T9B: Feed Lines and SWR – Ham Radio Technician License Study Guide
T9B covers the transmission lines, connectors, and impedance concepts that connect your radio to your antenna. A good feed line system keeps signal loss to a minimum and ensures the antenna is properly matched to the transmitter.
This group covers coaxial cable types and why coax dominates amateur installations, how loss changes with frequency, SWR and what it measures, the function of an antenna tuner, how to choose RF connectors for different frequencies, and what causes common feed line problems.
SWR: Standing Wave Ratio
Standing wave ratio (SWR) is a measure of how well a load is matched to a transmission line. When the antenna's impedance perfectly matches the feed line's impedance, all the transmitter's power flows forward into the antenna and none is reflected back. When there is a mismatch, some energy reflects back toward the transmitter — the forward and reflected waves create a standing wave pattern in the feed line. SWR is the ratio of the peak voltage to the minimum voltage of this standing wave.
The primary benefit of low SWR is reduced signal loss. At 1:1 SWR, the match is perfect and loss from mismatch is zero. As SWR rises, reflected power increases, feed line loss increases, and the transmitter has to work harder. Solid-state transmitters also reduce their output power at high SWR to protect their output amplifier transistors.
- SWR = a measure of how well a load is matched to a transmission line
- Benefit of low SWR = reduced signal loss
Coaxial Cable
Coaxial cable is the most common feed line for amateur radio antenna systems. Its main advantage is that it is easy to use and requires few special installation considerations — it can be routed along walls, through holes, and around corners without concern for the spacing or proximity to metal objects that balanced feed lines require.
The most common impedance for amateur coaxial cables is 50 ohms. This matches the output impedance of most amateur transceivers and the feedpoint impedance of common antennas.
- Most common feed line because it is easy to use and requires few special installation considerations
- Most common impedance = 50 ohms
Feed Line Types and Loss
Not all feed lines perform equally. Loss in a feed line is measured in decibels per 100 feet at a given frequency. Two important relationships govern coax performance:
Loss increases as frequency increases. As the frequency of a signal in coaxial cable rises, the loss in that cable also increases. A cable that performs acceptably on HF may have significantly more loss at VHF or UHF.
Different coax types have different loss characteristics:
| Cable Type | Characteristic |
|---|---|
| RG-58 | Thin, flexible; higher loss — acceptable for short HF runs |
| RG-213 | Larger diameter; less loss at a given frequency than RG-58 |
| Air-insulated hardline | Lowest loss at VHF and UHF; rigid, requires special fittings |
For VHF and UHF installations where every fraction of a dB matters, air-insulated hardline has the lowest loss of any common feed line type.
- Loss increases as frequency increases in coaxial cable
- RG-213 has less loss at a given frequency than RG-58
- Air-insulated hardline has the lowest loss at VHF and UHF
Antenna Tuner
An antenna tuner (also called an antenna coupler or transmatch) is a device whose major function is to match the antenna system impedance to the transceiver's output impedance. When an antenna's impedance at the operating frequency does not match the 50-ohm output of the radio, the antenna tuner adjusts the impedance seen by the transmitter so the radio sees a good match and can deliver full power. The antenna tuner does not change the antenna itself — it creates an electrical transformation between the transmitter and the feed line.
RF Connectors
Different RF connector types are suited to different frequency ranges:
- PL-259 (UHF connector, mating with SO-239) — despite its name, this connector is commonly used at HF and VHF frequencies. It is not watertight and is not well-suited for microwave operation. It is a threaded, twist-lock connector widely used on amateur HF and VHF equipment.
- Type N — this connector is most suitable for frequencies above 400 MHz. It is weatherproof, has low loss, and maintains its characteristics well into the UHF and microwave range.
- PL-259 = commonly used at HF and VHF frequencies (not watertight, not for microwave)
- Type N = most suitable for frequencies above 400 MHz
Feed Line Problems
Two common feed line problems appear in T9B:
Sources of Loss in Coaxial Feed Line
All three of the following are sources of loss in a coaxial feed line system:
- Water intrusion into coaxial connectors — moisture inside a connector dramatically increases loss and causes corrosion
- High SWR — mismatch causes additional resistive losses in the cable dielectric and conductors
- Multiple connectors in the line — each connector junction introduces a small amount of additional loss
Erratic Changes in SWR
When SWR readings change unpredictably — varying without a clear pattern as you operate — the most likely cause is a loose connection in the antenna or feed line. A connector that is not fully tightened, a corroded connector contact, or a broken solder joint can intermittently change the impedance seen by the SWR meter, producing erratic readings.
- Sources of coax loss = water intrusion, high SWR, multiple connectors (all three)
- Erratic SWR changes = loose connection in the antenna or feed line
T9B Practice Questions
Check Your Knowledge
T0: Safety →
← T9A: Antennas