#hamr m0xpd Shack Nasties is bookmarked and I like his sense of humor as a ham radio blogger including tech prowesshttp://t.co/vfxZllp6
Just found this resource:http://t.co/wlqb30CF Coverage down to 50mHz is possible with one type & down to 22mHz with another.
Wish I had the time to play with these things…..Dan
W7DTG: Another boatanchor receiver demonstration Hallicra…http://t.co/QXZn29Pe
I’m still not sold on digital voice, but if it were easy to set up, like say PSK-31, I’d give it a try, at least….Dan<
Three more articles from recent editions of the electronics trade magazines.
Heathkit: A right-time, right-place business. Heathkit was a popular electronics company for decades before its demise earlier this year. Former employees Lou Frenzel and Chas Gilmore share some memories and discuss the factors that led to its closing. Lou Frenzel is W5LEF.
In the article, he notes how he was instrumental in developing the Heath/Zenith line of computer kits. At that time, I was a fledgling test engineer working for Memorex (remember them?) making the 8-in. floppy drives that were an option for those computers.
Real-world testing of wi-fi hotspots. This article talks about both the RF testing and data communications testing needed to ensure a good wi-fi hotspot.
How to simulate cable in SPICE. This article covers the two main loss effects related to cables (the skin effect and dielectric losses) and presents a simple cable modeling method for use in standard SPICE simulators.
Morse Code is arguably the original digital mode. Morse code is a digital code consists of elements having unequal length. (E8C01) One advantage of using Morse Code is that it is very narrow bandwidth. The bandwidth necessary for a 13-WPM international Morse code transmission is approximately 52 Hz. (E8C05)
The next oldest digital mode is radioteletype, or RTTY. RTTY uses a five-bit code called Baudot. Most modern digital devices these days use ASCII, which is a 7-bit or 8-bit code. Some of the differences between the Baudot digital code and ASCII are that Baudot uses five data bits per character, ASCII uses seven or eight; Baudot uses two characters as shift codes, ASCII has no shift code. (E8C02) One advantage of using the ASCII code for data communications is that it is possible to transmit both upper and lower case text. (E8C03)
The reason that some ASCII transmissions have only seven bits, while others use eight bits is that the eighth bit is a parity bit. The advantage of including a parity bit with an ASCII character stream is that some types of errors can be detected. (E8C12)
The bandwidth needed for ASCII digital transmissions increases as the data rate increases. The bandwidth necessary for a 170-hertz shift, 300-baud ASCII transmission is 0.5 kHz. (E8C06) The bandwidth necessary for a 4800-Hz frequency shift, 9600-baud ASCII FM transmission is 15.36 kHz. (E8C07)
PSK has become a very popular digital mode. One reason for this is that it occupies a very narrow bandwidth – only 31 Hz. One technique used to minimize the bandwidth requirements of a PSK31 signal is the use of sinusoidal data pulses. (E8C04)
An up-and-coming digital mode is JT-65, named after its inventor, Nobel Prize winner and amateur radio operator, Joe Taylor, K1JT. It uses 65 different tones spread over a bandwidth of 175 Hz. One advantage of using JT-65 coding is the ability to decode signals which have a very low signal to noise ratio. (E8C13)
Spread-spectrum communication is a wide-bandwidth communications system in which the transmitted carrier frequency varies according to some predetermined sequence. (E8C08) Direct sequence is a spread-spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier. (E8C11) Frequency hopping is a spread-spectrum communications technique alters the center frequency of a conventional carrier many times per second in accordance with a pseudo-random list of channels. (E8C10) Spread-spectrum techniques causes a digital signal to appear as wide-band noise to a conventional receiver. (E8C09)
Perhaps the most popular digital mode these days is PSK31. PSK stands for “phase shift keying.” One of its main advantages is that it had a very narrow bandwidth—only 31 Hz. In fact, PSK31 is the digital communications mode that has the narrowest bandwidth. (E2E10)
One of the ways is achieves this narrow bandwidth is that uses variable length coding. That is to say, characters have different numbers of bits, depending on how frequently they appear in normal text. PSK31 is an HF digital mode that uses variable-length coding for bandwidth efficiency. (E2E09)
Another type of modulation commonly used on the HF bands is frequency-shift keying, or FSK. RTTY, for example uses FSK modulation. FSK is a type of modulation that is common for data emissions below 30 MHz. (E2E01) One type of FSK modulation is MFSK16. The typical bandwidth of a properly modulated MFSK16 signal is 316 Hz. (E2E07)
Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. The difference between direct FSK and audio FSK is that direct FSK applies the data signal to the transmitter VFO. (E2E11) When using audio FSK, audio, typically from a computer sound card, is used to shift the frequency of the transmitted signal.
To tune an FSK signal, one often uses a crossed-ellipse display. You have properly tuned a signal when one of the ellipses is as vertical as possible, and the other is as horizontal as possible. When one of the ellipses in an FSK crossed-ellipse display suddenly disappears, selective fading has occurred. (E2E04)
PACTOR is one digital mode that uses FSK. It also uses the ARQ protocol to detect errors. Because of this, PACTOR is an HF digital mode that can be used to transfer binary files. (E2E08) How does ARQ accomplish error correction? If errors are detected, a retransmission is requested. (E2E05)
Another way to detect and correct errors in a data transmission is forward error correction. The letters FEC mean Forward Error Correction when talking about digital operation. (E2E02) Forward Error Correction is implemented by transmitting extra data that may be used to detect and correct transmission errors. (E2E03)
No matter what type of modulation you use, data transmission over an HF radio link is very slow. 300 baud is the most common data rate used for HF packet communications. (E2E06) In fact, due to bandwidth limitations, 300 baud is the maximum data rate.
Many of the digital modes were designed to allow keyboard-to-keyboard operation. That is to say, that operators can type messages back and forth to one another, almost as if they were having a conversation using SSB. Winlink, however, does not support keyboard-to-keyboard operation. (E2E12)
One of the most commonly understood concepts in digital communications is the baud. A baud is not equal to a bit per second, except for very simple systems. Rather, the definition of baud is the number of data symbols transmitted per second. (E2D02) A data symbol may represent multiple bits.
The baud rate is a measure of how fast a digital communications system can transmit data. Under clear communications conditions, 300-baud packet is the digital communication mode that has the fastest data throughput. (E2D09)
In the past ten years or so, we’ve had an explosion of digital modes become available. JT65 is one example. JT65 is a digital mode especially useful for EME communications. (E2D03) JT65 improves EME communications because it can decode signals many dB below the noise floor using FEC.(E2D12) FSK441 is a digital mode especially designed for use for meteor scatter signals. (E2D01)
One of the most popular digital modes is the Automatic Packet Reporting System, or APRS. AX.25 is the digital protocol used by APRS. AX.25 is more commonly known as packet radio. (E2D07) Unnumbered Information is the type of packet frame used to transmit APRS beacon data. (E2D08)
APRS stations can be used to help support a public service communications activity. An APRS station with a GPS unit can automatically transmit information to show a mobile station’s position during the event. (E2D10) Latitude and longitude are used by the APRS network to communicate your location. (E2D11) 144.39 MHz is a commonly used 2-meter APRS frequency. (E2D06)
Amateurs that enjoy satellite communications also use digital modes. For example, store-and-forward is a technique normally used by low Earth orbiting digital satellites to relay messages around the world. (E2D05) The purpose of digital store-and-forward functions on an Amateur Radio satellite is to store digital messages in the satellite for later download by other stations. (E2D04)
Where: Atlanta, GA Sheraton Gateway Hotel Atlanta Airport 1900 Sullivan Road Atlanta, GA 30337
When: September 21 – 23, 2012
Technical / Introductory Sessions Schedule http://www.tapr.org/pdf/DCC_2012_Schedule.pdf
Technical Sessions Friday – Saturday: Introductory Sessions
Saturday Night Banquet Speaker & Topic: http://www.tapr.org/dcc#banquet
DCC Saturday Night Banquet Speaker will be Bdale Garbee, KB0G talking about the “Sharing the Joy of Making.”
Sunday Morning Seminar Speaker & Topic: http://www.tapr.org/dcc#seminar
DCC Sunday Morning Seminar will be a hands-on tutorial using Gnuradio to design and implement software defined radios on your laptop presented by Tom Rondeau, KB3UKZ, the leader of the Gnuradio project.
Extra Class question of the day: modulation methods; modulation index and deviation ratio; pulse modulation; frequency and time division multiplexing
In FM modulation, the two primary parameters of interest are deviation ratio and modulation index. Deviation ratio is the ratio of the maximum carrier frequency deviation to the highest audio modulating frequency. (E8B09) The deviation ratio of an FM-phone signal having a maximum frequency swing of plus-or-minus 5 kHz when the maximum modulation frequency is 3 kHz is 1.67. (E8B05)The deviation ratio of an FM-phone signal having a maximum frequency swing of plus or minus 7.5 kHz when the maximum modulation frequency is 3.5 kHz is 2.14. (E8B06)
The term for the ratio between the frequency deviation of an RF carrier wave, and the modulating frequency of its corresponding FM-phone signal is modulation index. (E8B01) The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index of a phase-modulated emission does not depend on the RF carrier frequency. (E8B02)
The modulation index of an FM-phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency, when the modulating frequency is 1000 Hz is 3. (E8B03) The modulation index of an FM-phone signal having a maximum carrier deviation of plus or minus 6 kHz when modulated with a 2-kHz modulating frequency is 3. (E8B04)
Some amateur radio communications are pulse-width modulated. That is to say that the information being sent is proportional to the time the carrier is on. When using a pulse-width modulation system, the transmitter’s peak power greater than its average power because the signal duty cycle is less than 100%. (E8B07)
Some signals are pulse-position modulated. That is to say, what is significant is when the pulse occurs. The time at which each pulse occurs is the parameter that the modulating signal varies in a pulse-position modulation system. (E8B08)
Frequency division multiplexing is one method that can be used to combine several separate analog information streams into a single analog radio frequency signal. (E8B10) When a system uses frequency division multiplexing, two or more information streams are merged into a “baseband,” which then modulates the transmitter. (E8B11)
When a system uses digital time division multiplexing, two or more signals are arranged to share discrete time slots of a data transmission. (E8B12)
The Air Force High Frequency Global Communications System Program Office at Tinker Air Force Base is looking for some folks to help tweak single sideband so it can function as a relatively wide band over the air data transmission system as well as handle voice over IP. Single sideband will never have the throughput of a fiber optic connection. On the other hand, there’s not a lot of fiber drops in the sky or a forward operating base in Afghanistan, so a 120 kbps connection from a radio looks real good.
This certainly sounds like a project for some enterprising hams. And, who knows, this new mode might even make its way into the ham radio bands someday.