Radio: Difference between revisions

Walkie-talkies operate on license-free broadcast frequency ranges in the UHF (Ultra High Frequency) band respective to the region the walkie talkie is purchased within. In the EU the allocation is known PMR446 (Private Mobile Radio, 446 MHz)<ref>https://en.wikipedia.org/wiki/PMR446</ref>, whereas in the US it is typically the Family Radio Service (FRS)<ref>https://en.wikipedia.org/wiki/Family_Radio_Service</ref> or GMRS<ref>https://www.fcc.gov/wireless/bureau-divisions/mobility-division/general-mobile-radio-service-gmrs</ref>, and in Australia UHF CB<ref>https://en.wikipedia.org/wiki/UHF_CB</ref>.  

When selecting a walkie-talkie it is good to ensure it has a high IPX water rating, is durable, has a 'squelch' function (allows for 'cleaning' up a noisy signal to hear it better), good battery life and has at least 3W of power. Importantly, some walkie-talkies can only be charged in a special charging dock, and so if you choose a walkie-talkie of this sort it is important that it has removable batteries such that you can bring pre-charged batteries with you. If not, it is good to get walkie-talkies that take AA batteries, that you can recharge in a standard battery charger, one of which may include a portable solar charger.

The length of a random wire antenna should correspond to the bands you wish to listen to, and should be at minimum 1/4 of the wavelength of the band at which you wish to receive.  

In a disaster situation with no access to mobile data nor the Internet, and a handheld radio does not provide sufficient reception or is unavailable, AM radio is often a good first option. If you are wanting to listen for local shortwave (AM/MW) broadcasts and have little resources and/or poor reception with a little handheld, even a rough 6 meters length of wire should give you opportunity to listen to many stations within about 500 kHz to 13 MHz.  

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.

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 and FM 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 on 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. An example is J. C. Sprott, a physicist at University of Wisconsin, USA<ref>https://sprott.physics.wisc.edu/technote/randwire.htm</ref>.

[[File:Delta-antenna-triband.jpg|thumb|320x320px|Example of a multi-band delta loop antenna design, here for the 80m, 40m and 30m bands. This file shared courtesy of Iulian Rosu (YVA3IUL), and as per the attribution in the image]]

The sides of a delta loop antenna are equal and typically correspond to a 1/2 or 1/4 wavelength. Many designs are multi-band.

A comprehensive source of designs can be found at the QSL hobby and amateur radio site<ref>https://www.qsl.net/va3iul/Antenna/Wire%2520Antennas%2520for%2520Ham%2520Radio/Wire_antennas_for_ham_radio.htm</ref>.

[[File:Wavelength-calculation.jpg|thumb|left]]

An all-in-one transceiver is recommended for those with little technical knowledge. Most can be quickly learned reading their respective technical documentation. A more technical but in some cases less inexpensive option is discussed below.

[[File:Rtl-sdr.jpg|thumb|left|Four RTL-SDR devices, two of them with the popular SMA style brass connectors, used to interface with antenna. Note the writing on pink tape on one citing that device's particular parts-per-million clock offset, referenced by the operator when preparing the device for use.]]

When working with RTL-SDR devices it is important to use a reference tone to determine the amount of 'clock shift' (in parts-per-million) that the device in use has, and shift it using your chosen tool before use. This is due to the lower quality of the clock (used to tune) on these devices. They will differ from device to device.

== Citations ==

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