All practical antennas have some directivity -- putting out more power in some directions than others.  Ideally, you would like to put power out (and receive from) only in the direction toward the station you're talking to.  A "beam" antenna, designed for directivity, can increase your signal by 1 S-unit (6 dB) or more, receiving and transmitting. Beam antennas work by using the "wave" nature of RF energy to create reflections and consentrations in certain relative directions, a process called beamforming, which narrows the RF radiated by the antenna to specific directions. The gain of the beam is the gain at the strongest concentration of RF energy. The "beam width" of an antenna is usually defined as the size of the azimuthal angle where the antenna's gain is at least 50% of its maximum and includes the "lobe" of maximum gain.

• Why A Beam Antenna?
  QST January 1972, pp. 36-39
  Some basic antenna information for the newcomer about Yagi antennas including a tutorial on antenna gain and construction of a 15-meter beam antenna. 
• Simple Gain Antenna for the Beginner
  QST August 1981, pp. 32-35
  A tutorial on the Yagi antenna with construction of a two element beam for 10-, 15-, or 20-meters. 
• The Building-Supply Yagi
  QST March 1991, pp. 22-24
  Here's a cheap, easy-to-assemble, two-element Yagi you can build for 10, 12, or 15 meters. 
• Two on 10
  QST April 1999, pp. 67-69
  A two element 10-Meter beam designed for portable or permanent installation.
• A Two-Element Duoband Beam
  QST April 1993, pp. 36-37
  Explore the 12- and 17-meter bands with this small, lightweight Yagi. 
• A 15-Meter Beam On A Budget
  QST February 1971, pp. 41-43
  A two element beam made from electrician’s thin wall tubing. 
• Basic Beams for 12 and 17 Meters
  QST August 2000, pp. 57-62
  Some well-designed and easy-to-build antennas for the 12- and 17-meter bands. 
• A Three Element Lightweight Monobander for 14 MHz
  QST July 2001, pp. 28-31
  A portable easy to build light weight antenna 
• A Portable 2-Element Triband Yagi
  QST November 2001, pp. 35-37
  This novel wire antenna is great for permanent or portable, QRO or QRP, and old-timer or beginner operation. 
• Practical High Performance HF Log Periodic Antennas
  QST September 2002, pp. 31-37
  The Electrical and mechanical design process for two Log Periodics that cover the HF bands from 10-30 MHz. 
• Simple Offset Feeding of Wire-Element Beams
  QST October 1999, pp. 45-46
  This approach to matching a feed line to an antenna uses the antenna itself as an impedance transformer.
• A Light and Sturdy Quad for 10 and 15 Meters
  QST July 1991, pp.30-32
  Here’s how you can build a two-element, lightweight 10- and 15-meter quad using parts available at local hardware and radio-parts stores and a sporting-goods mail-order supplier.
• A Five-Band, Two-Element Quad for 20 through 10 Meters 
  QST April 1992, pp. 52-56
  Want a small antenna that covers the ham bands between 14 and 29.7 MHz? Here's a solution with two alternatives for construction: using hardware-store parts or modifying an existing commercial triband quad

How to repair a CDE, Hygain, or MFJ Rotator by Jim Pickett, K5LAD.

A collection of antenna modeling files by K5TR. Mostly HF beams, including the trapped Hygain TH6DXX.