Which of the following is NOT true with respect to antennas?
Yagi-Uda antennas have high bandwidth and low gain
Antenna Fundamentals Explained
Antennas are essential components in radio frequency (RF) systems, converting electrical signals into electromagnetic waves for transmission and vice versa for reception. Understanding their characteristics, such as radiation patterns, gain, and bandwidth, is crucial for effective wireless communication. This solution will analyze common antenna properties to identify which statement is incorrect.
Antenna Radiation and Frequency Relationship
The location of maximum radiation on an antenna is closely related to the current distribution along its structure, which in turn depends on the operating frequency and the antenna's physical dimensions. Radiation primarily occurs from regions where the current is high.
- Higher frequencies are radiated near the feedpoint: For many resonant antennas, especially common designs like center-fed dipoles, the feedpoint is designed to be a point of maximum current (or current antinode) at the fundamental operating frequency and its odd harmonics. As frequency increases (wavelength decreases), current maxima appear closer to the feedpoint or at multiple points along the antenna. Thus, significant radiation often occurs near the feedpoint for higher frequencies, making this statement generally true.
- Lower frequencies are radiated near the end of transmission line: This statement is generally not true for efficient antenna operation. A properly designed antenna radiates efficiently from its entire structure, with radiation maxima typically occurring at current maxima. For lower frequencies (longer wavelengths), the antenna needs to be physically longer to be resonant. If a transmission line itself is radiating, it usually indicates an impedance mismatch or an unintentional antenna effect. Efficient radiation for lower frequencies from the "end of a transmission line" is not a standard characteristic for antennas, making this statement questionable but not as definitively false as another option we will discuss. However, in comparison to the other options, it's less universally true.
Horn Antenna Usage in Satellite Communication
Horn antennas are aperture antennas known for their directivity and efficient impedance matching. They are widely used as primary feeds for larger reflector antennas, such as parabolic dishes.
- Horn antennas are used to feed parabolic dish antennas: This statement is absolutely true. Horn antennas are commonly employed to illuminate parabolic reflector dishes because they provide a controlled beam that efficiently directs the energy onto the reflector, which then focuses it into a very narrow, high-gain beam.
Yagi-Uda Antenna: Gain and Bandwidth Characteristics
The Yagi-Uda antenna is a highly popular and widely used directional antenna, easily recognizable by its array of parallel elements (a driven element, a reflector, and one or more directors).
- Yagi-Uda antennas have high bandwidth and low gain: This statement is false. Yagi-Uda antennas are specifically known for their high gain and excellent directivity. However, they typically suffer from a relatively narrow bandwidth. This means they perform optimally over a limited range of frequencies. Their high gain makes them ideal for applications like television reception or point-to-point communication where a strong, directional signal is needed.
Detailed Option Analysis for Antennas
Let's summarize the truthfulness of each statement:
| Statement | Analysis | Truth Value |
|---|---|---|
| Lower frequencies are radiated near the end of transmission line | Radiation depends on current distribution. For efficient antennas, radiation occurs from current maxima, not necessarily the end of a transmission line, especially for lower frequencies where the antenna length is significant. This statement is generally misleading or incorrect in the context of efficient antenna design. | Likely False/Misleading |
| Horn antennas are used to feed parabolic dish antennas | Horn antennas are ideal primary feeds for parabolic reflectors due to their controlled radiation pattern. | True |
| Yagi-Uda antennas have high bandwidth and low gain | Yagi-Uda antennas are known for high gain and narrow bandwidth. Therefore, this statement contradicts their well-established properties. | False |
| Higher frequencies are radiated near the feedpoint | For many resonant antennas, the feedpoint is a current maximum or close to it, especially at fundamental and harmonic resonances. Radiation is highest where current is maximum. | True |
Comparing the options, the statement about Yagi-Uda antennas is definitively incorrect based on widely accepted antenna theory. While the statement about lower frequencies and transmission line ends might be debated in very specific contexts of unintended radiation, the Yagi-Uda statement is a direct contradiction of its fundamental characteristics.
Conclusion: Correcting Antenna Misconceptions
Based on the analysis, the statement that is NOT true with respect to antennas is that "Yagi-Uda antennas have high bandwidth and low gain". Yagi-Uda antennas are known for their high gain and narrow bandwidth, making them effective directional antennas for specific frequency ranges.
Important Questions from Basics of Antenna
The antenna which provides circularly polarized waves is
A dipole carries RMS current of about 300 A across the radiation resistance 2 Ω. What would be the power radiated by an antenna?
A Yagi antenna is a directional antenna consisting of parasitic elements-
Arrange the following antennas in ascending order of their radiation resistance.
A. Short dipole (L = \(\frac{\lambda}{10}\) )(I av = l o)
B. Short dipole (L= \(\frac{\lambda}{10}\) )( I av = l o/2)
C. Linear \(\frac{\lambda}{2}\) dipole (sinusoidal current distribution)
D. Small Loop (square loop) single turn of(L = \(\frac{\lambda}{10}\) )
Choose the correct answer from the options given below
If the frequency of the signal is 1 MHz, the minimum height of the transmitting antenna should be:
