering those questions.
Let me assure you: The antenna is REAL. The loop antenna on the left, of diameter 1 meter, weighs about 1kg, covers all ham bands from 40m to 10 m. And works amazingly well for its size.
In terms of transmit gain, a beam antenna on top of a tall tower might easily beat it, but for many of us with restricted antenna space, this could be a great solution. Perhaps an only one. When it comes to listening, this tiny loop might beat some good big antenna, by cutting through noise and interference.

The purpose of this writing is to answer these questions and explain what this mysterious antenna is all about, without getting too theoretical.

First, what is an antenna?
Antenna is an interface between a transmitter / receiver and a propagation medium (typically air). Simply stated, it converts RF energy generated by the transmitters to electromagnetic field. That EM field propagates the distance, depending on its frequency and the ionospheric weather in the sky.

Electromagnetic Field?

I mentioned “electromagnetic field. You might be familiar with the term electric field, what is electromagnetic field? Well they are the same thing. Electric field and magnetic field are light and shadow, ying and yang, different faces of the same phenomena. They always go neck and neck, as in one begets the other, and vice versa.

Two Antenna Types
To generate this EM field, there are two types of antennas. One is an E-field type, that generates electric field, which kicks off the EM-field train. Dipole antenna, and its derivatives including vertical and Yagi beams belong to this type. That’s like 99% of all the antenna we have experience dealing with.
The other type, generates magnetic field, to start the train. That’s the “magnetic” antenna, and the small loop belongs to this type.

An M-field antenna generates M-field using the current flowing through its element. To do this, the current must be (close to) constant across the element. Therefore its element length must be 10% of the wavelength or less. As you can see on the diagram above, 0.1 wave length loop keeps 95% of peak current at the edges. Longer loop has smaller current at the edges, resulting in more E-field effect mixed. Of course, It is NOT shorter the better situation. For one thing, just a wire of 0.1 wavelength cannot be matched to your transmitter. Matching mechanism must be added. But let’s keep that for a later discussion.

What’s So Good about Magnetic Antenna?
As it was mentioned, M-field antenna is not as easily affected by the environment. So it can be “tuned” to a near-perfect match in most cases. That’s a unique advantage of this type of antenna.
Due to its very high-Q characteristics of the antenna, this near-perfect match tends to be very narrow-banded. That sounds bad, but it also helps in that the antenna itself acts like a filter, so both transmission and receiving spurious signals are suppressed. Yes, it actually enhances the dynamic characteristics of your receiver.
That’s not all. Most of your receiving noise are man-made, and coming from you and your neighbor’s. I mentioned that a near-field effect extends to one wavelength. For 20 meter band, that’s 20 meter in every direction. Around here in high density residential area in Southern California, it is likely enough to cover your household and good deal of your neighbor’s.
Now, I bluntly mention that most of the man-made noise is E-field in nature. M-field antenna is impervious to E-field noise, due to its dual property. That is, it generates M-field, and receives M-field. This is the reason for the low noise characteristics of a magnetic antenna.
So that’s the receiving advantage. How about transmission? Well, there is. But for now, that remains to be a subject of a future topic discussion.

Questions, comments, opinions, etc. all e-mail to me

To be Continued...