Problems

1. Use the data in Fig. 5 and compute the maximum usable frequency for communicating a given distance at several different times of day.

2. A grounded vertical quarter-wave antenna is fed 5.0 kilowatts at 550 kilocycles. Calculate the field strength produced at one mile, assuming no losses. Draw a circle showing the area covered by this station, if 0.5 millivolt per meter is the limit of satisfactory reception and if earth conditions are good. Repeat for a similar antenna fed the same amount of power at 1600 kilocycles. Draw a similar circle using the same center. Is there any relation between the areas of these circles and the frequency assignment of standard amplitude-modulation broadcast stations?

3. Use curves such as given in reference 16, and, for the conditions of Problem 2, plot two comparative circles for 550 kilocycles with maximum and minimum earth conductivities, and plot two comparative circles for 1600 kilocycles for maximum and minimum conductivities.

4. If two ships have antennas mounted 150 feet above the water line, how far can they communicate by direct-ray transmission?

5. Draw diagrams such as shown in Fig. 12 for a ³/₄λ antenna, and a 1.0λ antenna. What will be the driving-point impedances?

6. Calculate the data for and plot the ground-wave reflection diagram for a horizontal half-wave antenna ³/₄λ above the earth.

7. Draw arrows for instantaneous current flow on the antenna of Fig. 22, proving that it has the directional properties explained.

8. Calculate the horizontal radiation pattern for the antenna array considered on page 473, except that the spacing is 120° and the current ratio is 0.8.

9. Repeat the calculations on page 476 using equation 16 for an azimuth angle of 45° and a zenith angle of 45°.

10. A broadcast antenna has an input impedance of 37 ohms resistance. If it is drawing 1.0 kilowatt, what is the current input, and the voltage across the base insulator? Repeat for 5, 10, and 50 kilowatts.

11. If each sloping wire of Fig. 33 is 2λ in length, what must be the height in wavelengths? What would be the height in feet at 10 megacycles? At 30 megacycles?

12. If each of the four wires of the rhombic antenna of Fig. 35 is 2λ in length and if maximum directivity in the horizontal plane is the only objective, determine the important angles and dimensions for the antenna of Fig. 35.