Urban Legends from the world of Antennas. Marc C. Tarple

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Urban Legends from the world of Antennas

• Marc C. Tarplee Ph.D., N4UFP
• ARRL South Carolina Section
• Technical Coordinator

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Introduction

• There are more myths, distortions, lies, and
  urban legends surrounding antennas than any
  other aspect of amateur radio.
• In the following slides, we will examine some of
  these myths to see how true they really are

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Myth Number 1 High SWR is bad Antennas that
show high SWR are inefficient and do not
radiate well.

• False!!!!! an antennas efficiency is
  determined by the ratio of its radiation
  resistance to its total input resistance. Power
  reflected by the antenna travels back to the
  transmitter and is re-reflected back to the
  antenna

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Myth Number 2 Low SWR is good Antennas that
show low SWR radiate better.

• False!!!!! a dummy load has an SWR of 1.0 and
  it doesnt radiate at all. A short vertical
  antenna with a radiation resistance of 0.1 ohm
  and a loss resistance of 49.9 ohms radiates only
  0.2 of its input power but has an SWR of 1.0

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Myth Number 3 A vertical antenna is an antenna
that radiates poorly in all directions.

• False!!!!! A properly installed vertical
  antenna can radiate very well at low takeoff
  angles. A good ground system is a must for a
  vertical to work well.

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Myth Number 4 A quad array is better than a yagi

• Yes, maybe If a quad and yagi have the same
  number of elements and are installed at the same
  height, the quad usually has a slightly lower
  angle of radiation and will have slightly higher
  gain, because each element is an actually an
  array of dipoles.

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Myth Number 5 My antenna has to be resonant in
order to work.

• False!!!!! The antenna radiates because a
  varying current flowing through it creates an
  electromagnetic field. Resonance only simplifies
  matching the antenna to the transmission line.

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Myth Number 6 To connect 450 ohm ladder line to
my matchbox, I need a 41 balun.

• False!!!!! Commercially available baluns are
  designed to convert 200 ohms balanced to 50 ohms
  unbalanced. Unless the input impedance of your
  antenna system is 200 ohms, the impedance the
  matchbox sees is not 50 ohms and there are
  additional losses in the balun

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Myth Number 7 The most cost-effective
improvement to an amateur station is a good
antenna.

• True A simple 3-element HF yagi can provide a
  gain of 6 8 dB on transmit and receive and
  costs 400 600, which is about 100/dB. A good
  linear amplifier can provide 10 12 dB gain on
  transmit only and costs 1500 - 6000, which is
  at least 150/dB.

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Myth Number 8 The SWR of my antenna system can
be improved by changing the length of the feed
line.

• False!! SWR depends only on the line and
  antenna impedances, not on line length. If
  changing the line length changes the SWR, there
  are probably unwanted currents flowing on the
  outside of the coax and the varying SWR that is
  measured is not the true antenna SWR. A balun may
  be required at the feedpoint, or the coax may
  have to be re-routed.

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Closing Comments

• Do your homework before buying a commercially
  made antenna. It is not possible for an antenna
  to be electrically small, have high gain, high
  efficiency and large bandwidth simultaneously. Be
  skeptical of manufacturers claims.
• If you build you own antennas dont be afraid
  to experiment the best antenna is the one that
  works for you