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.

• 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.