# Why is the Antenna Pattern Scaled Differently in This Diagram?

• onsightly
In summary, the antenna radiation pattern diagram is scaled differently, the bore sight of the main beam is at 2 dB, and all measurements should be in negative dB.
onsightly

Hello Physics Forums!

I'm not an engineer/scientist but I work with antennas and I'm having trouble deciphering the above antenna radiation pattern diagram.

I am used to seeing radiation patterns expressed in polar plots. Usually the scale is set to dBi (where the isotropic antenna = 0dBi), or it is set to main beam bore sight power (where the peak of the main beam is 0 dB). The diagram above is scaled differently.

As you can see, the isotropic level is '7 dB'. The bore sight of the main beam is at about 2 dB. The half-power points (5 dB) occur at roughly +/- 35 degrees off bore sight.

I have some questions:

1) Why is the diagram scaled this way? What does it mean to say that the bore sight of the main beam is at 2 dB? 2 dB less than what?

2) Shouldn't all of these measurements be in negative dB? e.g. shouldn't the isotropic level be -7 dB?

3) Could I go ahead and 'convert' this diagram to isotropic scale? So the bore sight is at 5 dBi, the power at 90 / 270 degrees is roughly -9 dbi, etc.?

Thanks!

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Welcome to PF.
onsightly said:
1) Why is the diagram scaled this way? What does it mean to say that the bore sight of the main beam is at 2 dB? 2 dB less than what?
2 dB down from the measurement system calibration reference. Avoid “less”, use “down”.

onsightly said:
2) Shouldn't all of these measurements be in negative dB? e.g. shouldn't the isotropic level be -7 dB?
Yes and No. It depends on if it plots gain or attenuation respectively. A gain of –3dB is an attenuation of +3dB.

onsightly said:
3) Could I go ahead and 'convert' this diagram to isotropic scale? So the bore sight is at 5 dBi, the power at 90 / 270 degrees is roughly -9 dbi, etc.?
All things are relative. The outer circle was the reference level on the chart.
If the isotropic reference is 7dB down and the main lobe is 2dB down, then the antenna main lobe gain is 7–2 = 5 dBi.

You might want to use a reference of either dBi or dBd depending on what you need to compare it with.
Since the 0, 5, 10, 15dB … rings are in bold, the chart cannot be simply renumbered.
You must draw it again to avoid confusion later, or simply subtract the selected reference when reading the numbers.

berkeman
I'll restate in different words what baluncore said:
It's a calibration thing. Their system, for whatever reason, must be calibrated manually. At the bottom of the plot they have indicated that 0 dBi falls on the "7 dB" line of the plotting system, and as you have pointed out, they have omitted the minus sign. So "0 dB" at the outside corresponds to +7 dBi, and you can figure out any reading from there. The peak antenna gain is +5 dBi.

## 1. What is antenna pattern scaling?

Antenna pattern scaling is a technique used to adjust the characteristics and behavior of an antenna based on its size and operating frequency. This allows for optimal performance across a range of frequencies and applications.

## 2. How is antenna pattern scaling achieved?

Antenna pattern scaling is achieved by modifying the dimensions of the antenna elements, such as length and spacing, in relation to the operating frequency. This can also involve adjusting the feed mechanism and placement of the antenna.

## 3. What are the benefits of antenna pattern scaling?

Antenna pattern scaling allows for increased flexibility and versatility in antenna design. It also helps to improve the overall performance of the antenna, including gain, directivity, and radiation efficiency, across a wider frequency bandwidth.

## 4. Are there any limitations to antenna pattern scaling?

While antenna pattern scaling can greatly enhance the performance of an antenna, it is not a one-size-fits-all solution. It is important to carefully consider the specific requirements and limitations of each application and make adjustments accordingly.

## 5. What are some common applications of antenna pattern scaling?

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