G9C: Directional Antennas – Ham Radio General License Study Guide
G9C covers the Yagi-Uda beam antenna — the most widely used directional antenna in amateur HF and VHF operation. The eleven active questions in this group address element functions and sizes, how gain is expressed and compared, what bandwidth depends on, what adding elements and extending the boom does, the meaning of front-to-back ratio and main lobe, the gain improvement from stacking two antennas, what can be optimized in a Yagi design, and two common feed-matching techniques.
Topics include what increases Yagi bandwidth, the length of the driven element, how reflector and director lengths compare to the driven element, dBi vs dBd gain comparison, the effect of adding directors, the definition of front-to-back ratio, the definition of the main lobe, the gain of two stacked three-element Yagis vs one, what can be adjusted to optimize a Yagi, what a beta/hairpin match is, and what characterizes a gamma match.
Yagi Elements and Their Functions
A Yagi-Uda (Yagi) antenna uses a single driven element connected to the feed line, plus one or more parasitic elements that are not directly connected but influence the radiation pattern through electromagnetic coupling.
| Element | Length vs. Driven Element | Position | Effect |
|---|---|---|---|
| Driven element | ≈ 1/2 wavelength | Center | Connected to feed line; radiates the signal |
| Reflector | Longer than driven element | Behind driven element | Reflects energy forward; reduces rear radiation |
| Director(s) | Shorter than driven element | In front of driven element | Focuses energy forward; increases forward gain |
Increasing the boom length and adding more directors is the primary way to increase Yagi gain — each additional director adds gain, with diminishing returns as more are added. Gain increases as the antenna becomes longer.
Bandwidth
Yagi bandwidth — the range of frequencies over which the antenna maintains acceptable SWR and pattern — is increased by using larger-diameter elements. A fatter conductor has a lower Q, which means it resonates across a broader frequency range. Closer element spacing or loading coils reduce bandwidth.
Antenna Gain: dBi vs dBd
Antenna gain is always relative to some reference antenna. Two references are used:
- dBi — gain relative to an isotropic radiator (a theoretical antenna that radiates equally in all directions)
- dBd — gain relative to a half-wave dipole in free space
A dipole in free space has a gain of 2.15 dBi over an isotropic radiator. Therefore, for any antenna:
Example: A Yagi with 7 dBd gain = 7 + 2.15 = 9.15 dBi
The dBi number is always higher — gain in dBi is 2.15 dB higher than the same gain in dBd.
When comparing specifications from different manufacturers, always verify whether the stated gain is in dBi or dBd — a 2.15 dB difference exists between the two units for the same antenna.
Radiation Pattern Terms
Main Lobe
The main lobe of a directive antenna is the direction of maximum radiated field strength. For a Yagi, the main lobe points in the direction the directors are facing — toward the front of the antenna. The main lobe represents the direction in which the antenna concentrates its radiated power.
Front-to-Back Ratio
The front-to-back ratio is the ratio of the power radiated in the major lobe compared to that radiated in the opposite direction. A high front-to-back ratio means the antenna radiates much more power forward than backward, reducing interference from stations behind the antenna. Front-to-back ratio is expressed in dB.
Optimizing Yagi Performance and Stacking
Three parameters can be adjusted to optimize a Yagi antenna's forward gain, front-to-back ratio, or SWR bandwidth:
- The physical length of the boom
- The number of elements on the boom
- The spacing of each element along the boom
The exam answer for what can be adjusted to optimize a Yagi is all these choices are correct. Different combinations of these parameters trade off between maximum gain, best front-to-back ratio, and widest SWR bandwidth — an optimal design for one metric may not be optimal for the others.
Stacking Two Yagis
When two identical three-element Yagi antennas are stacked vertically (spaced 1/2 wavelength apart, fed in phase), the combination provides approximately 3 dB more gain than a single three-element Yagi. The stacking gain comes from narrowing the elevation beam — the two antennas combine their patterns constructively in the forward direction while partially canceling rearward radiation.
Feed Matching: Beta and Gamma
Yagi antennas typically have feed point impedances below 50 ohms, requiring a matching network to mate with 50-ohm coaxial cable. Two techniques are commonly used:
Beta (Hairpin) Match
A beta match (also called a hairpin match) consists of a shorted transmission line stub placed at the feed point of the Yagi to provide impedance matching. The stub is a short length of wire or transmission line connected across the feed point and shorted at the far end. By adjusting the stub length and the driven element length, the system matches the antenna's impedance to 50 ohms. The beta match requires the driven element to be split (insulated from the boom) so the center can be fed.
Gamma Match
A gamma match uses an asymmetric tapping arrangement on the driven element. Its key characteristic is that it does not require the driven element to be insulated from the boom — the driven element can be electrically connected to (grounded through) the boom. This simplifies construction. The gamma rod taps the driven element at the appropriate point to achieve the required impedance transformation.
G9C Practice Questions
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