Lesson 31 Flashcards — Antennas

Click a question to reveal the answer.

1. How does an antenna radiate?

Separation of charge creates an E-field; moving charge creates an H-field. Drive a conductor with AC and you get both, oscillating and perpendicular.

2. What does antenna reciprocity mean?

The same element transmits and receives with the same gain and pattern.

3. Give the wavelength formula.

\(\lambda = c/f\) with \(c = 3\times10^8\) m/s.

4. What is antenna gain, physically?

The ratio of energy focused in the best direction to what an isotropic source would put there — a linear ratio with no units. An antenna does not create power — it redistributes it, so gain in one direction is paid for by a null elsewhere.

5. Give the gain of a half-wave dipole and a quarter-wave monopole.

Ideal dipole \(G = 1.64\); ideal monopole \(G = 3.28\) — twice the dipole, because it radiates into a hemisphere rather than a sphere. 'Ideal' is the equation sheet's word for these two values.

6. What is the pattern of a dipole or monopole?

Omnidirectional in the horizontal plane — a doughnut, with nulls off the ends of the element.

7. Is an omnidirectional antenna isotropic?

No. Omnidirectional means the same toward every compass heading; isotropic means the same in every direction, with \(G = 1\). An ideal monopole is omnidirectional and still has \(G = 3.28\).

8. How do you read a gain off a pattern plot?

Find what 0° is — the plot names it (the nose on a top view; the horizon ahead on a side view, with straight up at 90°). The angle is the direction you look and the distance from the center is the linear gain, read against the labeled rings. Read it toward the other end of the link, not at the biggest lobe.

9. Give the gain of a parabolic dish.

\(G = (2\pi r)^2\eta/\lambda^2\), where \(r\) is the radius and \(\eta\) the aperture efficiency (typically 0.5–0.7). The equation sheet writes it \(G_{dish} = (2\pi r_{dish}/\lambda)^2\,\eta\).

10. What is the most common error in a dish gain calculation?

Using the diameter where the formula wants the radius.

11. How do you size a dish for a gain you need?

Solve the dish equation for the radius, \(r = \dfrac{\lambda}{2\pi}\sqrt{G/\eta}\), then double it: the diameter is \(2r\).

12. How does dish gain change if you double the frequency?

Gain goes as \(1/\lambda^2\), so doubling \(f\) quadruples the gain for the same dish.

13. What does a phased array buy you over a mechanical dish?

Beam steering by controlling the phase fed to each element — electronic, with no moving parts, in microseconds rather than seconds. An AESA gives each element its own T/R module.