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Radio (view source)

Revision as of 04:21, 28 August 2023

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At the receiving end of the random wire antenna, a ''balun'' is ideally used. A balun 'balances' an unbalanced line, significantly improving reception quality and mitigating for many Standing Wave Ratio (SWR) challenges, typical of long lines of wire used in a radio context<ref>https://en.wikipedia.org/wiki/Balun</ref>. Baluns can be made using a simple toroidal core commonly harvested from electronics at hand, or a professional balun can be purchased suitable for the project.  
At the receiving end of the random wire antenna, a ''balun'' is ideally used. A balun 'balances' an unbalanced line, significantly improving reception quality and mitigating for many Standing Wave Ratio (SWR) challenges, typical of long lines of wire used in a radio context<ref>https://en.wikipedia.org/wiki/Balun</ref>. Baluns can be made using a simple toroidal core commonly harvested from electronics at hand, or a professional balun can be purchased suitable for the project.  


=== Random wire antennae for hobby and amateur radio bands ===
==== Random wire antennae for hobby and amateur radio bands ====


During emergencies hobby and amateur radio (HAM) bands can be a source of valuable information. Radio operators operating in their homes or 'radio shack' may send broadcasts on popular 20M, 40M and 80M bands. These are long-distance bands, sometimes covering hundreds or thousands of kilometers, and so this should be kept in context when seeking information. It is also important to beware the operators of these bands are not necessarily providing authoritative situational status information. For this reason, AM/MW bands should be the preferred choice for reception when mobile data and Internet connectivity is not possible.
During emergencies hobby and amateur radio (HAM) bands can be a source of valuable information. Radio operators operating in their homes or 'radio shack' may send broadcasts on popular 20M, 40M and 80M bands. These are long-distance bands, sometimes covering hundreds or thousands of kilometers, and so this should be kept in context when seeking information. It is also important to beware the operators of these bands are not necessarily providing authoritative situational status information. For this reason, AM/MW bands should be the preferred choice for reception when mobile data and Internet connectivity is not possible.


If you are wanting to listen to broadcasts from popular amateur radio bands, with far larger wavelengths (like 40M, 80M), you can try any very long wire you can get your hand on.  Very long wires can allow for tuning across several bands, as the wire will include 1/4, 1/2 and perhaps even full wavelength representations. However, reception will likely suffer from interference due to a phenomenon known as Standing Wave Ratio (SWR) due to differences in impedance between the antenna itself and the feedline to the receiver <ref>https://en.wikipedia.org/wiki/Standing_wave_ratio#Practical_implications_of_SWR</ref>. Physicists have studied random wire electromagnetic fields and with the aid of computer modeling have determined ideal lengths for minimal interference. J. C. Sprott, a physicist at University of Wisconsin, USA, has calculated the optimal lengths for reception of the most widely used bands of 1.8-2, 3.5-4, 7-7.3, 10.1-10.15, 14-14.350, 18.068-18.168, 21-21.45, 24.89-24.99, and 28-29.7 MHz<ref>https://sprott.physics.wisc.edu/technote/randwire.htm</ref>.
If you are wanting to listen to broadcasts from popular amateur radio bands, which have far larger wavelengths (like 40M, 80M), you can try any very long wire you can get your hand on.  Very long wires can allow for tuning across several bands, as the wire will include 1/4, 1/2 and perhaps even full wavelength representations. Using any long wire without a balun will often result in reception suffering from Standing Wave Ratio (SWR) interference due to differences in impedance between the antenna itself and the feedline to the receiver <ref>https://en.wikipedia.org/wiki/Standing_wave_ratio#Practical_implications_of_SWR</ref>. If you are without a balun, you can carefully select a wire for minimum interference. Physicists have studied random wire electromagnetic fields in relation to the SWR phenomenon, and with the aid of computer modeling have determined ideal lengths for minimal interference. J. C. Sprott, a physicist at University of Wisconsin, USA, has calculated the optimal lengths for reception of the most widely used bands of 1.8-2, 3.5-4, 7-7.3, 10.1-10.15, 14-14.350, 18.068-18.168, 21-21.45, 24.89-24.99, and 28-29.7 MHz<ref>https://sprott.physics.wisc.edu/technote/randwire.htm</ref>. As an example, a good wire length for broadcasts around 100kHz would be approximately 15 meters (49 feet).


A summary table produced by Sprott follows.
==== Wavelength calculation ====


{| class="wikitable"
If you know what frequency you need to tune into, you can calculate the full wavelength and build an antenna to match. The wavelength calculation is as follows:
|+ Caption text
 
|-
\lambda ={\frac {v}{f}}\,\,,
! Gap !!  !! Freq !! Length meters !! Length feet
 
|-
where v v is called the phase speed (magnitude of the phase velocity) of the wave and f f is the wave's frequency<ref>https://en.wikipedia.org/wiki/Wavelength</ref>. In the context of radio, 'v' is the speed of light, or 3×10<sup>8</sup> m/s. For instance, if the desired emergency broadcast station was at 1650kHz, the full wavelength would be 181.69m, and so a 1/4 wave of 45.42 meter (149 feet) or 1/2 wave antenna of 90.84 meter (298 feet) could be built.
| 1 kHz || .874265 MHz || 85.6 || 281
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== Citations ==
== Citations ==
<references />
<references />
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