T9B Feed lines, connectors, SWR and tuners

Feed lines: types, attenuation vs frequency, selecting; SWR concepts; Antenna tuners (couplers); RF Connectors: selecting, weather protection

Drill this topic (12 questions) Print flash cards

This group covers the cable and hardware connecting a radio to its antenna: coax types, impedance, loss, connectors, SWR and the antenna tuner.

Coaxial cable

Coax is the most common feed line in amateur radio because it is easy to use and needs few special installation precautions. It can be run near metal, buried or routed through a building without much concern. It is not the lowest-loss, highest-power or cheapest option in every case. The usual impedance is 50 ohms; 75-ohm cable is used mostly for video. Values like 8, 12 or 600 ohms are not typical coax.

Loss in coax rises as frequency increases. The characteristic impedance does not change with frequency. Air-insulated hardline has the lowest loss of the common types, better than flexible 50-ohm or 75-ohm coax.

RG-58 is thin and lossy. RG-213 is thicker, has less loss at a given frequency and handles more power. Both have a single braided shield.

Sources of loss and trouble

Loss in coax can come from water getting into connectors or the cable, high SWR, and extra connectors in the line. Each adds a little, and all are valid sources. A loose connection in the antenna or feed line is the classic cause of erratic SWR readings that jump around.

Connectors

Connector	Typical use
PL-259 (UHF)	HF and VHF, threaded, not weatherproof
BNC	Handhelds and test gear, bayonet type
Type N	UHF and microwave, above 400 MHz, weather-resistant

PL-259s are common at HF and VHF but are not watertight, and are poor choices for microwaves. Type N is the best choice above 400 MHz. Outdoors, PL-259, BNC and Type N connectors should all be carefully taped or sealed against weather. RS-213 is not a connector, and DB-25 is a computer data connector.

SWR

Standing wave ratio (SWR) measures how well a load, such as an antenna, is matched to the transmission line. A 1:1 ratio is a perfect match. A high SWR means reflected power and added line loss.

Antenna tuners

An antenna tuner (coupler) matches the antenna system’s impedance to the transceiver’s output impedance, typically 50 ohms. It does not change the antenna’s pattern, select bands or help receive weak signals.

Key facts to memorize

Common coax impedance: 50 ohms.
Coax loss increases as frequency increases; impedance does not change.
Air-insulated hardline has the lowest loss.
RG-213 has less loss than RG-58 and handles more power.
Type N: best for frequencies above 400 MHz.
PL-259: common at HF and VHF, not watertight.
Tape or seal all outdoor connectors.
Loss sources: water, high SWR, multiple connectors.
Erratic SWR: suspect a loose connection.
SWR: how well a load is matched to the line.
Antenna tuner: matches the system impedance to the transceiver.

Questions in this group (12): answers shown

T9B01

Which of the following connectors should be carefully taped for weather protection when used outdoors?
- A. PL259
- B. BNC
- C. Type N
- D. All these choices are correct ✓ Correct answer
PL-259, BNC and Type N connectors are not completely weatherproof, so each should be carefully taped or sealed when used outdoors. Water entering a connector increases loss and corrodes the contacts.
T9B02

What is the most common impedance of coaxial cables used in amateur radio?
- A. 8 ohms
- B. 50 ohms ✓ Correct answer
- C. 600 ohms
- D. 12 ohms
Most amateur coax, such as RG-58 and RG-213, has a 50-ohm characteristic impedance, matching the output of most transceivers. The 75-ohm type is mostly used for video, and 600 ohms is typical of open-wire line.
T9B03

Why is coaxial cable the most common feed line for amateur radio antenna systems?
- A. It is easy to use and requires few special installation considerations ✓ Correct answer
- B. It has less loss than any other type of feed line
- C. It can handle more power than any other type of feed line
- D. It is less expensive than any other type of feed line
Coax is popular because it is simple to install. It can be run near metal objects, buried or routed through buildings without much concern. Other line types may have less loss or handle more power.
T9B04

What is the major function of an antenna tuner (antenna coupler)?
- A. It matches the antenna system impedance to the transceiver's output impedance ✓ Correct answer
- B. It helps a receiver automatically tune in weak stations
- C. It allows an antenna to be used on both transmit and receive
- D. It automatically selects the proper antenna for the frequency band being used
An antenna tuner matches the impedance of the antenna and feed line system to the transceiver's output impedance, usually 50 ohms. It does not change the antenna's pattern or select antennas or bands.
T9B05

What happens as the frequency of a signal in coaxial cable is increased?
- A. The characteristic impedance decreases
- B. The loss decreases
- C. The characteristic impedance increases
- D. The loss increases ✓ Correct answer
Coax loss increases as frequency rises, which is why long runs at VHF and UHF need low-loss cable. The characteristic impedance stays essentially constant with frequency.
T9B06

Which of the following connector types is most suitable as an RF connector for frequencies above 400 MHz?
- A. PL-259
- B. Type N ✓ Correct answer
- C. RS-213
- D. DB-25
The Type N connector is built for UHF and microwave frequencies, and works well above 400 MHz. The PL-259 performs poorly above that range. RS-213 is not a connector, and DB-25 is a computer data connector.
T9B07

Which of the following is true of PL-259 type coax connectors?
- A. They are preferred for microwave operation
- B. They are watertight
- C. They are commonly used at HF and VHF frequencies ✓ Correct answer
- D. They are a bayonet-type connector
PL-259 connectors are common on HF and VHF equipment. They are threaded, not bayonet, are not waterproof, and perform poorly at microwave frequencies. BNC is the bayonet type.
T9B08

Which of the following is a source of loss in coaxial feed line?
- A. Water intrusion into coaxial connectors
- B. High SWR
- C. Multiple connectors in the line
- D. All these choices are correct ✓ Correct answer
All of these add loss. Water in connectors or the cable adds loss, high SWR causes additional line loss, and each connector adds a little loss. Together they reduce the power reaching the antenna.
T9B09

What can cause erratic changes in SWR?
- A. Local thunderstorm
- B. Loose connection in the antenna or feed line ✓ Correct answer
- C. Over-modulation
- D. Overload from a strong local station
A loose or corroded connection in the antenna or feed line makes the impedance change randomly, so the SWR reading jumps around. Thunderstorms, over-modulation and strong signals do not cause this.
T9B10

What is the electrical difference between RG-58 and RG-213 coaxial cable?
- A. There is no significant difference between the two types
- B. RG-58 cable has two shields
- C. RG-213 cable has less loss at a given frequency ✓ Correct answer
- D. RG-58 cable can handle higher power levels
RG-213 is a thicker cable than RG-58, so it has less loss at a given frequency and can handle more power. RG-58 has higher loss and lower power handling. Both have a single braided shield.
T9B11

Which of the following types of feed line has the lowest loss?
- A. 50-ohm flexible coax
- B. Multi-conductor unbalanced cable
- C. Air-insulated hardline ✓ Correct answer
- D. 75-ohm flexible coax
Air-insulated hardline has the lowest loss of the listed types, and is used for long runs or high frequencies. Flexible coax, whether 50-ohm or 75-ohm, has higher loss, and multi-conductor cable is not a low-loss line.
T9B12

What is standing wave ratio (SWR)?
- A. A measure of how well a load is matched to a transmission line ✓ Correct answer
- B. The ratio of amplifier power output to input
- C. The transmitter efficiency ratio
- D. An indication of the quality of your station’s ground connection
SWR is a measure of how well a load, such as an antenna, is matched to the transmission line. A 1:1 ratio indicates a perfect match, and higher ratios indicate more reflected power.