Shortwave broadband antenna

A shortwave broadband antenna is a radio antenna, that without adjustment, can be used for transmission (and reception) of a shortwave radio channel chosen from greater part of the shortwave radio spectrum. Some shortwave broadband antennas can even be used on the whole shortwave radio spectrum (1.6-30 MHz) which consist of the upper part of medium frequency (1.6-3 MHz) and the whole of high frequency (3-30 MHz). A true shortwave broadband antenna will work continuously across most of, if not all of the shortwave spectrum with good radiation efficiency and minimal compromise of the radiation pattern.

At the lower shortwave frequencies e.g. 1.8 MHz, the antennas need to be physically large to enable good coupling to "space" and hence efficient radiation. As an example, at 5 MHz a half wave dipole antenna is around 30 meter in length and 2 MHz it is 75 meter long. Half wave horisontal dipoles are efficient radiators, if they are high above ground in terms of electromagnetic waves wavelengths - if not then half wave horisontal dipoles are inefficient radiators.[1] Half wave dipoles are narrow band (only work over a very small frequency range) before serious impedance mismatch occurs. This mismatch can be accommodated using an antenna tuner but these add to costs and modern shortwave communication often uses frequency hopping techniques. Even automatic antenna tuners will not work with frequency hopping signals.

Broadband shortwave base antennas traditionally fall into two main categories:

The challenge for many years has been to devise an antenna which is an efficient radiator, compact, and also inexpensive. Previous solutions include the Barker Williamson folded dipole, the Australian traveling-wave antenna and other designs by Guertler etc. Many "broadband" designs have also been offered by the Amateur Radio community but these are generally not true broadband antennas as they only work well (without an antenna tuner) in the harmonically related Amateur bands.

Examples

Robinson Barnes HF Broadband Antenna

The Robinson Barnes HF Broadband Antenna was conceptualised and developed in the early 1990s by Graham Robinson and John Barnes and has become a widely used design for commercial and military shortwave base stations, where ground space is limited, yet a full 4 octave bandwidth (2-30 MHz) is required.

It is a centre fed wire antenna with two arms each consisting of 3 radiating elements and is generally tower mounted, either horizontally or as an "inverted V".

The design has almost double the radiation efficiency of the shortwave folded dipole of the same length and is nearly half the size of other traditional HF wire antennas such as the "Australian travelling wave dipole" The standard size Robinson Barnes antenna is only 27m in length. Various Models are available in lengths of 20, 27, 54 & 74 Metres and power ratings of 125w to 1 kW Continuous - Ruggedized versions are also available for extreme environments.

Graham Robinson was the founder of Bushcomm HF Antennas[3] located in Perth Western Australia. The "RB" Travelling Wave Antenna is popular for use by Governments, UN, Military, NGOs,and is marketed in various configurations as a BBA-100.[4]

References

  1. ↑ 160-Meter n0gw.net: Dipole Height. N0GW - Gary Wescom - 1 Nov 2006, backup Citat: "...Table 1 – Dipole performance over average ground...So, what is the “bottom line”?...Effectively, almost any horizontal antenna configuration for 160 meters is going to be a high angle radiator..."
  2. ↑ The Terminated Coaxial Cage Monopole (TC2M). A new design of Broadband HF vertical antenna. Martin Ehrenfried, G8JNJ
  3. ↑ Bushcomm HF Antennas
  4. ↑ BBA-100
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