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G9C

ANTENNAS AND FEED LINES

- Directional antennas

G9C011 of 11

Which of the following would increase the bandwidth of a Yagi antenna?

Why An antenna element behaves like a resonant circuit, and bandwidth depends on its Q: lower Q means a broader response. Making the elements fatter lowers the conductor's inductance-to-capacitance ratio, which lowers Q and widens the SWR and gain bandwidth. That is why wide-band Yagis and log periodics use thick tubing, cages, or multiple parallel wires per element.

Watch out Squeezing the elements closer together does the opposite: tight spacing raises Q, which can raise gain on one frequency but makes the feedpoint impedance and pattern change very rapidly with frequency. Loading coils electrically shorten an element and also narrow the bandwidth; tapered tubing is a mechanical and weight compromise, not a bandwidth booster.

Fat elements, fat bandwidth. Low Q equals wide; skinny elements and loading coils equal narrow.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C022 of 11

What is the approximate length of the driven element of a Yagi antenna?

Why A Yagi is built around a resonant dipole as its driven element, and a dipole is about a half wavelength long. In practical terms that is roughly 468 divided by the frequency in MHz, in feet. The parasitic elements are sized relative to that: the reflector is about 5 percent longer and the directors about 5 percent shorter.

Watch out The quarter wavelength choice describes a vertical or ground-plane element, which uses the ground or radials to supply the missing half of the dipole; a Yagi's driven element stands alone as a full dipole.

Yagi driven element = a dipole = 1/2 wavelength. Reflector longer, directors shorter.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C033 of 11

How do the lengths of a three-element Yagi reflector and director compare to that of the driven element?

Why In a Yagi, only the driven element is fed; the reflector and director are parasitic elements that re-radiate with a phase shift set by their length. An element about 5 percent longer than the driven element is inductive and acts as a reflector, pushing the pattern away from it; an element about 5 percent shorter is capacitive and acts as a director, pulling the pattern toward it. So the typical boom order is reflector (longest), driven element, director (shortest), with the main lobe going out the director end.

Watch out The idea that relative lengths depend on frequency confuses absolute size with proportion: all three elements scale with wavelength, but their length relative to each other stays the same.

Longest element is in the back: reflector long, director short, beam fires toward the short end.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C044 of 11

How does antenna gain in dBi compare to gain stated in dBd for the same antenna?

Why dBi compares an antenna to a theoretical isotropic radiator that spreads power equally in all directions, while dBd compares it to a half-wave dipole. The dipole itself already has 2.15 dB of gain over isotropic, so any antenna's gain figure is 2.15 dB larger when referenced to isotropic. For example, a Yagi rated 7 dBd is the same antenna rated 9.15 dBi.

Watch out Reversing the sign is the common trap: subtracting 2.15 would be the way to convert a dBi figure back to dBd, not the way dBi compares to dBd. The 1.25 dB choices are not a real reference conversion at all.

The isotropic reference is the weakest, so dBi numbers look biggest. Remember 2.15: dBi = dBd + 2.15.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C055 of 11

What is the primary effect of increasing boom length and adding directors to a Yagi antenna?

Why A Yagi's forward gain comes mainly from how long the boom is, because a longer boom lets you space more directors out in front of the driven element, each one reinforcing the wave traveling forward. Adding directors and stretching the boom therefore raises gain, roughly a few tenths of a dB per doubling of elements once the boom gets long. Beamwidth is the flip side: as gain rises, the main lobe gets narrower, not wider.

Watch out The choice about beamwidth increasing has the relationship backwards, since gain and beamwidth trade off inversely. Front-to-back ratio depends mostly on reflector tuning and element spacing, and resonant frequency is set by element lengths, not by boom length.

Longer boom, more directors, more gain and a narrower beam. Boom length buys gain; element length sets frequency.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C076 of 11

What does "front-to-back ratio" mean in reference to a Yagi antenna?

Why Front-to-back ratio compares the signal strength radiated (or received) in the direction of the main lobe with the strength radiated 180 degrees behind the antenna, expressed in dB. A typical HF Yagi might show 15 to 25 dB of front-to-back, meaning the rearward signal is that many dB weaker than the forward one. It is a measure of how well the antenna rejects interference coming from behind, not a measure of how much gain it has forward.

Watch out The choice comparing forward gain to a dipole is describing gain in dBd, a separate specification; an antenna can have high gain and still have poor rejection off the back.

Front-to-BACK: front lobe versus what's directly BEHIND you, in dB. Gain is forward, F/B is rejection.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C087 of 11

What is meant by the "main lobe" of a directive antenna?

Why An antenna's radiation pattern is a plot of relative field strength in every direction, and it typically has one dominant peak plus smaller side and rear lobes. The main lobe is that dominant peak, the direction in which the antenna concentrates the most energy, so it is where the radiated field strength is greatest. Gain figures and beamwidth are both measured with reference to this main lobe.

Watch out The choices about maximum current or maximum voltage standing wave points describe conditions along the wire of the element itself, not features of the radiation pattern in space. Vertical angle of radiation describes the elevation of the lobe, not the definition of the lobe.

Lobe = a bulge in the pattern; the main lobe is the biggest bulge, pointing where the signal is strongest.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C098 of 11

In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?

Why Stacking two identical antennas and feeding them in phase doubles the effective aperture, and a doubling of power or aperture is 3 dB. So two three-element Yagis stacked a half wavelength apart give about 3 dB more gain than one of them alone, with the extra gain coming from a narrower vertical pattern.

Watch out The choice saying about 6 dB corresponds to four stacked antennas (two doublings of aperture), not two; each additional doubling adds only another 3 dB.

Double the antennas, double the power: 2x = 3 dB. Four stacked would be 6 dB.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C109 of 11

Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?

Why A Yagi's performance comes from the mutual coupling among driven element, reflector and directors, so every geometric variable matters. Boom length sets the aperture available, more elements add gain (roughly 1 to 1.5 dB per added director early on) and let you trade gain against pattern, and element spacing along the boom controls the phasing that determines front-to-back ratio, feedpoint impedance and how wide the usable SWR range is. Designers juggle all three, plus element lengths, because optimizing for maximum gain usually costs front-to-back ratio or bandwidth.

Watch out Each single-item choice is true by itself, which is what makes them tempting, but none of them is the whole story, so the combined answer is the right pick.

Yagi design = boom length + element count + spacing. Change any one and gain, F/B and SWR bandwidth all shift.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C1110 of 11

What is a beta or hairpin match?

Why A Yagi driven element typically has a feed point impedance well below 50 ohms, so the element is deliberately cut slightly short to make it look capacitive. A short piece of shorted parallel line or a U-shaped wire (the hairpin) placed right across the feed point acts as a shunt inductor, and that inductor plus the element's capacitive reactance forms an L-network that transforms the low resistance up to about 50 ohms. Because the stub is shorted at the far end and much less than a quarter wavelength long, it behaves purely as an inductance.

Watch out The quarter-wavelength piece of 75-ohm coax describes a different technique, the quarter-wave matching transformer or Q-section, not a beta match.

Hairpin = shorted stub = shunt inductor across the Yagi feed point.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

G9C1211 of 11

Which of the following is a characteristic of using a gamma match with a Yagi antenna?

Why A gamma match feeds the driven element off-center: the coax shield connects to the middle of the element (the zero-voltage point) and the center conductor goes through a series capacitor to a gamma rod tapped out along the element. Because the element's center is at RF ground potential, the element can be a single unbroken tube bolted straight to the boom, with no insulating center section or split needed. That mechanical simplicity and ruggedness is the main reason gamma matches are popular on Yagis.

Watch out The claim about needing no inductors or capacitors is wrong because a gamma match normally includes a series capacitor, with the gamma rod itself supplying inductive reactance; the gamma match is also a narrowband, single-band matching device, not a multiband solution.

Gamma = Grounded element: no split, no insulator, but yes a series capacitor.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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