T9 — Antennas and Feed Lines
2 exam questions · 2 groups · 24 questions in pool
Getting the signal into and out of the radio: antenna types, polarization, gain, and length; then the feed lines that connect them — coax types, loss, connectors, SWR, and tuners. No FCC citations.
T9A — Antennas: Polarization, Gain, Beams, Loading, Length
12 questions
What this group tests: the common antenna types and the relationships between length, frequency, polarization, and gain.
Foundational concepts
Antenna gain is the increase in signal strength in a given direction compared to a reference antenna — gain isn’t free power, it’s focusing. A beam antenna concentrates energy in one direction, and the highest-gain common type is the Yagi. A 5/8-wave whip for VHF/UHF mobile has more gain than a 1/4-wave whip.
Polarization depends on orientation: a dipole parallel to the ground is horizontally polarized. A half-wave dipole radiates strongest broadside — out the sides, perpendicular to the wire.
Length and frequency are inversely linked (the same idea as λ = 300/f from T3B): shortening an antenna raises its resonant frequency. For VHF you can estimate physical length — a quarter-wave vertical for 146 MHz is about 19 inches, and a half-wave 6 m dipole is about 112 inches. When an antenna must be shorter than resonant, loading (inserting inductors in the radiating elements) electrically lengthens it back to resonance.
A couple of practical disadvantages: the stubby “rubber duck” on a handheld is low efficiency versus a full quarter-wave, and using a handheld inside a vehicle loses signal because the metal body shields the antenna.
Key facts to retain
- Gain = stronger signal in one direction vs. a reference; Yagi = highest common gain; 5/8-wave whip beats 1/4-wave mobile.
- Horizontal dipole = horizontal polarization; dipole radiates broadside.
- Shorter antenna → higher resonant frequency; loading uses inductors to electrically lengthen.
- 1/4-wave vertical @146 MHz ≈ 19 in; 1/2-wave 6 m dipole ≈ 112 in.
- Rubber duck = low efficiency; handheld in a car loses signal to shielding.
External reference anchors
- NCVEC syllabus: T9A — Antennas: polarization, gain, beam types, loading, portable/mobile antennas, resonant length vs. frequency, dipole pattern
Per-question map
| Q | Asks for | Resolved by |
|---|---|---|
| T9A01 | What a beam antenna is | Concentrates signal one direction |
| T9A02 | A type of antenna loading | Inductors in radiating elements |
| T9A03 | Horizontal dipole polarization | Horizontally polarized |
| T9A04 | Rubber-duck disadvantage | Low efficiency |
| T9A05 | Raising dipole resonant freq | Shortening it |
| T9A06 | Greatest gain | Yagi |
| T9A07 | Handheld inside vehicle | Vehicle shielding reduces signal |
| T9A08 | 1/4-wave vertical for 146 MHz | ~19 inches |
| T9A09 | 1/2-wave 6 m dipole length | ~112 inches |
| T9A10 | Dipole strongest direction | Broadside |
| T9A11 | What antenna gain is | Increase vs. a reference antenna |
| T9A12 | 5/8-wave whip advantage | More gain than 1/4-wave |
T9B — Feed Lines; SWR Concepts; Tuners; Connectors
12 questions
What this group tests: coax characteristics and loss, RF connectors, SWR, and the job of an antenna tuner.
Foundational concepts
Coax is the most common amateur feed line because it’s easy to use with few special installation considerations, and the standard impedance is 50 ohms. Loss in coax increases with frequency, and there are several loss sources at once — “all of these.” Cable choice trades size for loss: RG-213 has less loss than RG-58 at a given frequency, and air-insulated hardline has the lowest loss at VHF/UHF.
Connectors by frequency: PL-259 (UHF) connectors are common at HF and VHF, while Type N is preferred above 400 MHz (it’s weatherproof and lower-loss at UHF).
SWR describes how well the load (antenna) is matched to the line. Low SWR gives reduced signal loss (the benefit). Erratic SWR changes usually point to a loose connection in the antenna or feed line. An antenna tuner (coupler) doesn’t change the antenna — its job is to match the antenna-system impedance to the transceiver’s output so the rig sees a good load.
Key facts to retain
- Coax is easy to use; standard 50 Ω; loss rises with frequency.
- RG-213 < RG-58 loss; hardline lowest loss at VHF/UHF.
- PL-259 for HF/VHF; Type N above 400 MHz.
- Low SWR → less loss; erratic SWR = loose connection; tuner matches impedance.
External reference anchors
- NCVEC syllabus: T9B — Feed lines: types, attenuation vs frequency, selecting; SWR; antenna tuners; RF connectors
Per-question map
| Q | Asks for | Resolved by |
|---|---|---|
| T9B01 | Benefit of low SWR | Reduced signal loss |
| T9B02 | Common coax impedance | 50 ohms |
| T9B03 | Why coax is common | Easy to use, few special needs |
| T9B04 | Antenna tuner function | Matches antenna to transceiver impedance |
| T9B05 | Coax loss vs frequency | Loss increases |
| T9B06 | Connector above 400 MHz | Type N |
| T9B07 | PL-259 connectors | Common at HF and VHF |
| T9B08 | Source of coax loss | All these choices |
| T9B09 | Cause of erratic SWR | Loose connection |
| T9B10 | RG-58 vs RG-213 | RG-213 has less loss |
| T9B11 | Lowest-loss feed line at VHF/UHF | Air-insulated hardline |
| T9B12 | What SWR is | How well load matches the line |