“Magnetic Loop Antenna”


Chapter 1: Introduction

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Magnetic Loop Antenna
Source: Field Antenna http://www.purple.dti.ne.jp/fieldant/index.html
“©2013 Munekazu Yoshimura, used with permission

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 Wave has 2 components
Electromagnetic Field has two component
Dipole is an E-field type antenna
Dipole antenna is an E-field antenna

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.

What’s the Difference?
So there are two types of antenna, one generates E-field, the other, M-field. I said earlier that E-field begets the M-field, M-field begets E-field. If they are neck-and-neck, what is the difference? To explain that, I’ll have to talk about far-field and near-field.
When we talk about gain and directionality of antenna, we are always talking about far-field effect. I’m talking about distance between the antenna and the “other station”, miles and miles away. Where is the border between the near and far? For a practical purpose, 1 wave length in radius. That's how far the initial field is dominant.
It is difficult to imagine the complex behavior of near-field (see the image below: rather chaotic, is it no?). But imagine this: Within one wave length of a loop antenna, the emitted EM field is mostly magnetic, before the E-field is induced by it.
Near Field
Near Field plot of a loop antenna
We are more familiar with E-field behavior. It is easily bent, shielded, and/or attenuated with nearby conductors, i.e., metal surfaces. That’s why E-field antenna behaviors (radiation, directionality, SWR) are affected by its surroundings. We are not as accustomed to M-field. It is not affected by the surrounding metal. You can bend, but not as easily shielded, like E-field. It passes through most conductive object. It is affected by magnetic materials, but generally you don’t have much magnets around your household. Even if you have electric motors, compressors, etc., they are very concentrated in a small area, which are not likely to interfere with an antenna in a significant way.
This is why the magnetic antenna can be tuned to optimum match and radiation, mostly independent of its surroundings. Magic, isn’t it?
So What Makes an Antenna Magnetic?
You heard about the “small” magnetic antenna. What makes it magnetic, and why can that be so small? Well the truth is, it’s magnetic because it’s small.
An E-field antenna generates E-field using the voltage difference between points on the antenna element (wire). This is why its antenna element is related to its wavelength.
Current distribution on Mag Loop
Current distribution around loop antenna
Source: https://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm
©2012 Frank Doerenberg N4SPP, used with permission

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...

Home Chap 1
Introduction
Chap 2
How does it work?
Chap 3
Practical Aspect
Chap 4
Automating (pt1)
Chap5
Automating (pt2)
 
 
 
 
  Copyright 2017, Tak Asami / W6SI & OSO Technology