Yagi Uda and Log Periodic Antennas Questions and Answers

Answer: A

Only the driven element is fed; every other element is parasitic, excited by the field of its neighbours and re-radiating with a phase set by its own length and spacing. That is what makes the antenna so cheap: one connection, one balun and a row of aluminium rods. Feeding more elements would turn it into a driven array with quite different design rules. The whole behaviour of a Yagi follows from the fact that the parasites are excited by coupling rather than by wire.

Answer: B

The reflector is about five per cent longer than the driven element, which makes it inductive and gives the re-radiated field a phase that reinforces forward and cancels backward. Directors are shorter, and each is often slightly shorter than the one before. The boom is a mechanical member rather than an element, and its length is measured along the array rather than across it. Length relative to resonance decides whether a parasite reflects or directs.

Answer: C

Above resonance a dipole is inductive, so the current induced in it lags the driving field, and the re-radiated wave adds forward and subtracts backward. A shorter, capacitive element does the opposite and acts as a director. Nothing is absorbed and re-emitted in a directed sense, the element radiates perfectly well off resonance, and the extra interception is negligible. The whole design of a parasitic array rests on this relationship between element length and re-radiated phase.

Answer: D

The first reflector already reduces the rearward field substantially, so a second one sits in a weak field and is excited only feebly, contributing very little. Directors, by contrast, sit in the strong forward field and continue to add gain for many elements. Some designs do use a reflector screen or a pair of rods for extra front to back ratio. Where an element sits relative to the field it is meant to modify decides how much good it can do.

Answer: A

Each director adds less than the one before, so gain rises roughly with the logarithm of the boom length and a doubling of elements buys about three decibels. Beyond fifteen or twenty elements the return rarely justifies the length and the bandwidth becomes very narrow. Gain does not peak and collapse, nor does it saturate abruptly. Diminishing returns with aperture is a general feature of antennas and is worth expecting rather than being surprised by.

Answer: B

A ten element Yagi on a boom of about two wavelengths gives 12 to 13 dBi, and adding elements raises it slowly from there. A parabolic reflector reaches 25 dBi only with an aperture of many wavelengths, and 3 dBi would be barely better than a dipole. Very long Yagis do exceed 18 dBi, so that is not a ceiling. Having a realistic figure in mind lets an extravagant advertised gain be recognised for what it is.

Answer: C

Mutual coupling from the closely spaced parasites loads the driven element heavily and pulls its resistance down, sometimes to ten or twenty ohms on a long array. That is why a folded dipole, with its four to one step up, is so often used as the driven element, or a gamma or T match is fitted. The impedance remains resistive at the design frequency once the element is trimmed. Coupling that improves the pattern also complicates the match, which is the recurring tension in parasitic arrays.

Answer: D

Coupling drops the driving point resistance to perhaps eighteen ohms, and the folded dipole's factor of four raises that to about seventy, which a four to one balun then converts to fifty. It also widens the bandwidth a little. Its radiation is the same as a plain dipole's, and mechanical strength and pattern width are incidental. Choosing a driven element whose impedance transformation cancels the loading is neater than adding a separate matching network.

Answer: A

A gamma match connects the inner conductor of the coaxial cable part way along one half of the driven element, with a series capacitor to cancel the resulting inductance. Tapping further out raises the impedance, so any value can be reached, and the element need not be split. Quarter wave transformers, coaxial baluns and stubs are all separate devices used elsewhere in antenna practice. Its virtue is that it matches and balances an unsplit element at once.

Answer: B

A T match is the balanced version, tapping symmetrically either side of the centre, and it needs a balun where a gamma match does not. Both use series capacitors to cancel the tap inductance. Neither is restricted to a folded dipole, and neither gives a fixed transformation ratio, since the tap position sets it. Symmetric and asymmetric versions of the same idea recur throughout feed design, and the choice usually turns on whether a balun is wanted anyway.