Consider the following statements about the maximum usable frequency for radio communication between two specified points by reflection from an ionosphere layer:
1. It is equal to the critical frequency
2. It is more than the critical frequency
3. It depends upon the distance between the two points
4. It depends upon the height of the ionosphere layer
Of these statements

Coupling into and out of a traveling-wave tube can be accompanied by a

A. Waveguide match

B. Cavity match

C. Direct coax-helix match

D. All of the above

When it is desired that short-range targets be clearly seen on a pulsed-radar set, it is important that the receiver and display system have

A. A long time constant

B. Low-pass filters

C. The shortest possible time

D. The restricted high-frequency response

The antenna separations (in meters) required for optimum operation of a space diversity system can be calculated from:
where R = effective earth radius (m) and L = path length (m)

A. $${\text{S}} = \frac{{2\lambda {\text{R}}}}{{\text{L}}}$$

B. $${\text{S}} = \frac{{3\lambda {\text{R}}}}{{\text{L}}}$$

C. $${\text{S}} = \frac{{\lambda {\text{R}}}}{{\text{L}}}$$

D. $${\text{S}} = \frac{{\text{L}}}{{\lambda {\text{R}}}}$$

The aquadag coating on the inside of PPI tube is used

A. To focus the beam of primary electrons

B. To shield the electron beam from unidirectional magnetic

C. As a second anode and to prevent the build-up of secondary field

D. All of these