Why do these antennas have different radiation patterns?

[qnach]

In Kraus book on antenna (3ed page 203)
example 7-4.1
the difference between those antenna is (to my understanding) only the wave length.
How could they have different radiation pattern?

 

[davenn]

In Kraus book on antenna (3ed page 203)
example 7-4.1
the difference between those antenna is (to my understanding) only the wave length.
How could they have different radiation pattern?

can you please photo the diagrams and post here
few of us have easy access to that book

 

[Stephen Tashi]

Or perhaps you could refer to a diagram in the 2nd edition of Kraus's book, which is available as a PDF from several web sites.

 

[qnach]

can you please photo the diagrams and post here
few of us have easy access to that book

you can simply remove the word of "Kraus" the page number...etc and change the question into:

Why does an antenna has different radiation pattern at different wave-length?

 

[qnach]

Or perhaps you could refer to a diagram in the 2nd edition of Kraus's book, which is available as a PDF from several web sites.

It is on the chapter about small-loop antenna, i.e. chapter 6 of 2nd ed.
The example does not seems appear in the 2nd ed. But it is in the Figure 6-10.

 

[Stephen Tashi]

It is on the chapter about small-loop antenna, i.e. chapter 6 of 2nd ed.
The example does not seems appear in the 2nd ed. But it is in the Figure 6-10.

From page 248

 

[qnach]

From page 248

yes, thanks. but
Why does an antenna has different radiation pattern at different wave-length?

 

[tech99]

In Kraus book on antenna (3ed page 203)
example 7-4.1
the difference between those antenna is (to my understanding) only the wave length.
How could they have different radiation pattern?

A small loop may be regarded as two closely spaced rod antennas driven in anti-phase. This applies when looking from any direction in the plane of the loop. It occurs because there is no phase shift around the loop, because it is small compared with the wavelength, and the rods are being driven at opposite ends.
As the wavelength is reduced, there starts to be a phase difference between the two antennas, and the various patterns are simply the array factor of two antennas having the particular phase and spacing. For instance, when the circumference becomes a full wavelength, the two antennas become in-phase and radiation is then broadside to the plane of the loop. I can explain this in more detail if wanted.
The radiation from all antennas is fundamentally caused by the acceleration of charges in the conductor. This also applies to loop antennas. From memory, Kraus does not use this approach, whilst Terman does.

 

[Aaron Crowl]

the difference between those antenna is (to my understanding) only the wave length.

Is there a confusion here? The antennas in the illustration are operating at the same wavelength but they have different diameters.

 

[tech99]

Is there a confusion here? The antennas in the illustration are operating at the same wavelength but they have different diameters.

My slight error. I think we mean loop diameter, which is equivalent to altering the wavelength.

 

[davenn]

Is there a confusion here? The antennas in the illustration are operating at the same wavelength but they have different diameters.

where the diameters are different portions or multiples of the (fixed) operating freq and are expressed in wavelengths or parts there of, of that frequency

 

[vk6kro]

The diameter of the loop doesn't matter, by itself, but the circumference does and this depends on the diameter.

As you go around the circumference of a loop which is resonant, you would observe a standing wave pattern with alternate high and low voltage and current points.
This spacing of voltage and current points will vary with frequency.

It is the way that these are spaced that affects the radiation pattern.

In some directions, the resultant radiation will cancel due to the source radiations being out of phase with each other.
In other directions, the radiation will be in phase and the radiation will be enhanced.

There is an excellent antenna simulator called Eznec which is free to download in a demo version. It is great for learning about simple antennas and it produces results you could build if you wished.

 

[tech99]

If doing manual calculations, be careful to remember that two polarizations need to be considered, each having its own radiation pattern. Of course, they are generally in phase and add to just give a single polarization, but the problem still needs to be analysed.