Explain Yagi element roles, what raises gain and bandwidth, the dBi and dBd scales, and how hairpin and gamma matches work.
Syllabus: Directional antennas
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
The driven element is about ½ wavelength. A reflector slightly longer sits behind it, and directors slightly shorter sit in front. The main lobe is the direction of maximum radiated field. Front-to-back ratio compares the main lobe to the signal radiated the opposite way.
RememberReflector longer, directors shorter. Main lobe forward.
Adding directors and lengthening the boom raises gain and narrows the beam. Gain, front-to-back ratio and SWR bandwidth all depend on boom length, number of elements and element spacing, so designers trade between them. Larger-diameter elements widen the SWR bandwidth.
RememberLonger boom, more directors: more gain. Fatter elements: more bandwidth.
dBi measures gain against an isotropic radiator, dBd against a half-wave dipole. A dipole is already 2.15 dBi, so the same antenna reads 2.15 higher in dBi.
Stack two identical Yagis ½ λ apart and you get about twice the power: 10 log 2 ≈ 3 dB.
RememberdBi = dBd + 2.15. Double the antennas, plus 3 dB.
A Yagi's feed point is lower than a feed line's impedance, so it needs a match. A hairpin (beta) match is a shorted stub across the split driven element. A gamma match uses a rod and a series capacitor, and the driven element can be bonded to the boom.
RememberHairpin: shorted stub. Gamma: no insulation from the boom.