If you visit:
http://en.wikipedia.org/wiki/Radio-wave_propagation
you can see the radio spectrum broken down into bands. The VLF and LF bands are low frequency and travel by ground waves, following the curvature of the Earth. The are used to send short messages to ships and submarines at sea. (Flash traffic in the movie "Crimson Tide".) Used by MP to wake teams. The radio equipment the teams have can not send signals at the proper frequency to respond with a message back to PB.
The PRC-70 operates across the HF band and about 40MHz into the VHF band. The PRC-70 has a range of 2500 miles if it is properly set up to operate in the CW mode, in the lower end of the HF band, with the proper antenna, the doublet, erected at a sufficient height. It does this by skywave.
http://en.wikipedia.org/wiki/Skywave
HF can also use relay stations to increase the range of signals.
The VHF band on the PRC-70 is for local communication with units close to the radio. From a few miles up to tens of miles depending upon how high the antenna is and ground clutter. This is a direct line of propagation communication mode. A relay or repeater station can be erected on a tall tower, hill top, etc. that recieves and re-transmits the signal at a higher power level. The combination of height (few hundred feet) and increased power expands the signals coverage area to fifty or sixy miles in diameter. Relays can be simplex, one frequency for both recieving and re-transmitting, or duplex, a frequency for recieving and another about 500kz from the other for re-transmission.
The doublet antenna is a half wave dipole. It is made of two wires set in a single line, one wire is connected to the radio and the other is connected to the ground. A horizontal orientied dipole radio radiation pattern is like a doughnut (with a very tiny hole) and the wire goes through the center of the doughnut hole. If you set the wire of the antenna on a north-south axis most of the energy is transmitted to the east and west. A vertical whip antenna is a quarter wave antenna (half of a dipole). The ground of the Earth forms the other half of the "dipole" of a vertical antenna. They radiate in a 360 degrees around the vertical antenna but at reduced power from a dipole.
A type of directional antenna is referred to as the Yagi-Uda. To learn more about it read this article:
http://en.wikipedia.org/wiki/Yagi-Uda_antenna
Whereas the horizontal dipole has a radiation pattern like a doughnut, the Uda-Yagi has one side of the "doughnut" at a smaller, less powerful size while the other side is bigger, more powerful and shaped like ovaloid or egg. They are often the easiest directional antenna to make and are use for radio direction finding.