Tuesday, December 20, 2011

SWR: Does it matter?

SWR and baluns were discussed during several of our recent morning QSOs. We combined personal experiences and many on-line references in order to better appreciate the topic.
  • Understanding SWR by Example by K5DVW is a must-read article. (You can skip the Smith Chart section.) The article illustrates how transmission line loss can easily be a greater issue than SWR when trying to transfer maximum power to the antenna. 
  • VSWR Myths includes a similar set of examples and goes on to discuss various common myths. 
  • The Easy Way is a wordy article that also makes the case for non-resonant antennas and tuners to operate across multiple HF bands. Four topics covered in separate sections of the article are: Antennas of non-resonant length, Line attenuation,the Transmatch, and the Balun.

A few online calculators enable you to estimate the losses due to SWR:

Many articles explore the use of baluns in all sorts of HF antennas:
  • Choosing the Correct Balun by W8JI provides guidelines for several antenna types. An area of particular interest in our group discussions is the multi-band  dipole with balanced feeder. A table in this article includes recommended element and feed line dimensions. I found a second version of this article that has a copy of Table 1 with data for both 300-ohm and 450-ohm feeders.W8JI has lots of other content on his web site.
  • Baluns in Antenna Systems by VK1OD analyzes several balun usage scenarios and compares their modeled behaviors, with conclusions for each. Other references are provided. The author's home page has links to lots of content.
  • Baluns: What They Do and How They Do It by W7EL gives a detailed explanation, with sketches, of what issues a balun is intended to solve. The article includes the results of field experiments to support conclusions about balun use, but also with further questions. W7EL is the author of EZNEC software.
Finally, from G4NVH comes HF Balanced Transmitting Systems, a 75-page compilation of materials from various sources. See the table of contents and pick your favorite topics!


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