Radio: Difference between revisions

== AM(MW)/FM radio reception ==

In an emergency where critical infrastructure is experiencing outage, local information will almost always be broadcast over AM (also referred to as Medium Wave) and/or FM. It is for this reason, at the least, a simple AM(MW)/FM radio with charged batteries and spares should be acquired and stored in an accessible place. They can usually be bought very cheaply. [[File:Am-fm-radio.jpg|thumb|left|A photo of 2 AM(MW)/FM radios, each with extendable antenna and headphone jack.]]

It is good to buy am AM(MW)/FM radio with a mini-jack socket and extendable antenna. The mini-jack socket allows for plugging in headphones which can be useful when reception quality is poor. The extendable antenna can greatly aid in finding and receiving a broadcast.

If you are caught out without a radio and have access to a vehicle, it very likely has a built-in AM(MW)/FM radio. Note that AM broadcasts travel over vastly longer distances than FM, and so it is important to be aware of this in relation to your geographical context. As always the best reception with radio is line-of-sight. While it is rare we can see the antenna(e) we are receiving a broadcast from, getting to a position of altitude away from RF (radio-frequency) occlusions like buildings and hills can make all the difference in reception.

Unlike AM(MW)/FM radios detailed above, walkie-talkies are both broadcast station and receiver, and so known as 'two-way' radios. The range of a modern walkie-talkie is anything from a few hundred meters in a city and in dense vegetation, to many kilometers when used on a peak or high rooftop. This communication tool can be especially invaluable when you are in a group and need to split up, staying in contact and relaying information.

Regardless, if you are in a region using 2 or more walkie-talkies operating on unlicensed bands, they will still function. Typically walkie-talkies include a channel selector for selecting a pre-agreed channel to operate on. This selector is either a button or dial. It is important to agree on a channel to use before splitting up your group and using the walkie-talkies. Some walkie talkies also include an AM/FM radio scanner, allowing the operator to quickly find and tune into a local radio station. Some even include NOAA weather radio receivers.

==== Wire antenna for AM/MW reception ====

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/MW radio is 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 an AM/MW range (about 500 kHz to 13 MHz).  

==== 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.

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

==== Delta Loop Antenna ====

==== Other wire antenna designs ====

==== Wavelength calculation ====

[[File:Transceiver.png|thumb|A modern HF transceiver with a spectrum analyzer and DSP capabilities]]

== Software Defined Radio ==

[[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.]]

An inexpensive option for those wishing to build a quality receiver for their long-range antenna project is working with RTL-SDR devices and compatible tuners. RTL-SDR devices are low-cost digital video tuner (DVB-T) USB sticks using the Realtek RTL2832U controller that can be repurposed for receiving a broad spectrum of radio signal<ref>https://www.rtl-sdr.com/</ref>. Some RTL-SDR models will provide reception from 500 kHz up to 1.75 GHz. For those wishing to work with lower signals, for instance in the MW, an 'upconverter' can be used. A popular upconverter is the Nooelec 'Ham it up'.

[[File:Full-wave-loop-listeners.jpg|thumb|right|240px|Radio enthusiasts in Aotearoa New Zealand listening to an AM radio broadcast from Austria, received on a 20M Full Wave Loop wire antenna connected to an RTL-SDR tuned by the GQRX software]]

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.