Getting Started: Introduction

2011/12/27

by heaty (Twitter: @mzerod)

This document was drafted to help introduce people to the realm of Citizens Band radio, specifically with packet radio usage in mind. The original intent behind this document was to establish some grounding in radio theory, to align expectations with reality, and to help educate those inspired by the free nature of the airwaves - specifically those set aside for citizens use without needing to hold a permit (where legal).

The motivation to write this article came from questions posed by the Alternative MeshNet Plan community interested in emergency citizens communications - specifically freeband meshnets which can do long distance store and forward for basic data. An additional article is available which talks about some aspects and limitations directly relating to the subject here.


Interface Hardware

To begin, you will need to understand some of the principles behind how radio works so that you don't waste time and money on expensive and useless crap. There aren't any plug-and-play solutions, there aren't any products on the market tailored specifically for CB packet usage either, so we make do with whats out there.

Below are two different methods of getting packet radio working.

Soundcard Packet

Using your computer's sound interface as a modem is a fairly 'new' approach to modulating packet radio signals. Since computers have increased in CPU power since the early 90s it has been possible to replace the hardware modulation circuitry with a 100% software solution.

A radio typically interfaces with a computer via Line-In/Mic inputs and Line-Out/Speaker/Headphone output connected to the radio's mic-in and external speaker socket. The transmission keying, that means PTT (push to talk) is done via a simple circuit connected to the computer's serial port (or a USB-SERIAL converter).

For windows users, a soundmodem driver exists called AGWPE. For linux, there is a userspace driver called 'soundmodem' which is available on-line. Note: We plan to create a linux set-up article sometime in the future as a lot of the info for setting up AX25 on Linux is now scattered, old and pretty confusing.

Soundcardpacket.com - A site with plenty of information regarding packet radio operation using a PC soundcard instead of dedicated packet hardware. The published information is intended for Amateur Radio operation, however the information can be appied for CB Packet use. The site includes instructions on how to set up the wiring between PC and radio, a lot of CB radios are compatible with the PTT switching method employed using the soundcard packet system.

Dedicated Packet Hardware: TNCs

Using dedicated hardware for packet radio is probably the most simple way to go about setting up packet if you wish to run a semi-autonomous station without a PC attached.

TNCs, or Terminal Node Controllers are little boxes that attach directly to the radio and often have internal CPU, memory and ROM that loads a basic operating system enabling you to simply use a serial terminal program to access packet radio.

Many TNCs have an internal mailbox/personal BBS, node and digital-repeater functionality as well as a host of other features. These devices do not need a PC connected permanently to operate.

Additionally, many TNCs support a transparent 'through' mode called 'KISS' which enables software running on a PC to directly control the TNC as if it were just a dumb packet modem - this is useful when you wish to run software that is far more advanced than the software offered on the ROM, such as NETROM node, BBS, Converse and internet-to-packet gateways.

Kantronics KPC3 TNCs come highly recommended, though these are fairly rare to find nowadays.


Expectations

First, "OMG FREE WIFI OVER 1000 MILES!!!!!". If this is your thinking, I'm really sorry but you're out of luck. That isn't achievable on CB radio, there just simply isn't the bandwidth in 10khz of channel spacing to accommodate more than 1200 baud... Yeah. It's slow.

Secondly, radio is a specialist field, do not expect to be making international long distance contacts by just hooking up a CB radio and antenna bought at a store while the antenna is propped up the wall or lying on the grass in the yard. You have to put in quite a considerable bit of work to get the most out of what is essentially a low powered device intended for short range communications.

Long Distance Communication (DX)

Long ago, the American FCC chose 27Mhz in the shortwave range for the home of a personal communications medium which today is known as CB, or Citizens Band. Due to exports to many countries these devices were eventually adopted elsewhere, illegally, but were eventually aligned with legislation, often requiring new frequency coverage or modulation type. This caused some fragmentation and is responsible for the varied types of CB on the market around the world today.

The choice of frequency is actually key to the long distance (DX) possibilities with a CB radio. There is a phenomena that causes the high altitude naturally occurring wrapper that encompasses the Earth - known as the ionosphere - to reflect radio signals that are below a certain threshold. That threshold is around 30Mhz.

Any signals with a frequency above 30Mhz penetrate the ionosphere and continue radiating out into space, those below (such as 27Mhz for CB) are often reflected under the right conditions to enable communications over great distance.

One of the challenges of DX communications on CB is that the ionosphere is dynamic, it is constantly changing and as such propagation of signals can never be easily predicted nor forced. The ionosphere also treats higher frequency signals less efficiently than lower frequency ones, and as you may have noticed, 27Mhz is at the upper limit of the shortwave band, close to the 30Mhz threshold

During solar events such as sunspot flares, the ionosphere is bombarded with particles from the Sun. These particle collisions cause sporadic ionization at lower altitudes in the atmosphere which can create temporary reflective 'mirrors'. This phenomena often enables 'skipping' propagation of radio waves at frequencies that are even higher than the usual threshold.

This sporadic ionization is usually responsible for long distance communication possibilities on 27Mhz CB radio. Due to the sporadic nature of the ionization, this type of radio propagation is not at all reliable, it's sporadic. A strong sporadic event can open up an inter-continental or even transatlantic pathway for several hours, then suddenly disappear. It's pretty sporadic, maaan.

With this in mind, you need to evaluate whether CB is the right medium for you. Operating DX is a challenge even for the most experienced of radio amateurs. That said, DX is only a small part of the overall CB radio appeal.

Local Communication

Another aspect of CB is local communication. A group of stations within a few miles of each other will be able to communicate with each other on CB - as long as the antenna is sufficiently high and clear of obstacles. Generally speaking, CB works as a line-of-sight medium when it isn't bouncing off the ionosphere, though due to the physical properties of 27mhz CB antennas and the wavelength of the frequency - CB is capable of significantly better propagation in an urban setting compared to the UHF (Ultra High Frequency) portable devices such as PMR446 (FRS in the USA). These devices are usually much lower power but have some advantages in their compact antenna size (a few centimeters, instead of meters).


Radio Hardware

Those who have no clue about how radio works will certainly have a difficult time setting up a station for the first time. Setting up a CB radio is not an entirely plug and play affair. There are quite a few quirks that go into getting RF (radio frequency) to radiate from an antenna properly without damaging your radio, so it is probably best that you learn some basic theory.

There is also a lot of kit out there floating around on the 2nd hand market which is unsuitable for your needs. It is not uncommon for unscrupulous people to sell receive-only scanner antennas as antennas for CB, or antennas meant for different types of CB radio.

The range of CB equipment available today is now very diverse after 30 or so years of CB radio popularity, some of the equipment won't have the necessary features, or it may not even be on the correct frequency band - some of it may even be illegal to own.

This document will try and give you some hints which may help you determine what you need.

The Antenna

This is the most important bit of kit in the system. You can have the world's most pimped out radio, but it is the antenna that does the actual work. If you skimp on the antenna setup, you will already be putting a limitation on what your radio will be able to do.

Often the easiest practice is to buy a pre-made vertical CB antenna and have it mounted as high up as possible, away from neighbours and over head wiring. You may have to be aware of local planning regulations if you have to install it on a property, there may even be laws governing antennas in your area too.

Be warned - If you think about opting for a mobile-magnet mounted antenna for home operation, you will be severely disappointed when you find out that half the antenna is missing - the metal body of the car. So always opt for a homebase antenna for home operation. Popular commercially available CB antennas usually state what frequency range they are intended for, since this site is geared towards CEPT CB band usage, that is therefore assumed to be 27Mhz.

The size of the antenna is a consideration you cannot ignore. For base station operation, you can expect the antenna - usually a vertical pole - to be about 3-4 meters (10-15 foot) long. Sure, you can get compact antennas that are shorter, but they are very very inefficient and use coils of wire to make up for the electrical length of the antenna. This is called 'loading'.

A recommended antenna is a simple 1/2 wave vertical, whilst being pretty large - they are more efficient with the little power you have available, and they are also far better for reception than shorter, loaded antennas simply because they have a larger footprint to capture the radio signals with.

If you want to learn more before making any decision to buy an antenna, we recommend you read this article (locally mirrored here).

The Radio

Which band?

Pick a radio that covers your frequency band allocated for Citizens Band. You can check this by looking for information on your government radio agency's frequency plans, or look up CB gear being sold in local electronics shops or 2nd hand market place. There are some countries where the 27Mhz CB band has not been implemented - for example Australia.

For the sake of sticking with the theme of this document - CEPT 27mhz band - we shall assume that this is the norm. Note that the CEPT band has some cross-over compatibility with the USA allocation, though due to the legacy of CB in the USA, The US utilizes AM modulation rather than FM. Additionally, operating packet on American CB is illegal.

There is also a UK-only allocation which sits above the CEPT 40, these radios have a special mark on them - a circle with "27/81". These radios are unsuitable for packet usage as they are for use within the UK only - and do not cover the international packet frequency, 27.235Mhz. Since 2006, The UK has deregulated CB radio and has adopted the CEPT model giving the UK access to 80 channels from 26.965-27.405Mhz and 27.601-27.991Mhz respectively. On newer CB radios, both blocks of 40 channels are often available

Circuit compatibility

When choosing a radio for packet usage, the method that causes the radio to switch between receive and transmit has to be considered. It is a game stopper if you get it wrong.

Really old and cheap radios used to (and to some degree, still do) use the microphone itself to do the actual switching off of the receiver, and switching on of the transmitter. This is done via a dual-pole sliding microphone switch and it is NOT compatible with packet radio Push-to-talk (PTT) transistor switching methods, unless you wire up a relay to do the manual double-switching automatically (very impractical, so just avoid). Typical radios that suffer from this problem are those based on the Cybernet motherboard design.

The PTT method you need to look for is the type that allows just 1 microphone pin to be shorted to ground to cause the radio to flip flop between transmit and receive. For instance, in many Cobra and Superstar brand radios (Uniden motherboards) the PTT pin is often pin 3. By shorting pin 3 to the ground pin (often pin 1), the radio switches to transmit and turns off the receiver automatically.

Converted Amateur Radios

Operating equipment not intended for CB use is usually against the law. The reason being is that CB gear has to go through technical type approval and these tests ensure the devices operate within the restrictions of the Citizens Band legislation.

However, it is quite obvious from observation that a lot of people decide to use Amateur Radio equipment modified for CB radio frequency coverage, these radios tend to output more power, are often more modern in design and occasionally have dedicated inputs for radio-data. They are worth considering if you don't mind the risk, nor the expense.

A modern popular conversion for CB use is the Icom 706. Older equipment is often converted 10-meter amateur band gear, (admittedly, a lot of the gear marketed as converted 10-meter band gear was in fact manufactured intentionally for illegal CB use).

Known working radios

From personal experience of the author, illegal CB radios such as the Cobra 148 GTL DX, Superstar 3900, Super Jopix 2000, Superstar 360 radios all have the ability to run packet radio both on 27.235Mhz, with the correct PTT wiring, FM and (optional) SSB modulation methods. Basically any radio that can be keyed as mentioned earlier with single-wire grounding is compatible if it also meets the frequency requirements.

The choice to use radios which fall into the illegal category of multimode radios which have non-regulatory compliant features is left entirely at the users own risk.

A list of compatible radios along with their frequency coverage is under construction as of December 2011. However that will requires crowd sourcing the information as the author has only limited experience with the aforementioned radios. If you wish to get involved, this list is work in progress, to contribute a known compatible radio to the list, please get in touch via the contact form.


This concludes the document. We may introduce an additional article covering Software in greater depth, if there is demand. If you have any questions that are not answered here but you feel may be of benefit to be answered here, get in touch via the contact form.