Wednesday, February 25, 2009

The Hexagonal Beam Antenna, Bob, session 40

Today's Net was prompted by an article in the March 2009 QST. It discusses a variation on the hexagonal beam antenna designed by Steve Hunt, G3TXQ. I like this antenna and think it will make a good one for my rooftop. I intend to build it this Spring and install it this Summer. I will have an update when it is up.

The hexagonal beam antenna is a variation on the Yagi. It is essentially a two element Yagi, just the driven element and the reflector, where the elements are not straight. Instead the elements are bent in the zigzag shape of a W. I will refer to the top or bottom of the W. By that I mean the top or bottom of a letter W on a line. The bends are horizontal so from the side it looks flat. Looking down from above, the driven element radiates toward the "bottom" of the W. The top of the W of the driven element is toward the center of the antenna. The reflector is essentially the shape of the driven element but facing the opposite direction and of course behind the driven element. So from above it looks like two W's, one upside down and on top of the other.

This arrangement makes construction of this antenna easy. What you do is create a base that holds six spreaders radiating out evenly at 60 degree intervals, like spokes on a wagon wheel except that you use somewhat flexible spreaders and put tension on the ends to bend them upward so they look like an upside down umbrella. The antenna elements can then be made by stringing wire either from tip to tip or along the spreaders. Trying to explain the construction would be too difficult for this net. Look at the blog to get links that explain it with diagrams.

This is the original Hexagonal Beam. The QST article discusses a modified design by G3TXQ. In that design, you use the same basic construction but instead of the reflector being in the shape of a W, it is in the shape of an arc. This design was arrived at by experimentation and by using the design software, EZNEC. The resulting design is slightly larger, 21.5 feet diameter for 20 meters vs. 19 feet, but it has much better SWR characteristics over a broader frequency range as well as slightly better gain and a better front to back ratio.

These can be nested to create multiband antennas without the need of traps. This is similar to the concept of a multiband dipole that consists of dipoles cut to each band that are connected at the feedpoint.

HEX-BEAM by Traffie Technology
A homemade Hexagonal Beam in 3 hours
Good Site on Original and G3TXQ Versions

Wednesday, February 18, 2009

APRS -- Brian Daly, WB7OML -- week 39

An Introduction to the Automatic Packet Reporting System (APRS) by Brian Daly, WB7OML

The Automatic Packet Reporting System (, or APRS, was developed by Bob Bruninga, WB4ARP, who is a senior research engineer at the U.S. Naval Academy. Everyone is probably aware that APRS can be used for position tracking of stations, but this is only one of the capabilities APRS has to offer. WB4ARP defines APRS as "a two-way tactical real-time digital communications system between all assets in a network sharing information about everything going on in the local area". APRS allows not only position reporting, but also provides the capability for short messaging, including the ability to send a message to another APRS user, or to provide weather information via APRS. Note that I called APRS the Automatic Packet Reporting System. When GPS started making its way into the mainstream, APRS was called by some the Automatic Position Reporting System - this misnomer really skewed the perception that APRS was about "position only" and did not emphasize that it can be used for other applications. The "correct" name according to WB4ARP is Automatic Packet Reporting System.

Now that I mentioned GPS, let me give a quick overview of how that works - GPS is one component which is used in an APRS system. GPS is a satellite-based navigation system that uses 24 Department of Defense satellites which are in orbit roughly 12,000 miles above earth. Each of these satellites orbit the earth twice a day at roughly 7,000 miles per hour - this orbit is very precise. These satellites transmit a signal down to earth. The GPS receiver uses this information to perform a triangulation to determine the location. The GPS receiver determines the time difference from when the signal is transmitted by the satellite to the time received, which is used to determine the distance to the satellite. If the GPS receiver has time measurements from a few more satellites, thus knowing the distance to each of those satellites, it can triangulate to give the location of the receiver.

The current GPS consists of three major segments - the space segment (SS), a control segment (CS), and a user segment (US). The space segment is the satellites. The control segment is the master control station in Colorado Springs which provides precise tracking information for the satellites. The user segment is the GPS receiver composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly-stable clock.

There are several signals transmitted by the satellite - the L1 signal is at 1575.42MHz and is the signal used by civilian receivers. Since this signal is at 1575MHz, signal propagation is by line of site, and will pass through clouds, glass and plastic, but will not go through solid objects such as buildings. The signal has three pieces of information - the pseudorandom code, ephemeris data, and almanac data. The pseudorandom code is simply an I.D. code that identifies which satellite is transmitting information. You can view this number on most GPS unit's satellite page, as it identifies which satellites it is receiving. Ephemeris data tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits ephemeris data showing the orbital information for that satellite and for every other satellite in the system. Almanac data, which is constantly transmitted by each satellite, contains important information about the status of the satellite (healthy or unhealthy), current date and time. This part of the signal is essential for determining a position.

It might seem three satellites are enough to solve for position, since space has three dimensions. However a very small clock error multiplied by the very large speed of light-the speed at which satellite signals propagate-results in a large positional error. The receiver uses a fourth satellite to solve for x, y, z, and t which is used to correct the receiver's clock. Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known (for example, a ship or plane may have known elevation), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible.

So given this overview of GPS, how does that apply to APRS? Many GPS receivers have the ability to tap into the GPS data stream and extract position information. This data stream can be fed into a packet data stream over amateur radio, and used by the receiving end to decode and plot on a computer-generated map. If this is done periodically as the amateur radio is moving, a new position report can be sent and plotted on the map. This is thus the position tracking capability of APRS.

So, APRS needs packet radio? Yes! You cannot have APRS without packet. Without going too deep into packet radio operation (that is a subject all to itself!), let's just say APRS requires a Terminal Node Controller (TNC) which will create the AX.25 protocol packets transformed into audio signals for sending on your radio. This TNC can be free-standing, such as the Kantronics KPC-3+, or built into the radio, such as in the Kenwood D7A. In addition, for position reporting, APRS requires the ability to interface your GPS receiver to the TNC to extract the position information and put onto the APRS protocol. The interface is a standard interface defined by the National Marine Electronics Association, or NMEA. Most computer programs that provide real time position information understand and expect data to be in NMEA format. This data includes the complete PVT (position, velocity, time) solution computed by the GPS receiver. If you are looking to build an APRS station, make sure your GPS unit has an NMEA interface.

Virtually all APRS activity takes place at 144.39MHz using 1200 baud packet. To set up an APRS station, an ordinary 2-meter FM transceiver is the primary component. Since APRS uses packet mode, you will need an APRS-compatible TNC to decode APRS traffic on 144.39MHz - this TNC can either be a stand alone, a built-in TNC in your radio, or a sound-card TNC. With this configuration you can monitor APRS traffic. If you want to send APRS traffic, you don't necessarily have to connect a GPS unit to your TNC - you can "hardcode" your latitude/longitude for fixed applications (such as weather stations). However, if you want to do position tracking, then you will need a GPS receiver as well (you can get a basic GPS unit with an NMEA output extremely cheap, especially used).

We are not going to talk about connecting a TNC to your radio or basic packet operation - again, this is an entire lesson in itself!

A typical APRS station will use a Secondary Station ID, or SSID, to distinguish the APRS station from their "normal" station. For example, I have my Kenwood D7A call set to WB7OML-5. SSID is not a requirement but a useful indicator.

The other important APRS component is software. Software is important to display positions of APRS stations as well as getting any additional information that may be included in their transmission, such as weather information. You do not need this software to transmit APRS information. Popular packages are UI-View, FindU, WinAPRS, and MacAPRS, using map programs such as Streets, Street Atlas, or Precision Mapping.

In packet radio, you normally want to send packet data to another station via a digipeater (or multiple digipeaters). For example, I can command my TNC to "connect" my station to W7ACS "via" W7ACS-10 (which is a digipeater in this example), or "via WB7OML-10, W7ACS-10" to indicate multiple digipeaters. However, APRS does not establish "connections" with each other - the packets are sent to no one in particular, but sent to everyone. To allow APRS packets to be repeated by digipeaters, an "alias" convention was established such that APRS users would not have to know which digipeaters to send the APRS packets to. The most common APRS digipeater alias is WIDEn-n where "n" represents a number. The first "n" designates the type of WIDE digipeater that will relay your packets. For example, a WIDE1 digipeater is a limited coverage "fill in". A WIDE2 is for wide coverage. The second "n" is the SSID and is used to provide a means of limiting how often or how far a packet can be repeated; think of this as a count of the number of times your packet will be repeated. There needs to be a limit or else the packets will ping-pong throughout the network over and over, and cause congestion. Each time your packet hits another WIDEn-n digipeater, the digipeater will subtract 1 from the SSID before it retransmits it. If the SSID is zero, it will not be retransmitted.

The suggested setting for a home APRS station is WIDE2-2. If you are in an area where fill-in digipeaters are needed to cover areas that are shadowed from the main digipeater's receiver, a suggested path is WIDE1-1, WIDE2-1. This will provide you with two hops via the RF network. WIDE1-1 will activate the fill-in digipeaters, which will boost you to the main digipeaters. This path asks the fill-in digipeaters for help of the first hop, as well as the main digipeaters, but only the main digipeaters will respond to the second hop. A suggested standard path for a mobile APRS station is WIDE1-1, WIDE2-1. This will provide you with two hops via the RF network. WIDE1-1 will activate the fill-in digipeaters, which will boost you to the main digipeaters. This path asks the fill-in digipeaters for help of the first hop, as well as the main digipeaters, but only the main digipeaters will respond to the second hop.

The other question is how often do you need to transmit? Obviously you don't want to transmit too often or there will be packet collisions with other users on the network. However, for moving stations, you want to transmit often enough to get goods tracking data. Home stations should transmit a position report to WIDE2-2 every 30 minutes. This is usually a default setting when configuring APRS for home use. Home weather stations should transmit a position report to WIDE2-2 every 15 minutes. Mobile stations should transmit a position report depending on the configuration of equipment they are using, to WIDE1-1, WIDE2-1, perhaps 3-5 minutes periodicity should be considered. Typically a shorter path of WIDE1-1 with a periodicity of once every two minutes is acceptable.

As I mentioned earlier, APRS not only is used for sending position information. APRS can send telemetry, short two-way text messages, bulletins, weather data, etc. For two-way text messaging, you can send these to a specific station and also request an acknowledgement.

APRS is another packet application you should consider, especially in emcomm operations. Small self-contained trackers can be obtained at reasonable cost. Examples include the Tiny Trackers from Byonics APRS applications include*:

Immediate local digital and graphical information exchange between all participants in a local area or event. This includes:
  • Positions of all stations and objects
  • Status of all stations
  • Messages, Bulletins and Announcements
  • Weather data and telemetry
  • DF bearings and signal strengths for quick transmitter hunting
  • RF Connectivity plots of all stations
  • Local OBJECTS on a common map display for all users
  • Local Freqs, IRLP, ECHOlink, Winlink, Nets, Meetings
Typical applications are:
  • Routine local awareness of all ham radio events and assets around you
  • Marathons, races, events and public service
  • Search and rescue
  • Family communications and tracking and one-line emails
  • Mobile-to-mobile global text messaging
  • Weather data exchange and display
  • Efficient multi-user Satellite communications
* - from Bob Bruninga, WB4APR

Links for More Information:
WB4APR's APRS Website:
GPS guide for Beginners:
WinAPRS and MacAPRS:
Northwest APRS:
Self contained APRS:,

Wednesday, February 11, 2009

Total Harmonic Distortion & SINAD

Harmonic Distortion & SINAD by Lee Bond, N7KC

February 11, 2009 Educational Radio Net, PSRG

For the 38th session of the Educational Radio Net, I have chosen to continue a review of basic and important concepts that cannot be avoided when dealing with radio equipment. Earlier sessions dealt with the relationship of energy, power, time, voltage, current, and resistance. The 13 parts of the Impedance series is a good starting point for individual review. This week I will delve into harmonic distortion, total harmonic distortion, and SINAD.

Lets get started. I have talked previously about 'linear' amplifiers. Imagine some 'box' which has gain properties. This box simply makes whatever goes into it on one side come out the other side larger. For example, if the box is 'linear', a man entering the box would emerge looking larger and retain whatever proportions he had before entering the box. This means that all parts of the man's body have been scaled by the same number. If the man's arm were scaled by 2 then so goes the foot or nose.

On the other hand, if the man emerged with one arm scaled by 2 and the other by 3, or if he were 3 times as tall but only 2 times as wide then we would know that the properties of the box were nonlinear.

Extending this idea to an electronic amplifier goes as follows. We need to consider 3 situations. Situation 1 considers a small signal, situation 2 considers a medium signal, and situation 3 considers a large signal. From situation 1 we plot a point on graph paper which represents the gain for small signals. Then, from situation 3, we plot another point on the paper which represents the gain for large signals. Now draw a straight line between the two points. If results from situation 2 plot on this straight line then we can say, with some assurance, that the amplifier is 'linear' over the operating region tested. All test points plot on a straight line so the amplifier is 'linear'.

Clearly there must be a signal which will not plot on the line so we are right to assume that some limits exist on linearity. Very large signals which are compressed or clipped certainly will not plot on a straight line so the amplifier is nonlinear in the over driving region defined by very large input signals.

A 'perfect' amplifier should only make a signal larger and not distort proportionality. We can come close to perfection but, alas, the completely perfect amplifier does not exist. We do have, however, instrumentation which will tell us how close our amplifier comes to perfection. Enter the spectrum analyzer and total harmonic analyzer.

Now it is time to talk about various signals. Everyone knows that square waves or triangle waves can be synthesized by carefully selecting sine waves of various frequencies and adding them together properly. If you were to look at a square wave on a spectrum analyzer... in the frequency domain... you could see the various frequency components associated with the input waveform. Now understand that the same waveform viewed on an oscilloscope in the 'time' domain looks like a square wave. The spectrum analyzer shifts the point of view from the oscilloscopic amplitude vs time to amplitude vs frequency. The various frequency components are completely exposed and can be easily measured. The central idea that I want to impart is that only one waveform, the sine wave, has no side spectra. A perfect sine wave produces only one line on a spectrum analyzer. Of course there is no such thing as a 'perfect' sine wave but carefully engineered circuits can come very close to the perfect waveform.

Now, back to our amplifier driven with an input signal which is a near perfect single frequency sine wave. We would expect our linear amplifier to only make this single frequency sine wave signal larger but the spectrum analyzer shows a different situation. There is a 'bump' or 'spike' on the analyzer which is harmonically related to the input signal. With the amplifier out of the test circuit there is no harmonic bump on the analyzer but, with the amplifier in place, the bump is clearly there. Harmonic means integer multiples so if the input frequency is f then the 'second' harmonic is 2f and the third would be 3f, etc. So, we fed our amplifier with a near perfect signal and the output is a larger signal but which includes some trash in the form of harmonic energy. The height or amplitude of the second harmonic component compared to the amplitude of the input signal is a measure of how linear the circuit under test is. By using a spectrum analyzer we can easily determine individual amplitudes of what might be called trash frequency components.

Historically, before the spectrum analyzer became widespread, there was an instrument called a Total Harmonic Distortion Analyzer or THD Analyzer for short. This instrument had a generator which produced a near perfect sine wave for test purposes. The instrument also had a very good tunable 'notch' filter circuit aboard. In use, the THD analyzer supplied near perfect, very clean, sine waves to the amplifier under test. The amplifier output went back into the analyzer and the notch filter removed the input signal. Anything left over was considered to be produced by nonlinear circuit performance in the amplifier under test. By comparing total left over energy to the original signal energy one could compute total harmonic distortion. This is important in both high performance high fidelity audio amplifiers and radio SSB linear amplifiers where one desires distortion free output signals.

Closely allied with THD measurements is the SINAD or Signal plus Noise and Distortion specification which you may have noticed listed in your radio manual. This is a common test performed on FM receivers. SINAD is the ratio of signal plus thermal noise plus distortion to thermal noise plus distortion expressed as decibels. A perfect radio receiver would produce no thermal noise or distortion but, alas, the perfect radio receiver does not exist. Assuming that we have a receiver with some inherent thermal noise and distortion the question becomes one of determining just how weak a signal must be before losing intelligibility. Careful testing has shown that the signal must be at the least 4 times the inherent thermal noise plus distortion to be easily tolerated by the ear and for ease of understanding. The common log of 4 times 20 is 12 so a 12 db ratio indicates the point of maximum thermal noise and distortion that can be tolerated in voice communications.

The SINAD test goes as follows: First the testing instrument is frequency modulated by a 1 Khz sine wave which is very clean with respect to harmonic distortion. This frequency modulated RF signal is then applied to the radio under test. The audio frequency output of the radio under test is then fed back into the testing instrument where a good notch filter removes the original modulation. Any residual frequency components are a result of distortion anyplace in the radio signal processing circuits and become a part of the ratio of signal plus thermal noise plus distortion to thermal noise plus distortion. During the test one starts with ample RF signal strength then reduces it until the SINAD meter indicates 12 db. At this point we know that the signal is 4 times as strong as the sum of thermal noise and distortion. Noting the number of signal microvolts to establish the 12 db ratio is the measurement. Typical VHF FM radios will come in at about 0.25 uv for 12 db SINAD and typical UHF FM radios will come in at about 0.35 uv for 12 db SINAD.

Simply measuring the 20 db quieting sensitivity of an FM receiver is not the full story since distortion products certainly will interfere with voice intelligibility. Both SINAD and quieting sensitivity are useful performance parameters to check when looking for a new FM transceiver. One point to note is that the testing audio frequency is a standard 1 Khz note. Since the audio passband seldom exceeds 2.5 Khz the 1 Khz fundamental and the 2 Khz second harmonic fit in the expected linear region of the receiver audio chain. Since the 3rd and higher harmonic energy is outside the passband it is pointless to attempt any measurement of components beyond the 2nd harmonic.

In summary, thermal noise and harmonic distortion are very likely present in any electrical system including radio frequency equipment. The relationship of signal energy to thermal noise energy plus distortion products sets the performance standard for a particular radio.

This concludes the set up discussion of Total Harmonic Distortion & SINAD. Are there any questions with regard to tonight's discussion?

This is N7KC for the Wednesday night Educational Radio Net

Wednesday, February 4, 2009

Near Vertical Incidence Skywave (NVIS)

So what is Near Vertical Incidence Skywave anyway?

Brian Daly

Has anyone seen pictures of military HUMVEEs with an antenna folded over? Ever wonder why? Do you think it is so they can get into parking garages or under low bridges? Let’s think about that and come back to it later…

Has anyone had the frustration of being able to talk on HF to Southern California but can’t reach a station in Portland, Oregon? Well, the medical services team has had this problem. We had an HF vertical antenna on top of the VA Hospital on Beacon Hill and could not communicate with hospitals in Portland. Anyone want to venture a guess why? To understand, let’s explore HF for a moment…

For HF propagation, there are a few modes of transmission through the atmosphere that are important to understand. Depending on the goal of your communication mission, the type of mode you use will determine whether your communication mission will be successful, or be a failure.

First, there is the traditional “skywave” propagation – skywave is the propagation of electromagnetic waves that are bent ( more technically know as “refracted”) back to the Earth's surface by a layer of the atmosphere called the ionosphere. If you tune your AM radio to 660kHz at night, you can typically hear the station 660News from Calgary, Alberta, Canada here is the Seattle area. If you are lucky and tune to 640kHz, you can also sometimes hear KFI from Los Angeles, California. The radio waves from these stations are bouncing off the ionosphere and landing on your AM radio antenna through skywave propagation. Most long-distance HF radio communication (between 3 and 30 MHz) is a result of skywave propagation. Amateur radio operators take advantage of skywave for long distance or DX communication.

Skywave is an excellent mode for long distance communication, say from several hundred miles and out. A skywave results in the signal going off your antenna at a fairly low angle, hitting the ionosphere, and bouncing back to earth at some distance away. As you can imagine, the angle that the wave hits the atmosphere will directly affect where the signal ends up – remember from basic physics the angle of incidence is equal to the angle of reflection. A good way to think about this is a pool table; if you want to bounce the ball off the side of the table to hit another ball, you need to adjust the angle such that the incident angle equals the reflected angle to hit your mark. The same is true of radio waves.

The second mode of HF propagation is known as a “ground wave”. A ground wave is a wave that travels along the surface of the earth. The distance that ground waves propagate depends on the frequency, which really is dependent on how well the wave is diffracted by the earth’s surface. Most local communication in the 30kHz to 300kHz is via “ground wave” and have been used in such applications as over-the-horizon radar. Ground wave propagation is typically on the order of tens of miles and is also dependent on sun spots, solar flares, and day or night operation.

The problem we now have is if you are trying to communicate in between the ground wave and skywave coverage areas – you are either further out then the ground wave will propagate, or too close such that the incident and reflected wave leaves a “hole” where the signal effectively bounces over you. This is what is known as the “skip zone”. If you were sitting in the skip zone and the skywave was such that the radio wave bounced over you and the ground wave did not reach you, you would not be able to receive the station sending that signal.

So back to our HF vertical on the VA Hospital -- we propagated a signal such that Portland is in the skip zone. Oops!

For many emergency communications activities (as well as military operations), you are not necessarily interested in talking across the continent, or around the world. However, you do need to talk farther then the distance a groundwave signal can reach. So what do we do?

The answer is Near Vertical Incidence Skywave, or NVIS (NE-VIS) for short. NVIS is a electromagnetic wave propagation method that provides usable signals in the range between groundwave and skywave distances, usually in the range of 30 to 400 miles. This easily covers Portland, Vancouver BC, Spokane, and beyond from the Seattle area. NVIS provides effective regional communications.

Although not all radio amateurs have heard the term NVIS, many have used that mode when making nearby contacts on 160 meters or 80 meters at night, or 80 meters or 40 meters during the day. You may have thought these nearby contacts resulted from groundwave propagation, but many such contacts involve no groundwave signal at all and actually used NVIS. You may actually have used NVIS without even knowing it!

A good way to visualize NVIS is to consider a mirror on the ceiling directly above you. You shine a flashlight straight up at the mirror. What happens to the light? It is reflected directly back down at you. This is what we want to do with NVIS – we want to send a radio wave straight up and have it come back down. Since the incident wave is not a perfect beam and the reflection is not perfect, the reflected signal “speads out” to cover an area from 30 to 400 miles.

Can any frequency be used for NVIS propagation? No, there is a limited range of frequencies that will provide NVIS – if the frequency is too high (above what is known as the critical frequency or “f0F2”), there will be no reflection and the radio wave will pass through into space. If the frequency is too low, there will be too much absorption of the signal in the atmosphere. The usable frequencies for NVIS communications are between 1.8 MHz and 10-15 MHz. The most common bands used in amateur radio are 3.5 MHz (75/80 meters) and 7 MHz (40 meters), with some experimental use of 5 MHz (60 meters) and 160 meter frequencies. Always use the highest frequency possible to get away from the critical frequency. Military NVIS communications mostly take place on 2-4 MHz at night and on 5-7 MHz during daylight. The lowest layer of the ionosphere, called the D layer, causes attenuation of low frequencies during the day. This layer disappears at night enabling improved communications at the lower frequencies during the night.

The rule of thumb for NVIS is – higher frequencies during the day, middle bands in afternoon/evening transition, lower at night. For emergency communication use, it is best to have a couple of frequencies in mind in 75/80 meters and 40 meters so you can use the optimal frequency for a given time of day which is below the “critical frequency”.

For HF, you may have heard that “height is best” when it comes to antennas – get your antenna as high off the ground as possible. This is true for long distance HF work, since the height above the ground will give an antenna pattern with low angle of radiation. Well for NVIS, forget this. We actually want to do the opposite – keep the antenna low to maximize the radiation straight up.

An NVIS antenna configuration is a horizontally polarized (parallel with the surface of the earth) radiating element that is from 1/20th wavelength to 1/8th wavelength above the ground. - hang a dipole at less than ¼ wavelength above the ground. According to US Army studies, 1/8th wavelength is the optimum height. For 80 meters, this is about 30', and for 40 meters it is about 15'. The height is not super-critical, as long as it is below ¼ wavelength. Antennas have been laid on the ground or in the bushes with usable results. The Army even has antennas that are buried two feet under the sand! However, antennas that are very close to (on in) the ground will be less efficient than ones hung at 1/8th wavelength.

Lowering the height also reduces the background noise level. That proximity to the ground forces the majority of the radiation to go straight up. Overall efficiency of the antenna can be increased by placing a ground wire which is slightly longer (5%) then the antenna, and placed parallel to and directly underneath the antenna (about 0.15 wavelength below); some designs also have three ground wires – one directly under the antenna, and one on each side 0.15 wavelengths away. While the ground wire is not necessary under good to excellent propagation conditions, antenna gain in the 3 dB to 6 dB range are common when the ground wire is used. This gain gives this antenna configuration its unique name- the “cloud burner” or “cloud warmer”. Heights of 5 to 10 feet above ground are not unusual for NVIS setups, and some people use dipoles as low as two feet high with good results (relatively weak signals, but a very low noise floor). I’ve even experimented with running a wire along the ground with success.

You will probably need a good antenna tuner. Antennas that are resonant at ¼ wavelength above the ground will have a higher SWR (standing wave ratio) when hung close to the ground. If ground conductivity is poor (dry sand, for instance), try running a counterpoise wire on the ground directly below the antenna. The counterpoise is actually a good idea with any NVIS installation, since it improves the system's overall efficiency. The counterpoise should be slightly longer (5%) than the antenna. Unbalanced antennas should have a balun installed at the feedpoint to prevent the coaxial cable from radiating.

When both the transmitting station and the receiving station use NVIS configuration for their antennas you can maximize the communications. Why? One potential problem with NVIS operation is "groundwave interference". If two stations are close enough to hear each other's groundwave signal, multipath interference can cause significant distortion. To reduce this effect, both stations should keep their antennas as low as possible, and point the ends of the antennas at each other to minimize groundwave radiation in that direction.

There is also an advantage inherent in the use of NVIS antennas which applies to receiving. The frequencies which are useful for NVIS are the same frequencies which are most susceptible to atmospheric noise. A major source of atmospheric noise is distant thunderstorms. Nearby thunderstorms are the worst, of course, but the noise from all possible sources adds together. Unless there is a nearby thunderstorm, most noise will be the sum of the noise from distant sources which are all propagated to the receiving antenna. Since an antenna optimized for NVIS is listening mostly to signals propagated from relatively nearby areas, and does not favor the reception of signals, static crashes, and other sources of noise and interference from more distant sources, it will not hear as much noise or interference as an antenna optimized for DX operation. The result is a better signal/noise ratio.

Almost any type of wire antenna can be used for NVIS – a random length wire antenna, a dipole, square loop, inverted vee, multi-band dipole, or a hamstick dipole. I’ve seen mobile installations where you run a wire off the back of a pickup truck at about 3 or 4 feet off the ground provide effective NVIS operation. For random wire antennas and inverted Ls, remember to run out a counterpoise wire along the ground to avoid RF burns from your equipment! Place the counterpoise on the ground directly below the radiating element. Avoid stringing long-wire antennas with any significant vertical radiating sections to keep ground-wave propagation to a minimum.

Other proven NVIS antennas include the Shirley dipole and the Patterson Loop (used by the military), but these are more complex to build and install. Some Amateurs have experimented with horizontal fiberglass loaded-whip dipoles (such as those from Hamstick, Iron Horse, and Valor), although they are far less efficient than full-size dipoles. In an electrically noisy environment, they do not have enough gain to work well.
So which frequency do I choose? The selection of the optimum frequency for NVIS operation depends upon many variables, including time of day, time of year, sunspot activity, and type of antenna used, atmospheric noise, and atmospheric absorption. To select a frequency to try, one may use recent experience on the air, trial and error (with some sort of coordination scheme agreed upon in advance – this is a popular method used in Automatic Link Establishment systems), you can use propagation prediction software, or probably the easiest is to use near real-time propagation charts which are available on the Internet and show the current critical frequency. A good site is from the Australian Government IPS Radio and Space Services website, and provides the optimum NVIS Frequency Map Based upon hourly ionosphere soundings. See Look for the f0F2 plot.
High power is not required for NVIS either. We are not trying to talk around the world, just out to a few hundred miles. I’ve used an FT817 on an alkaline battery pack at low power with success.
Among the many advantages of NVIS are:
  • NVIS covers the area which is normally in the skip zone, that is, which is normally too far away to receive groundwave signals, but not yet far enough away to receive skywaves reflected from the ionosphere.
  • NVIS requires no infrastructure such as repeaters or satellites. Two stations employing NVIS techniques can establish reliable communications without the support of any third party.
  • Pure NVIS propagation is relatively free from fading.
  • Antennas optimized for NVIS are usually low. Simple dipoles work very well. A good NVIS antenna can be erected easily, in a short amount of time, by a small team (or just one person).
  • Low areas and valleys are no problem for NVIS propagation.
  • The path to and from the ionosphere is short and direct, resulting in lower path losses due to factors such as absorption by the D layer.
  • NVIS techniques can dramatically reduce noise and interference, resulting in an improved signal/noise ratio.
  • With its improved signal/noise ratio and low path loss, NVIS works well with low power.
Disadvantages of NVIS operation include:
  • Only good for distances <>
  • For best results, both stations should be optimized for NVIS operation. If one station's antenna emphasizes groundwave propagation, while another's emphasizes NVIS propagation, the results may be poor. Some stations do have antennas which are good for NVIS (such as relatively low dipoles) but many do not.
  • NVIS doesn't work on all HF frequencies. Care must be exercised to pick an appropriate frequency, and the frequencies which are best for NVIS are the frequencies where atmospheric noise is a problem, antenna lengths are long, and bandwidths are relatively small for digital transmissions.
  • Due to differences between daytime and nighttime propagation, a minimum of two different frequencies must be used to ensure reliable around-the-clock communications.
  • So let’s end with answering the initial question – why do the HUMVEEs have the antennas folded over? Because they are operating NVIS!
Here are some useful links to explore NVIS further:

Yahoo! Group NVIS Discussion:

Modulation - Amplitude (AM), Frequency (FM) and Phase

For Ham Radio purposes, modulation is a small variation of a sine wave (also called carrier wave) that carries information. In the case of Amplitude Modulation (AM) and Frequency Modulation (FM) the information that is carried is generally a wave itself. Phase modulation is commonly used for digital information and you can have large jumps in the phase to represent a digital signal but the carrier wave isn't changed drastically.

Let's start from a basic sine wave, or carrier wave. Incidently, carrier wave is where the abbreviation CW comes from, indicating Morse Code communication. A carrier wave imparts no information other than that it is present. CW communication imparts it's information by turning the carrier wave on and off. In a sense, this is modulating the carrier wave. An ideal carrier wave does not change at all in frequency or amplitude. For AM or FM signals this is the frequency shown on the radio. In order to send anything interesting with this wave we need to vary it or modulate it.

Amplitude is a measure of the strength of a wave. The stronger the signal is, the greater the amplitude will be. Now imagine that you have a radio where you can change the signal strength, the amplitude, by simply turning a knob, much like a volume knob for audio. If you are transmitting a simple carrier wave then as you turn the knob up and down you are varying the amplitude. Now let's say you have really fast hands. If you turn the knob up and down just right, 440 times a second you would produce an output that was the carrier wave, modulated at 440Hz which happens to be the tone A above middle C. This is an AM signal. To produce a pure sine wave of 440Hz you would have to turn the knob just right so that the amplitude would go up and down over time in the shape of a sine wave. This is, in essence what an AM transmitter does. An AM receiver gets the sent 440Hz wave by filtering out the carrier wave.

You can extend the idea so that whatever audio frequency signal you put in, a human voice for example, you can vary the amplitude of the carrier wave to transmit that signal and the receiver can recover it.

Let's take the same carrier wave as in Amplitude Modulation; but now when you turn the knob you change the frequency by a little bit instead of the amplitude. Once again, if you turn the knob just right you can modulate the signal with a 440Hz sine wave and a receiver can recover that pure tone. In this case the receiver has to use a different method to recover the audio signal but it is still a matter of detecting the modulation of the carrier and producing only the audio signal without the carrier. One term you have probably heard is deviation. If you don't have enough deviation, your audio signal is weak; if you have too much your audio signal can be distorted by overloading the detector circuits. Deviation is the measure of how much the frequency changes from the carrier frequency when the change is at it's most. That is, what is the largest amount that the frequency deviates from the carrier when the audio signal is applied.

Phase modulation changes the phase of the carrier and that is how it imparts information. The phase is an indication of how far into the sine wave you have progressed. We measure phase in degrees based on the mathematical sine function. The wave starts at zero degrees goes to a maximum at 90 degrees, back to zero a 180 degrees, etc. With digital signals you can suddenly change the phase to indicate a digital change from 0 to 1. This is a topic that really needs it's own session so I will hold off on saying more about digital modes. One thing to note is that phase and frequency are very closely related and varying the phase will vary the frequency and vice versa. In fact, there has been equipment used to create FM signals by using phase variation in conjunction with other signal processing.