LED Animated GIFThe light emitting diode (LED) comes in a wide variety of sizes, shapes, and colors, and is something every ham should be familiar with.  There are a number of exam questions related to LEDs.


LEDs are more common than you may realize.  Not only used on ham radio equipment, they are typically found on most electronic gadgets, primarily as indicator lights, most often as power or status indicators.


Developed in the early 1960s, visible LEDs became practical replacements for miniature incandescent lamps in the 1980s.  Their main advantage is in efficiency (wasting little power), but they also last many years and illuminate instantly, all compared to incandescent light bulbs.


The above question phased out of the General class exam pool in 2019 but it speaks to the benefits of LEDs over other technology.  This example from the Tech license exam pool shows an LED being used in a traditional power supply circuit as a power indicator:

T6C07-2018Figure T2

Useful as more than just on/off indicators, LEDs, when grouped, really open up interesting applications.  When formed as bars in a figure 8 arrangement, the classic seven segment display is created:

seven seg display

This revolutionized electronics to display numbers quickly and inexpensively and were an early use of the technology starting in the 1970s.

Also, when different color LEDs are bunched together their combined light can form a different color, often to achieve white.  RGBThis is the principle of LED back lighting for flat panel displays and for the recent adoption for home and general lighting (replacing light bulbs).

LED Bulb White LEDs (or a blend of other colors to make white) weren’t practical until the mid-1990s, and it took years to become economical.  The widespread use of LEDs for display back lighting and general lighting is now less than 15 years old.

For those interested in the technology, we’ll dive into some of the details of LEDs now.  As the name implies, an LED is a form of diode, the simplest possible semiconductor device formed by Continue reading

Selective Calling & Tone Squelch

In a previous post we introduced the term squelch and how it was used in basic operation.  As mentioned there modern ham radio work (especially with repeaters) often involves other technologies that fall under a different category named selective calling, tone squelch being the most common form.

Selective calling is different from ordinary carrier or noise squelch.  In practice it is more of a security or channel sharing function.  Selective calling encompasses several similar technologies which largely do the same thing: prevent a transmitted signal from being received by other radios unless a particular code is entered by the sender.

From the Wikipedia article, “Selective calling is akin to the use of a lock on a door. A radio with carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct “key”, in this case a specific code.”

The most common form of selective calling in use by hams is a tone squelch system with the awkward name of continuous tone-coded squelch system (CTCSS).  Squelch in general is used to keep commercial and amateur radio repeaters from continually transmitting. Since a carrier squelch receiver cannot tell a valid carrier from a spurious signal (noise, etc.), CTCSS is often used as well, as it avoids false key-ups. Use of CTCSS is especially helpful where nearby repeaters may share the same frequency or in a high electrical noise or RF environment.


As the name implies, CTCSS sends out a continuous tone along with the transmit audio.  The tone is termed, “sub-audible”, although it is often a low audible frequency.  Most radio systems filter out these low frequencies so it is unlikely that you will hear the tone when listening to another ham unless you have an unusual radio and/or are wearing high fidelity headphones.

The CTCSS tone is selected by a repeater operator to avoid duplication with nearby repeaters on the same frequency.  In most cases, the objective is to reduce interference and not restrict legitimate access.  There are 100 established CTCSS frequencies but some are more commonly used than others.

CTCSS is often referred to as PL because it’s easier to say.  They mean the same thing but PL® (stands for Private Line) is a registered trademark of Motorola’s implementation and was the original employment of CTCSS.


Besides CTCSS, other forms of selective calling in use by hams include:

  • Selcall (mostly European)
  • Digital-Coded Squelch (DCS)

Why it matters to us

T2B04-2018Because various forms of selective calling prevent a signal from being re-transmitted (repeater) or received (simplex) without the proper code or tone, use of CTCSS or DCS is a possible reason other stations cannot receive you.  Especially on a repeater, if others cannot hear you it’s quite likely that you have the wrong code or your tone squelch is turned off.  How to know what the proper setting is?  Consult the repeater directory.

Some Useful Links

CTCSS – RadioReference

How radio “privacy tones” or CTCSS tones work – YouTube video


Get the Right Signal Tone – Ham Radio School

Hey, Why Can’t I Access the Repeater? – Ham Radio School

Differences between CTCSS and DCS – Retevis

CTCSS & Tone Burst Ham Radio Repeater Access – electronicsnotes

A Historical and Technical Overview of Tone Squelch Systems – WA6ILQ

Repeater Related Terminology – Repeater Builder group


Squelch is a funny word that is familiar to many of us without understanding what it really means.  Hard to improve on the definition beyond that in the Technician license exam question:


Most useful when using voice modes (phone), squelch makes radio operation more bearable by turning off the audio when there is no valid signal.  Without squelch our radios would be cranking out a lot of unwanted background noise.

The reality of both AM and FM radio (which we covered recently) is that there is electrical noise in the bands from many sources, natural and man-made.  This noise is often randomized so that it appears as hiss or fuzz (white noise) from radio receivers.  A squelch circuit mutes receiver audio to block the noise when there is no real signal.  Squelch acts as a noise gate which closes for random noise and opens when a real signal (such as modulated voice) appears on frequency.  How the squelch circuit determines what is a valid signal and what is noise varies; there are a few common techniques (refer to algorithm link in references below).

Squelch is built into more expensive broadcast receivers.  It may also be found on AM transceivers (particularly CB radios) and is a feature of nearly every FM amateur rig.

While less prone to electrical noise, FM technology is susceptible to a lack of true signal.  Traditional FM receivers use an LC tuning circuit that generates ‘hiss’ with no signal present.  You may have noticed this while tuning between stations  on your broadcast FM radio.  Since hams commonly use VHF/UHF FM transceivers for local chat and EmComm work, the squelch feature of our rigs is of particular interest.

Squelch setting is important because if you make it too tight you may not hear a weak signal; too loose and you get constant noise.  General good practice (at least a starting point) is to turn down the squelch until you hear background noise (hiss), then increase the threshold until the noise goes away, then just a little more.

Squelch threshold is always settable on your FM transceiver.  There may be a knob to turn as in the example below, or it may be through keypad menu.

Squelch Adj FT7900

Squelch tail is a common term that is related to all this.  The tail is the brief “pfffft” sound heard when another station stops transmitting.  Audio example on YouTube here.  It is the natural presence of noise during the delay between the time a signal drops and the squelch kicks in.  On a repeater it will often followed by an audible beep or similar courtesy tone.

So far we have discussed the basic squelch feature of common ham radio transceivers. There are other squelch techniques in common use with repeaters such as CTCSS, DCS, and PL.  These fall into a separate category called selective calling or one specific variant known as tone squelch, mainly because they have a different purpose and functionality.  We may discuss this in a future post.

Some Useful Links

Squelch setting – W3ATB blog

Squelch algorithms – PA3FWM site

Squelch Tail – DAP-COM reference


Voice Modulation- SSB

In the last post we explored voice modulation and learned that AM and FM are the two primary modes.  We also noted that single sideband (SSB), a variant of AM, is the dominant form of voice communication on the ham bands below 50 MHz.   This post will delve into the details of SSB.


SSB is important because it significantly improves the plain AM signal in both efficiency and bandwidth.

G2A06-2019In context of voice modulation, bandwidth means the range of frequencies the signal occupies.  In general, wider bandwidth means higher audio quality but less efficiency, and vice versa.




The SSB signal is generated using AM but before it is transmitted the carrier and one sideband are removed.  A diagram of an AM signal helps visualize how it starts:


SSB transmission reduces the power required (more efficient) and occupies a smaller slice of the RF spectrum (narrower bandwidth, ~3kHz), when compared to AM.


The transmitter circuitry is essentially AM but adds carrier and sideband filters at nominal cost.  The SSB receiver is more complex and expensive than AM because it must reconstruct the missing (mirror image) sideband to create a full voice signal, as well as replace the carrier with a local signal.

Because it has to recover the opposite sideband the SSB receiver is somewhat sensitive to tuning to make the voice sound normal or natural.  You will find that voices sound quite different in clarity and pitch when tuning around the transmit frequency.   A common reaction to hearing off-tune SSB is that it sounds like a quacking duck.  A great video demonstrating some of the nuances of SSB sound and tuning can be found in the links at the bottom.


So SSB transmits only one sideband, but which one, you ask?  The answer is either.  When the upper sideband is transmitted and received, the operation is called upper sideband (USB).

USB waveform


Conversely, when the lower sideband is used, the operation is called lower sideband (LSB).

LSB waveform

G2A02-2019In ham radio either one is legal to use in the phone portion of the band plan but by convention LSB is used below 10MHz, and USB is used above 10MHz (note that Continue reading

Voice Modulation- AM & FM

In the last post we introduced the concept of modulation as one of the five meanings of the term mode.  Voice is the most common and obvious mode of modulation so let’s dive into that a bit as it is one of the basic concepts of amateur radio.  

Modulation is the means to impress information on a radio signal.  It’s how a circuit puts our voice onto the radio signal through a microphone.


If you are (or were) fortunate enough to receive US Technician license training using the ARRL course book and companion slides you would find some good info as follows:

When information is added to the radio wave, (the carrier) we modulate the wave.
     •Turn the wave on and off (Morse code)
     •Speech or music

In the physics of waves there are three characteristics we might control to impart information (three possible modulation methods or modes).

Different modulation techniques vary different properties of the wave to add the information:   amplitude, frequency, or phase.

These three are commonly known as Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM).

AM is relatively easy to understand and visualize.  The RF carrier wave is varied in amplitude according to the audio waveform (as from a microphone) to create a modulated signal.

AM Graphic


Amplitude modulation is commonly used on the familiar AM broadcast band.  It may occasionally be found on lower frequencies in the HF ham bands.  While it used to be the primary mode of voice communication in the early years, very little pure AM is used nowadays in favor of single sideband (SSB), which is a type of AM. 


SSB is a big and important topic which merits its own write-up so we will post that soon.

FM is less intuitive and harder to visualize than AM.  The RF carrier is varied in frequency according to the audio waveform (as from a microphone) to create a modulated signal.

FM Graphic

Frequency modulation is commonly used on the familiar FM broadcast band and in ham radio above 28MHz for high quality simplex and repeater operation.


PM is very similar to FM in the way it operates with only a technical difference between the two (integration of the baseband signal).  Here the RF carrier is varied in phase according to the audio waveform, instead of the frequency.


Despite the terminology, many (most?) FM radios we routinely use for local communications over repeaters use PM circuitry.   The two are effectively equivalent for ham radio use; interesting discussion here and even more detailed one here.

For the ham radio operator, AM and FM are the two main voice modulation modes to understand.  SSB and PM are simply sub-sets or derivations.  This animated graphic is particularly helpful in visualizing AM vs FM:

AM vs FM animation

Useful comparisons between the two (applies equally to broadcast and ham radio):

  • AM has poorer sound quality (narrow bandwidth); FM better quality (wider).
  • AM is prone to noise interference; FM is relatively immune to electrical noise.
  • AM has narrower bandwidth (more efficient) than FM (trade-off with sound quality)
  • AM circuitry is simpler and cheaper than FM
  • With AM, transmitted power level varies with the amplitude of the signal; with FM transmitted power level is constant, regardless how much modulation (deviation) is applied.

In context of voice modulation, bandwidth means the range of frequencies the signal occupies.

Because of these differences in bandwidth and efficiency, AM and FM are generally used on different Continue reading

Mode Madness

In ham radio the term mode has at least five distinct meanings.  It’s confusing for even experienced hams so we’ll try to tame some of this madness.


If you’re a new ham with a handheld or mobile transceiver to talk on the local repeater, mode doesn’t mean much to you.  Your basic radio has no mode controls because it can only do one thing.  In this case you are operating in voice mode using frequency modulation (FM).  Guess what?  These are the first two—and most important—of the definitions of mode.  These are operating and modulation modes.


Below we will explore the five different contexts for mode found in US license exam questions:

  • Operating
  • Modulation
  • Propagation
  • Satellite
  • Split

Operating Mode-  The most basic definition of mode; a general category of radio transmission and reception.  There are three or four operating modes, depending on how they are categorized.  The common three are:

  • CW (continuous wave), typically for Morse code (radiotelegraphy)
  • Radiotelephony (phone), a fancy term for voice communications
  • Digital, where data is exchanged over the air, requiring computers or machines to interpret signals

For logging and awards these three categories are CW, Phone, and Digital modes.  The ARRL Logbook of The World (LoTW) adds a fourth category:  Image.  Collectively these 3 or 4 are known as mode Groups.



Modulation Mode-  Modulation is the means to impress information on a radio signal.  It’s how a circuit puts our voice onto the radio signal through a microphone.  There are different forms (modes) of modulation which can be employed within each basic operating mode.

For example, typical modern HF transceivers support voice modes using AM, FM, and SSB modulation modes.  There are a few flavors of CW and dozens of digital modes (and the list keeps growing).  Just look at that mode group list link above.


To further complicate matters we now have both traditional analog modulation for phone (voice) signals, and digital voice modulation as well.  Many digital modes simply modulate a SSB waveform using specific tones to represent data characters.  We live in an era where computers and radios are really working together to do amazing things.

Operating and modulation modes are hard to separate.  In fact, they sort of overlap and mash together.  Context of the discussion is key here; often it doesn’t really matter.

These two also play a role in the ITU classification of RF signals.  Refer to Types of Radio Emissions link.  Hams may occasionally log their mode according to this or a similar scheme.

Why do so many operational and modulation modes exist?  It’s largely for historic reasons as technology and electronics have advanced over the years.  In the earliest days of radio, only radiotelegraphy existed. Mode had no meaning as CW was the only possibility.   Then came voice technology and a second operating mode was born.  Going from original AM to SSB,  we then had modulation modes, adding FM as an improvement later.  Digital mode entered the scene after voice once people discovered they could encode audio signals to represent data; computer technology has made the digital mode wildly successful, if less personal, in recent years.  Image modes have been around since the early days of television but here again, computers have made them better and easier.

One other reason for different modes is Continue reading

Generator Use & Safety

Spring is [hopefully] coming soon with expected turbulent weather.  A new hurricane season is also upon us.  Both are liable to cause disruptions to utility electrical power.  Annual ARRL Field Day is also approaching.  Time to think about auxiliary power generators.

Small gasoline-powered generators are relatively common and widely available for emergency or portable electrical power.  It’s a good idea for the prepared homeowner to have one, mainly to keep the fridge/freezer cold during times of sustained power outage.  The savvy radio amateur also recognizes the importance of communications capability in a blackout scenario as well.  Power is needed to make our radios work beyond what limited battery capacity we have, particularly in emergency situations.  Most EmComm groups include generators in their plan and have them on hand.  Not every ham does.

Honda Generator

Consider having a small generator for essential power when the lights are out for hours.  Having a generator is good; knowing how to use it safely is the focus of this topic.  We will look at four related safety considerations:

  1. Carbon Monoxide hazards— CO
  2. Fuel handling and storage (fire)
  3. Generator grounding
  4. Shock hazard

Carbon Monoxide (CO) is the primary hazard with generators, since  CO is produced in the exhaust of all gasoline and natural gas combustion engines. 


NEVER, ever run a generator inside a dwelling or garage where exhaust can seep into the occupied space.  CO is a colorless and odorless gas which can kill or injure humans and animals.   It can be detected only with chemical or electronic CO detectors; it’s wise to have detectors in every home.


Always operate a power generator outdoors in a well-ventilated area. Continue reading