Saturday, July 26, 2014

Standing Wave Ratio, or SWR, A Ham Radio Exam Study Page

When a transmitter sends radio  frequency energy down a transmission line to an antenna, some of that energy can be reflected back up the transmission line from the antenna towards the transmitter.

The amount of energy reflected is determined by how well the impedances of the antenna,  the transmission line, and the transmitter match.  The reflected rf energy can enter the transmitter and damage the final radio frequency amplifier stage.
The standing wave ratio, (SWR), is a measure of how much of the RF is reflected by the antenna.  An SWR or 1:1 indicates that none of the RF is reflected.  With an SWR of 1:1, the transmission line, (feedline), and antenna are perfectly matched.  Ratios higher than 1:1 such as 1.5:1 or 2:1 indicate that there is an impedance mismatch and that RF is being reflected back up to the transmitter.  Remember, SWR depends on how well the impedances of the antenna, (also called the load), and the transmission line are matched.
In modern transmitters, there is often a circuit that will begin to reduce transmit power if an SWR of 2:1 or greater is detected.  This is done to protect the circuits in the transmitter.

The number or SWR in this question pool is up from 5 to 8.

Here are the exam questions

154|T|4|A|05|A|Where should an in-line SWR meter be connected to monitor the standing wave ratio of the station antenna system?
A. In series with the feed line, between the transmitter and antenna
B. In series with the station's ground
C. In parallel with the push-to-talk line and the antenna
D. In series with the power supply cable, as close as possible to the radio

297|T|7|C|03|A|What, in general terms, is standing wave ratio (SWR)?
A. A measure of how well a load is matched to a transmission line
B. The ratio of high to low impedance in a feed line
C. The transmitter efficiency ratio
D. An indication of the quality of your station’s ground connection

298|T|7|C|04|C|What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?
A. 2 to 1
B. 1 to 3
C. 1 to 1
D. 10 to 1

299|T|7|C|05|A|What is the approximate SWR value above which the protection circuits in most solid-state transmitters begin to reduce transmitter power?
A. 2 to 1
B. 1 to 2
C. 6 to 1
D. 10 to 1

300|T|7|C|06|D|What does an SWR reading of 4:1 indicate?
A. Loss of -4dB
B. Good impedance match
C. Gain of +4dB
D. Impedance mismatch

302|T|7|C|08|D|What instrument other than an SWR meter could you use to determine if a feed line and antenna are properly matched?
A. Voltmeter
B. Ohmmeter
C. Iambic pentameter
D. Directional wattmeter

380|T|9|B|01|B|Why is it important to have a low SWR in an antenna system that uses coaxial cable feed line?
A. To reduce television interference
B. To allow the efficient transfer of power and reduce losses
C. To prolong antenna life
D. All of these choices are correct

388|T|9|B|09|B|What might cause erratic changes in SWR readings?
A. The transmitter is being modulated
B. A loose connection in an antenna or a feed line
C. The transmitter is being over-modulated
D. Interference from other stations is distorting your signal

1 comment:

Cindy Dy said...

It's enjoyable to learn more and more from your blog. Thanks for sharing.