Radiation from a Dipole Questions and Answers

Q: Why does a folded dipole present about four times the impedance of a plain one?

The current divides between the two conductors, so for the same radiated power the current at the terminals is halved and, since power goes as current squared times resistance, the impedance rises by four. The radiation is that of a single dipole carrying the total current. The wires carry current in the same sense rather than in antiphase, which is why the arrangement radiates at all, and its resonant frequency is essentially that of the equivalent dipole.

Q: What is the chief advantage of a folded dipole besides its impedance?

The larger effective conductor diameter lowers the quality factor, so the impedance varies more slowly with frequency and the useful band widens. Its pattern and directivity are essentially those of an ordinary dipole, since the radiating current distribution is the same. It is still a balanced structure and still needs a balun when fed with coaxial cable, usually a four to one type which also completes the transformation to fifty ohms.

Q: What is the directivity of a full wave dipole compared with a half wave one?

Lengthening to a full wavelength narrows the broadside lobe and raises the directivity to about 2.4, though the impedance becomes awkward. Beyond about 1.25 wavelengths the main lobe begins to split and the broadside directivity falls again. Cancellation is complete only in particular directions rather than everywhere. The general lesson is that longer is better only up to the point where the current distribution develops regions of opposing phase.

Q: What distinguishes a travelling wave antenna from a standing wave one?

A resistively terminated wire carries current in one direction only, with no reflection and so no standing wave, which gives a broadband unidirectional pattern at the cost of the power absorbed in the termination. Rhombic and Beverage antennas work this way. Length and feed position are secondary, and no antenna radiates from direct current. The trade of efficiency for bandwidth and pattern is the defining characteristic of the whole travelling wave family.

Q: What is a biconical antenna's chief property?

An infinite biconical structure is defined entirely by angles, so it presents the same impedance at every frequency, and a truncated practical version is broadband over many octaves. Its pattern resembles a fat dipole, broadside with axial nulls, rather than a narrow beam. It radiates linear polarisation and needs a feed like anything else. The idea that a structure specified only by angles is frequency independent also underlies the log periodic and spiral antennas.

Q: A small loop's radiation resistance is \( 31171(A/\lambda^{2})^{2} \). What is it for an area of 0.01 square wavelengths?

Substituting the area ratio directly:\[ R_{r} = 31171 \times (0.01)^{2} \approx 3.1 \]giving about 3.1 ohms. The fourth power dependence on linear size is even more severe than the dipole's square law, which is why small loops are notoriously inefficient transmitting antennas and are used chiefly for receiving, where efficiency matters less than pattern and noise pickup.

Q: Why is a small loop widely used as a receiving antenna despite poor efficiency?

The loop's deep nulls perpendicular to its plane make it excellent for direction finding and for nulling a local interferer, and at low frequencies external noise dominates so inefficiency costs nothing in signal to noise ratio. Its gain is not higher, it is strongly inductive rather than resistive, and it responds primarily to the magnetic field, which is why it is less affected by local electric field noise. The last point is itself a practical advantage.

21.

Why does a folded dipole present about four times the impedance of a plain one?

A.The loop resonates at a quarter of the frequency
B.The two wires each carry the full current in antiphase
C.The same radiated power is drawn at half the terminal current
D.The two conductors double the radiation resistance twice over

Answer: C

The current divides between the two conductors, so for the same radiated power the current at the terminals is halved and, since power goes as current squared times resistance, the impedance rises by four. The radiation is that of a single dipole carrying the total current. The wires carry current in the same sense rather than in antiphase, which is why the arrangement radiates at all, and its resonant frequency is essentially that of the equivalent dipole.

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22.

What is the chief advantage of a folded dipole besides its impedance?

A.A narrower beamwidth in the principal plane
B.A higher directivity than a plain dipole
C.Freedom from any need for a balun at the feed point
D.A wider impedance bandwidth than a plain dipole

Answer: D

The larger effective conductor diameter lowers the quality factor, so the impedance varies more slowly with frequency and the useful band widens. Its pattern and directivity are essentially those of an ordinary dipole, since the radiating current distribution is the same. It is still a balanced structure and still needs a balun when fed with coaxial cable, usually a four to one type which also completes the transformation to fifty ohms.

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23.

What happens to the input impedance of a dipole as its length approaches one wavelength?

A.It rises to a very high resistive value
B.It falls towards zero ohms
C.It stays close to 73 ohms throughout
D.It becomes purely capacitive and very small

Answer: A

At a full wavelength the current at the feed point is a minimum of the standing wave, so the impedance rises to several thousand ohms, which is very hard to match. This is why full wave dipoles are fed off centre or at a current maximum instead. The impedance passes through resonances every half wavelength, alternating between low values at current maxima and very high ones at current minima, and knowing where the feed point falls on the standing wave predicts the whole behaviour.

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24.

What is the directivity of a full wave dipole compared with a half wave one?

A.Zero, because the contributions cancel in every direction
B.Somewhat higher, because the beam is narrower
C.Somewhat lower, because the beam is broader
D.Exactly the same, because the pattern shape is unchanged

Answer: B

Lengthening to a full wavelength narrows the broadside lobe and raises the directivity to about 2.4, though the impedance becomes awkward. Beyond about 1.25 wavelengths the main lobe begins to split and the broadside directivity falls again. Cancellation is complete only in particular directions rather than everywhere. The general lesson is that longer is better only up to the point where the current distribution develops regions of opposing phase.

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25.

Why does a dipole longer than about 1.25 wavelengths show a split main lobe?

A.The feed point moves to a voltage maximum
B.The wire begins to behave as a terminated travelling wave antenna
C.Parts of the current are in antiphase and cancel broadside
D.The radiation resistance becomes negative

Answer: C

The standing wave reverses direction every half wavelength along the wire, so once the antenna is long enough, oppositely directed currents cancel in the broadside direction and the energy appears in lobes to either side. Radiation resistance never goes negative for an isolated element, feed position affects impedance rather than pattern, and a travelling wave requires a terminated rather than an open wire. The same reasoning predicts the many lobed patterns of long wire antennas.

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26.

What distinguishes a travelling wave antenna from a standing wave one?

A.The wire is longer than one wavelength in every case
B.The wire is fed at the end rather than the centre
C.The wire carries direct as well as alternating current
D.The wire is terminated so no reflected current returns

Answer: D

A resistively terminated wire carries current in one direction only, with no reflection and so no standing wave, which gives a broadband unidirectional pattern at the cost of the power absorbed in the termination. Rhombic and Beverage antennas work this way. Length and feed position are secondary, and no antenna radiates from direct current. The trade of efficiency for bandwidth and pattern is the defining characteristic of the whole travelling wave family.

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27.

What is a biconical antenna's chief property?

A.Its impedance is set by cone angle and so is broadband
B.Its pattern is a single very narrow beam along the axis
C.It radiates only circular polarisation
D.It requires no feed at all, being self excited

Answer: A

An infinite biconical structure is defined entirely by angles, so it presents the same impedance at every frequency, and a truncated practical version is broadband over many octaves. Its pattern resembles a fat dipole, broadside with axial nulls, rather than a narrow beam. It radiates linear polarisation and needs a feed like anything else. The idea that a structure specified only by angles is frequency independent also underlies the log periodic and spiral antennas.

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28.

What is a small loop antenna equivalent to?

A.A travelling wave structure of one wavelength
B.A magnetic dipole
C.An electric dipole of the same length of wire
D.A quarter wave monopole over ground

Answer: B

A loop much smaller than a wavelength carries essentially uniform current and radiates like a magnetic dipole, with its electric and magnetic fields interchanged relative to the short electric dipole. Its pattern is the same doughnut shape but with the null perpendicular to the plane of the loop rather than along a wire. This duality between the electric and magnetic cases is one of the tidiest results in antenna theory and makes the loop's fields obtainable without fresh integration.

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29.

A small loop's radiation resistance is \( 31171(A/\lambda^{2})^{2} \). What is it for an area of 0.01 square wavelengths?

A.About 0.31 ohms
B.About 312 ohms, which would match a folded dipole
C.About 3.1 ohms
D.About 31 ohms

Answer: C

Substituting the area ratio directly:

\[ R_{r} = 31171 \times (0.01)^{2} \approx 3.1 \]

giving about 3.1 ohms. The fourth power dependence on linear size is even more severe than the dipole's square law, which is why small loops are notoriously inefficient transmitting antennas and are used chiefly for receiving, where efficiency matters less than pattern and noise pickup.

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30.

Why is a small loop widely used as a receiving antenna despite poor efficiency?

A.Its gain is higher than that of a dipole of the same size
B.It has no reactive component at any frequency
C.It responds to the electric field rather than the magnetic
D.Its sharp nulls allow direction finding and interference rejection

Answer: D

The loop's deep nulls perpendicular to its plane make it excellent for direction finding and for nulling a local interferer, and at low frequencies external noise dominates so inefficiency costs nothing in signal to noise ratio. Its gain is not higher, it is strongly inductive rather than resistive, and it responds primarily to the magnetic field, which is why it is less affected by local electric field noise. The last point is itself a practical advantage.

Discuss in Forum

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