# Why does Kraus include both A and r in his equation for a small-loop antenna?

• qnach
In summary, the author discusses the use of a small loop antenna for transmitting and receiving radio waves. He uses two equations to calculate the far field of the antenna. One equation includes the area of the antenna, while the other includes the distance from the antenna to the far field point. The author states that if the area and the distance could be canceled out, it would simplify the equation. However, he keeps the area and the distance in the equation because they are both necessary for understanding the far field of the antenna.
qnach
I am reading Kraus' book on Antenna, Chapter 6.
I think Eq. (8) and (9) is his solution for the far field of a small-loop antenna.
However, why does he need A (the area) and r both in the equation (8)?
A= \pi r^2 and can be simplified?

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It would be best if you attach the equation in question.

My guess is:
"A" is the area of the loop antenna
"r" is the distance from the center of the loop antenna to the point of far field measurement

the_emi_guy said:
It would be best if you attach the equation in question.

My guess is:
"A" is the area of the loop antenna
"r" is the distance from the center of the loop antenna to the point of far field measurement
o.k. I attached that page of Eq. (8) and (9)
But I think that is no use. I know the definition of A and r
But my question is why did he keep it without canceling out.

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qnach said:
However, why does he need A (the area) and r both in the equation (8)?
A= \pi r^2 and can be simplified?
The loop radius was a. The distance from the dipole was r. The radius of the loop was NOT r.
Since a does not appear in the equation, what can you possibly cancel ?

The term A / λ2 is a pure ratio. It is the aperture in wavelengths. It appears as a recognisable term with many antenna configurations. If it could be canceled now it would have to be extracted again later, when any shaped loop is considered.

qnach
qnach said:
...
But I think that is no use. I know the definition of A and r
But my question is why did he keep it without canceling out.

I think we are having a hard time figuring out why you think A and r can cancel. You are right, the definitions of A and r are clearly stated in the text that you attached:
"r is the distance from the dipole" , "...the area A of the loop..."

But A=πr2 only applies if A and r are part of the same circle right?

Let A be the area of my yard, and r be the distance from the Earth to the Sun, can these cancel?

the_emi_guy said:
I think we are having a hard time figuring out why you think A and r can cancel. You are right, the definitions of A and r are clearly stated in the text that you attached:
"r is the distance from the dipole" , "...the area A of the loop..."

But A=πr2 only applies if A and r are part of the same circle right?

Let A be the area of my yard, and r be the distance from the Earth to the Sun, can these cancel?
sorry, I mixed up the symbols, since I can only read the book from my screen...

Baluncore said:
The loop radius was a. The distance from the dipole was r. The radius of the loop was NOT r.
Since a does not appear in the equation, what can you possibly cancel ?

The term A / λ2 is a pure ratio. It is the aperture in wavelengths. It appears as a recognisable term with many antenna configurations. If it could be canceled now it would have to be extracted again later, when any shaped loop is considered.
I have a further question about the r and the retarded current I.
Since I am considering the receiving antenna, the wave come from infinity.
So, how should the r be defined? Should it be infinity? And, the retarded I does not seems to be meaningful?

Consider a reference plane, perpendicular to the direction of r, in or near to the antenna. The retarded element currents are sinusoidal currents with phase shifts measured relative to that reference plane. All retarded element currents can then be summed as phasors in the direction of r. That gives both the receive and transmit antenna pattern.

The range, r, allows for the reduction in energy per square meter with range as the energy is radiated over a greater spherical surface. If you make r infinite the energy will be zero.

Now go back to chapter 5 in Kraus, on short dipoles. On page 203, fig 5-3b you will see a reference plane and the independent definition of r and the retarded currents.

Learning about loop antennas backwards is a very inefficient way of modelling photosynthesis.

## 1. What is a small-loop antenna in Kraus?

A small-loop antenna is a type of antenna that is small in size and designed to operate at high frequencies. It was first introduced by John Kraus in his book "Antennas" and is commonly used in portable radios and communication devices.

## 2. How does a small-loop antenna work?

A small-loop antenna works by creating a magnetic field when an alternating current is passed through it. This magnetic field then radiates energy, allowing the antenna to send and receive radio signals.

## 3. What are the advantages of using a small-loop antenna in Kraus?

Some advantages of using a small-loop antenna in Kraus include its compact size, low cost, and ease of installation. It also has a high gain and efficient performance at high frequencies.

## 4. What are the limitations of a small-loop antenna in Kraus?

One limitation of using a small-loop antenna in Kraus is its low bandwidth, which means it can only transmit and receive signals within a narrow range of frequencies. It also has a lower power handling capacity compared to larger antennas.

## 5. How can I improve the performance of a small-loop antenna in Kraus?

To improve the performance of a small-loop antenna in Kraus, you can use multiple antennas in an array or use a larger loop size. It is also important to carefully tune and match the antenna to the desired frequency for optimal performance.

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