Ham College episode 135 is now available for download.
Technician Exam Questions Part 22.
T7C – Antenna and transmission line measurements and troubleshooting: measuring SWR, effects of high SWR, causes of feed line failures, Basic coaxial cable characteristics, Use of dummy loads when testing.
T7D – Using basic test instruments: voltmeter, ammeter, and ohmmeter, Soldering.
This video, “Ham College 135,” (0:00-0:03) focuses on preparing viewers for the amateur technician exam, specifically covering antenna and transmission line measurements, troubleshooting, and the use of basic test instruments.
Key topics include:
Dummy Loads The primary purpose of a dummy load is to prevent transmitting signals over the air during tests (2:08-2:12). It consists of a non-inductive resistor mounted to a heat sink (6:06-6:09).
SWR (Standing Wave Ratio) An SWR meter is used to determine if an antenna is resonant at the desired operating frequency (3:17-3:21). A reading of 1:1 indicates a perfect impedance match (7:27-7:34). High SWR indicates an impedance mismatch (12:46-13:08) and can cause solid-state transmitters to reduce output power to protect circuitry (8:30-8:35).
Power lost in a feed line due to high SWR is converted into heat (13:34-13:47). A directional watt meter is used to measure SWR (15:23-15:27).
Coaxial Cable Moisture contamination is a primary cause of coaxial cable failure (21:11-21:15). The outer jacket of coaxial cable should be resistant to ultraviolet light to prevent damage and water entry (23:23-23:50).
Air core coax cable requires special techniques to prevent moisture (26:13-26:21) but has less loss per foot than foam or solid dielectric types (25:26-25:33).
Test Instruments
A voltmeter measures electric potential (35:06-35:16) and is connected in parallel to a component (38:03-38:19).
An ammeter measures current and is connected in series with a component (39:16-39:20).
An ohmmeter is used to measure resistance (54:04-54:09). When connected across a large, discharged capacitor, it shows an increasing resistance over time (57:36-58:11). When measuring in-circuit resistance, the circuit must not be powered (1:01:31-1:02:14).
Soldering A cold tin-lead solder joint typically appears with a rough or lumpy surface (47:22-47:34).