chapter 5 digital communications 6/3/20151technician - 1 jul 10 - 30 jun 14 digital modes combine...

Chapter 5Digital Communications

04/18/23 1Technician - 1 Jul 10 - 30 Jun 14

Digital modes combine modulation with a protocol (the rules by which the data is packaged and exchanged). The protocol also controls how the transmitter and receiver coordinate the exchange of data.

Why use digital modes?

Special codes and characters embedded in the stream of data allow the receiving equipment to detect and sometimes correct errors which can’t be done with voice or CW modes.



Communications using digital modes requires additional equipment and software.

You will need a “modem” which changes data signals to and from audio signals.

• Can be a stand alone device

• Can be a device built into your radio• Can be combined with computer software and a sound card



If a microprocessor that performs the protocol rules is combined with a modem it is called a “terminal node controller” or TNC. A TNC that can perform several protocols is called a “multiple protocol controller” of MPC.

A modem, TNC or MPC is installed between a computer and the radio.

Software modems or TNCs use a sound card to do the work of the TNC. The power of soundcards in every computer allows ham protocol developers to experiment and develop new modes such as PSK31 and MFSK.



Different combinations of protocols, codes and modulation methods are used to create digital modes in several freq ranges. The combination determines the speed and the reliability of communications.

On HF, where SSB is the norm, popular digital modes include:

• Radioteletype (RTTY) that uses the 5-bit Baudot code• Winlink 2000 using the PACTOR or WINMOR protocols• Keyboard-to-keyboard modes such as PSK31 or MFSK



On VHF, where FM is the standard, popular digital modes include:

• Packet radio using the AX.25 protocol• Winlink 2000 using the B2R code to enable e-mail to be exchanged via packet radio.• Even CW is considered a type of digital mode using Mores code and AM



Radio signals experience many disruptions between the transmitter and receiver (e.g., fading, interference, and noise), which can introduce errors into the stream of data.

These errors are measured as the “bit error rate”.

These disruptions could cause errors in the received information if precautions are not taken.



Some codes include extra data elements to allow the receiver to detect an error. A “parity bit” can be used to detect simple errors in a sing character of data.

Some protocols add special data so that common errors can be detected and sometimes corrected.

On VHF and UHF, the most common digital mode is “packet radio”.



Packets are transmitted in noise-like bursts. “Frequency-shift keying” or “FSK” is used to transmit the individual characters in a series of rapidly alternating radio tones.

The receiving modem and TNC reassemble the data from the received packets.

Packets are transmitted at 1200 or 9600 baud so that the overall speed is about 400 or 3000 bits per second.



Each packet consists of a “header” and “data”.

The header contains information about the packet and the call sign of the destination station as well as a “checksum” that allows the receiver to detect errors. If an error is detected, the receiver automatically requests that the packet be retransmitted until it is received properly.

This is called “ARQ” for “automatic repeat request”. The rules of the AX.25 protocol insure the data accepted by the receiver is error free.



Individual packet stations can connect to each other directly with operators typing messages to each other – just like “instant messaging”.

Node stations act as routing centers for packet connections.

Stations can also use relays called “digipeaters” to connect to out-of-range stations.



Keyboard-to-keyboard modes are designed for real-time communications.

Most popular on HF.

Radioteletype (RTTY) is the oldest keyboard-to-keyboard mode, invented in the 1930’s.

Uses the “Baudot code”

RTTY signals today are generated by computer software and a sound card.



Model 28 Radioteletype

machine



The most popular keyboard-to-keyboard mode today is PSK31 which stands for “phase shift keying, 31 baud”.

PSK31 uses precise signal timing to aid the receiving modem in recovering the signal from noise and interference. Although it sends data at a slow rate, it works well in noisy conditions.

The software to use PSK31 is free and runs on most computers. A screenshot of the popular DigiPan software is shown on the next slide.



The “Automatic Position Reporting System” or APRS, uses packet radio to transmit the location information from a moving or portable station to a system of APRS digipeaters.

The location information is forwarded and stored on servers for access via the Internet.

The mobile or portable station is basically a packet radio station with a “Global Positioning System” or GPS receiver attached to the TNC. Along with location information some stations also transmit weather information.



Bob Bruninga (WB4APR) – a member of this club – designed and developed the APRS®.

Dr. Steve Dimse (K4HG) first introduced the APRS interface to the Internet in 1999.

This enabled the display of APRS maps on Internet Browsers and permitted RF originated APRS information from mobile ham stations to be accessible to non-hams too.



Winlink is another amateur radio innovation gaining popularity for emergency communications and traveling hams, especially boaters.

Winlink combines “Airmail” or “Sailmail” (e-mail programs) with a digital mode such as PACTOR III or WINMOR on HF and packet radio on VHF/UHF to allow the sending and receiving of e-mail.



This graphic is an example of how a station is configured to use digital modes.

If a sound card is used, its output is connected to the radio’s microphone input and the speaker or headphone output is connected to the soundcard input.



Use a digital communications interface to isolate the radio and computer to prevent hum or RF feedback from interfering with the data signals.



To make sure your digital signal is transmitted and received correctly you’ll need to adjust or configure the following:

• Transmit audio level• Receive audio level• Digital interface• Transceiver control

Refer to the data interface or software operating manual.

To return to phone operation, physically swap connectors or us a phone/data switch.



Repeater

Gateway

Gateway

Internet

Repeater

PC User

An amateur radio station used to connect other amateur radio stations to the internet is called a “gateway”.



Most gateways are set up to forward messages. The most common examples are packet radio “bulletin boards systems (BBS) and the Winlink RMS stations.

Messages with a recognized Internet e-mail address can be sent and received over these systems.

Note: The rules and regulations about commercial and business-related messages and communications apply to Internet gateways. Take extra care to comply with the amateur radio operating rules.

Chapter 5Power Supplies & Batteries


Most modern amateur radio transceivers run best off of 13.8 volts DC. They can also operate using a 12 volt car battery. A power supply converts 120 volt AC from the power company into 13.8 volts DC for amateur radio equipment. Power supplies have two ratings: Output voltage and the amount of current it can supply continuously.

A regulated power supply can protect equipment from voltage fluctuations through the use of a regulator circuit.



A single power supply can be shared between two or more pieces of equipment if it can supply enough current.



Installing an amateur radio in a vehicle requires special attention to safety. General guidelines include:

• Fuse both positive an negative leads near the power source.

• Connect the radio’s negative lead to the negative battery terminal or where the battery ground lead is connected to the vehicle body.



• Use grommets or sleeves to protect wiring from chafing or rubbing on exposed metal (e.g., bulkhead or firewall).

• Don’t assume all metal is connected to the battery ground – today’s vehicles are made of a mix or plastic and metal



Vehicle power wiring often carries noise that can affect your radio’s operation. The most common is example is “alternator whine” which is caused by noise on the power system inside your vehicle. You might hear not hear it but others will, every time you transmit. It will sound like a high-pitched whine that varies with your engine speed.

You can eliminate alternator whine by installing a dc power filter at your radio.



When power from the utility grid is unavailable, hams often rely on generators and inverters.

• A generator converts energy from an engine into ac or dc power.

• An inverter converts dc power into ac power.

Because voltage regulation is important, you should check the specifications of a generator to be sure the output voltage is acceptable at “no load” and “full load”.



Batteries supply dc power in place of power supplies when ac power is not available.

Batteries are made up of one or more “cells” which contain chemicals that produce current.

The cells of a multiple-cell battery are connected in series so that the voltage from each cell add together. The common 9-volt battery consists of six 1.5-volt cells



A 12-volt automobile battery is made up of six cells each of which produces two volts.

Batteries fall into three basic groups:

• Disposable

• Rechargeable

• Storage



Disposable and rechargeable batteries come in various sizes and types.

Battery PackCoin Cell



Alkaline and Carbon-Zinc batteries and Coin Cells are not rechargeable.

Nickel-Cadmium (NiCd), Nickel-Metal Hydride (NiMH) batteries and Lithium Ion battery packs are rechargeable.

You may put batteries in the refrigerator but never freeze them. Heat accelerates the chemical reaction in a battery.

When charging batteries only use a charger designed for that particular type of battery.



Storage batteries such as deep-cycle marine or RV batteries, are often used as an emergency power source by hams.



Storage batteries are often left connected to a charger so they are kept fully charged by a small flow of current. This is called “trickle” or “float” charging.

Be sure your charger will automatically switch to this lower current or it can overcharge and ruin your expensive batteries.

During an emergency, a 12-V lead-acid storage battery can be recharged by connecting it to a vehicle’s battery and running the engine.



Storage batteries must be treated with respect.

They contain strong acids that can be hazardous if spilled or allowed to leak.

Storage batteries can also release flammable hydrogen gas that can cause an explosion so be sure to store and charge these batteries in a well-ventilated area.



Battery packs are packages of several individual rechargeable batteries connected together in a single package and treated as a single battery



Rechargeable batteries or battery packs are convenient and less expensive than disposable batteries over time, but require a charger. Most hand-held radios come with a simple charger. A more sophisticated fast charger or drop-in charger is a convenient accessory.



For operations during emergencies, disposable batteries are preferred because they don’t rely on a battery charger for power.

Chapter 5RF Interference (RFI)


The increasing number of electronic devices and appliances put into use every day results in more interference between them and radios. This interference is called “radio frequency interference” or “RFI”.

RFI can be caused by amateur radio equipment or can be inflicted upon amateur radio equipment.



Types of common interference include:

• Fundamental Overload• Harmonics• Spurious Emissions

Filters are a critical element in preventing and eliminating RFI. They keep unwanted signals from being radiated and keep unwanted signals from being received.



“AC power line filters” are “low-pass filters” that keep RF signals from getting into or out of equipment via the ac power connection.

Low-pass and high-pass filters are installed in the antenna feed lines and reject interfering signals above or below the desired signal, respectively.



“RF choke” or “common-mode filters” made of ferrite material are used to reduce RF currents flowing on unshielded wires such as speaker wires, ac power cords, and telephone modular cords.

“Ferrite chokes” are also used to reduce RF current flowing on the outside of shield audio and computer cables.



Lots of devices are susceptible to RF interference: Telephones, doorbells, music players, touch-sensitive lamps to name a few. This type of interference is called “direct detection”.

Direct detection is most common in telephones because they are rarely designed to reject RF signals.

Low pass filters connected at the modular jack will reduce RFI from direct detection



When very strong signals overwhelm a receiver’s ability to reject them, this is called “fundamental overload”. When this occurs, you may hear interference on all channels of a TV or FM receiver, or across an entire band where the strong signal is present.

A high-pass feed line filter can be connected to the antenna input of FM and TV receivers to reject lower frequency signals from amateur HF and Citizen’s Band stations.

DO NOT use feed line filters of any sort on cable TV feed lines!



Placing a filter at the amateur’s transmitter will not solve overload problems.

The owner of the receiver is responsible to solve overload problems from a properly functioning transmitter.

When a TV receiver is overloaded by a nearby 2 meter transmitter, a “notch” or “band-reject” filter is required. These filters attenuate the 2 meter signal without filtering out the over-the-air broadcast (non-cable) channels 2-13.



Every transmitter’s output signal contains weak harmonics and other spurious emissions that can cause interference.

To prevent harmonics from being radiated, a low-pass or high-pass filter must be installed at the transmitter’s connection to the antenna feed line.

As a matter of good amateur practice, amateur HF stations use a low-pass filter to keep harmonics from being radiated in the VHF bands.



Interference to amateur stations is not usually caused by a transmitter. It’s more likely caused by “unintentional radiators”:

• Electrical arcs from motors, thermostats, electric fences, neon lights, etc.

• Cracked or dirty insulators, loose connections, wires rubbing together.

• Ignition noise



• Alternator whine.

• Switching power supplies used by computers and consumer electronics.

• Computer and networking electronics

Sometimes amateur will experience interference (intentional or unintentional) from unknown sources. In either case it’s important to locate the source of the interference. This is done by “radio direction finding” or RDF. By using directional antennas and maps it is possible to quickly find the source.



The following are guidelines for dealing with interference:

• Make sure your station follows good amateur practices for grounding and filtering.

• Eliminate interference to your own home appliances. Demonstrating that you are not interfering with you own devices is a good start and is considered good practice.

• Eliminate sources of interference in your own home.



You may encounter a situation where your signals are causing interference to a neighbor or a device the neighbor owns is causing interference to you.

Diplomacy is often required in these situations.

Visit the ARRL’s RFI Resources website:

www.arrl.org/radio-frequency-interference-rfi



Always check to make sure your transmitter is really causing the problem.

There have been many incidents where the mere presence of an antenna has resulted in reports of interference.

Offer to help determine the nature of the interference.

Detection, overload, or harmonics -- Knowing the source leads to solutions.



If a neighbor’s equipment is determined to determine the noise, offer to help determine the actual source of the interference.

Again, knowing the source leads to solutions.

When offering solutions, DO NOT offer to fix the problem.

As soon as you touch the equipment you could be blamed for it’s failure to operate correctly. Always let the owner fix the problem.



Always be diplomatic with your neighbors even though it may be their responsibility to deal with interference to or from their devices.

It is very doubtful that they are aware of the FCC rules.

Getting the FCC involved should be the last resort.



Part 15 of the FCC’s rules govern the responsibilities of owners of unlicensed devices that use RF communications (e.g., cordless phones, wireless data transceivers) or devices that radiate RF energy unintentionally (e.g., power lines, electric fences, or computers).

As long as your amateur station is operating properly under the FCC’s rules, then your station is protected against interference caused by Part 15 devices and complaints of interference to unlicensed equipment.



In other words, the owner of a Part 15 device can not seek redress from a properly operating amateur radio station.

The owners of Part 15 equipment do not get protection from interference and must accept interference.

It is the responsibility of the owners of Part 15 equipment to eliminate interference

caused by their equipment of devices.

These rules are printed in the owner’s manual of all unlicensed devices and are available on the FCC web site.

Chapter 5RF Grounding


In addition to ac safety grounding, it is also necessary to consider RF grounding.

RF grounding does not keep your equipment exactly at earth ground potential. It refers to keeping all of the radio equipment at the same RF voltage.

By keeping all of the equipment at the same RF voltage, RF current will not flow between them.



RF current in your station can cause audio distortion, erratic operation of computer equipment and occasionally RF “burns” where the RF voltage happens to be high.

RF current flowing in sensitive audio cables or data cables is more likely to interfere with your station’s normal functions. For example, RF feedback via a microphone cable can cause distorted, garbled or unintelligible transmitted audio.



Every station is different and will have to create a unique RF ground.



Never substitute an RF ground rod for a properly wired ac safety ground.

Local building codes will specify how ac safety grounds should be wired and bonded together.

Bond all metal equipment enclosures to a common ground bus.

Connect the ground bus to a ground rode with a short, wide conductor.



Keep all connections, straps and wires as short as possible.

Braid removed from coax should not be used for RF grounding.

• Too loose• Subject to oxidation

Just use the ground bus if your station is above the ground floor.

You can use a metal cold water pipe for a ground as long as the pipe takes a short, direct route to exit the house.



Questions?Read Chapter 6 for the next session.

chapter 5 digital communications 6/3/20151technician - 1 jul 10 - 30 jun 14 digital modes combine...

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