Ham Radio General Class Practice Test

What would be the SWR if you feed an antenna that has a 300-ohm feed-point impedance with 50-ohm coaxial cable?

1. 1.5:1

2. 3:1

3. 6:1

4. You cannot determine SWR from impedance values

The correct answer is: 6:1

To determine the Standing Wave Ratio (SWR), we use the formula: \[ \text{SWR} = \frac{Z_L}{Z_0} \] where \( Z_L \) is the load impedance (the antenna's feed-point impedance) and \( Z_0 \) is the characteristic impedance of the transmission line (the coaxial cable in this case). When feeding a 300-ohm antenna with 50-ohm coaxial cable, you can calculate the SWR as follows: 1. Begin by plugging in the values: \( Z_L = 300 \, \text{ohms} \) and \( Z_0 = 50 \, \text{ohms} \). 2. Apply the formula for SWR: \[ \text{SWR} = \frac{300}{50} = 6.0 \] This means that the SWR is 6:1. A high SWR indicates a mismatch between the antenna and the feed line, which can result in reflected power being sent back towards the transmitter, potentially causing damage if the mismatch is significant and not managed properly. The other options provide different ratios that do not accurately reflect the impedance values given, and the