A RTTY Tutorial for beginners

RTTY Tutorial

This document was originally posted by K4WW on rttyinfo.net but this domain has expired since some months at time writing. I’ve archived a copy that I believe could be usefull for beginners.
This is not intended to be an “etched in stone” indication of how to operate a RTTY contest, so please don’t take it that way! Whatever works the best for you, is what you should do, as long as it allows your participation to be fun! These hints were obtained from RTTY contesters, world wide, and only reflect how they try to make their operation more efficient! The more efficient “we” make our contest exchanges, the more efficient we make it for all involved! Band conditions certainly play an important part in the exchanged data, so establishing a “different” buffer could make your exchange more efficient in “not so good” band conditions!

There are many web sites to obtain RTTY contesting information, and for fear of slighting anyone, I will only suggest that if you intend to participate, regardless of the level, that you visit rttyjournal and acquaint yourself with the rules, or at least the necessary exchange! Subscribing to rtty@contesting.com is also advisable, as most all RTTY operators, especially those primarily interested in contesting, can be found here. Someone there can answer most any question that you may have! Some contest managers do not allow points for “unique” callsigns in the submitted logs! There are varying definitions of “unique”, or the specific number of logs that a callsign must appear in to not be classified as “unique”! Having this knowledge, if you choose to just work your friend just to give them a point, make sure that you work a few (4-5) others, to insure that the contest manager will allow the contact with your friend!

RTTY For Newcomers

by Bill, W7TI

RTTY is a fun mode and easy to operate, but there are some questions every newcomer has. Please take a moment to read the following and much of the mystery will disappear.

MARK AND SPACE

A RTTY transmitter sends out a carrier that shifts back and forth between two frequencies. There is no amplitude modulation, only a pure carrier which shifts frequency. The lower RF frequency is known as the SPACE frequency and the upper RF frequency is known as the MARK frequency. The difference between the two is known as the SHIFT. For amateur radio, the SHIFT has been standardized at 170 Hz. It is customary to refer to the MARK frequency as your operating frequency. For example, if you say you are transmitting RTTY on 14080.00 kHz, which means your MARK frequency is 14080.00 kHz and your SPACE frequency is 170 Hz lower, or 14079.83 kHz. While 170 Hz is the standard shift, sometimes you will find stations using a shift of 200 Hz, but don’t worry about it. Your equipment will copy it fine in almost all cases.

FSK and AFSK

You will often hear the terms FSK and AFSK when talking about RTTY on the HF bands. FSK means Frequency Shift Keying and AFSK means Audio Frequency Shift Keying. Regardless of which method is used, the RF signal sent out over the air is identical. MARK is always the higher RF frequency and SPACE is always the lower RF frequency. If the transmitter is operating properly, the station receiving the RTTY signal can not tell any difference at all. So what is the difference? It’s the way your transmitter generates the RF signal.

With FSK, your transmitter receives a simple on-off signal which causes the carrier frequency to shift back and forth. This signal may come from a TNC (Terminal Node Controller) such as a Kantronics KAM, an AEA PK-232, HAL DXP-38 or some other, or it may come from a soundcard program via one of your computer’s com ports. FSK is simpler, easier and more foolproof than AFSK and is highly recommended if your transmitter supports FSK input. Check your owner’s manual if you’re not sure.

Since not all transmitters support FSK input, there is another method available – AFSK. AFSK can be used with any SSB transmitter. AFSK is a bit trickier to set up and use, but when it is done correctly, it works just as well as FSK and will transmit a perfect RTTY signal. Also, AFSK can do some things FSK can not, such as Automatic Frequency Control (AFC) of the transmitter. With modern transmitters, drift is not a problem and transmit AFC is generally not needed. With older tube-type equipment, it may offer some advantage.

To operate with AFSK, you put your transmitter in the SSB mode instead of FSK mode, and you put an audio signal into the microphone input (some transceivers have a rear connector for audio data input). When you transmit your TNC or soundcard will put out a pair of audio tones that cause your transmitter to send the required RF output. Sounds simple, right? Here’s the tricky part: The tones are two simple sine waves, but the frequency and amplitude of the tones is critical. Let’s say you want to transmit on 14080.00 kHz, as in the previous example. Remember that your MARK signal has to be on 14080.00 kHz and SPACE 170 Hz lower. How do you do that with AFSK? Here’s how. With your transmitter in the LSB mode (Lower Side Band), whatever frequency goes into the microphone input will be subtracted from what your dial says and be transmitted on that frequency. For example if your dial says 14080.00 kHz and you put in a 1000 Hz audio tone, your transmitter will put out an RF signal at 14079.00 kHz, exactly 1000 Hz lower than your dial. So in this case, if the 1000 Hz represented your MARK signal, you would have to set your transmitter to 14081.00 on the dial, and your MARK signal would be transmitted on 14080.00, just as you wanted. Ok so far? Now, what about SPACE? Remember, you want your SPACE signal to be transmitted 170 Hz lower, on 14079.83 kHz. What audio tone will give you 14079.83? Simple – 14081.00 minus 14079.83, or 1170 Hz. So the MARK audio frequency is 1000 Hz and SPACE is 1170 Hz.

There you have the basics of AFSK. Your TNC or soundcard generates the two audio frequencies and your transmitter converts them into two RF frequencies. For technical reasons related to harmonic generation, audio frequencies of 1000 Hz and 1170 Hz are NOT recommended. They are used in this example just to keep the math simple. The recommended audio frequencies are 2125 Hz for the MARK audio frequency and 2295 Hz for the SPACE audio frequency. Making the frequencies higher like this will reduce second harmonics while keeping the tones within the passband of your SSB transceiver.

If you’ve been paying close attention, you may have noticed the SPACE audio frequency is higher than the MARK audio frequency, just the opposite of the RF frequency. This happens because you’re using lower sideband. If you happen to forget and set your transmitter to USB instead, two things will happen. Because your MARK and SPACE are now reversed in your receiver, any RTTY signals you hear will not print correctly. All you will see is random characters that make no sense at all. The other thing is that YOUR transmissions will also be nonsense to the other guy. So just remember – always use LSB. In the real world of course, sometimes USB gets selected accidentally. Nearly all software has a means to quickly reverse the tones, either a keyboard command or an on-screen button to click. When you have a station tuned correctly but all you see is nonsense printing reverse the tones. Now you can print the other fellow and tell him he is “upside down”, as it’s called. After he reverses himself, just reverse again and you will both be back to normal.

Also, you should know that in some parts of the world, especially Europe, the standard is to use USB instead of LSB. This works fine as long as you also reverse the two audio tones. In the US, nearly all equipment defaults to LSB. If you prefer to use USB, be sure your tones are reversed all the time.

The really critical part about AFSK is the amplitude of the signal fed into the microphone connector (or rear panel connector), together with the microphone gain setting. You must NOT overdrive your transmitter or spurious signals will be transmitted. In general, keep the audio drive low enough that your transmitter does not generate any ALC voltage. Never try to drive your transmitter to maximum output. Around 80 to 90 percent of maximum is about right. Consult your owner’s manual for more information on how to do this. If you ever hear a station at two or more frequencies at the same time, the cause is almost always overdrive. None of this applies to FSK, of course. With FSK, you can run full power and not worry about overdrive.

FIGURES SHIFT and LETTERS SHIFT

RTTY uses the Baudot code, invented before radio even existed, and still widely used throughout the world. The Baudot code uses data bits to represent letters, numbers and punctuation, much like your computer does. Unlike your computer, which uses eight bits for each character, the Baudot code uses only five, plus a start bit and stop bit. Using fewer bits is good because it speeds up transmission and reduces the chance of errors, but there is a complication. Five data bits can only represent 32 different characters. Since there are 26 letters in the English alphabet plus ten numbers, plus some punctuation, 32 different characters is not enough, even if you only use capital letters, as Baudot does.

Mr. Baudot could have chosen to use six data bits or even more, but he found a better solution. He reasoned that most of what is sent would be letters rather than numbers or punctuation, so he assigned all the letters to the basic 32. He then had six characters left over and he did a very clever thing with two of them. He made one of them a FIGURES SHIFT and another a LETTERS SHIFT. The way it works is this: When sending one of the basic 32 characters, nothing special happens. But when a number or punctuation is to be sent, a FIGURES SHIFT character is sent first (it’s a non-printing character – you won’t see it on your screen). Whatever follows will still be one of the basic 32 characters, but the receiver will interpret it differently. For example the letter Q uses the same five data bits as the number 1, but when the receiver gets a FIGURES SHIFT, it prints the following character as a 1, not a Q. This continues until a LETTERS SHIFT character is received, at which time the receiver goes back to “normal” printing. All of this shifting is done by the system – there is no key marked LETTERS SHIFT or FIGURES SHIFT. It’s all-automatic and you will scarcely notice it happening.

In fact, the only reason to mention it at all is because we are using radio instead of wires, and radio is susceptible to interference from various sources such as lightning static, man-made noise, QRM, etc. If a burst of static should happen to wipe out a LETTERS SHIFT or FIGURES SHIFT character, the characters following will not print correctly until another LETTERS SHIFT or FIGURES SHIFT is received. For example, suppose you are sending a signal report of 599, but the FIGURES SHIFT character gets wiped out by a burst of static. Instead of printing 599, the other fellow’s computer will print TOO. TOO is exactly the same as 599, without the FIGURES SHIFT. So how can he read what you sent? It’s easy if he knows the secret. Here it is: Look at the top row of letter keys on your keyboard – QWERTYUIOP. Now look just above each key and to the left. Each of those number keys is the same as the letter key below and to the right, plus the FIGURES SHIFT. In our example, TOO = 599. Likewise, the word PIPE, if the LETTERS SHIFT were missed, would print as 0803. If 0803 lost its FIGURES SHIFT, it would print as PIPE. Some RTTY programs have a way to highlight the erroneous characters and change them to the correct shift.

TUNING INDICATORS

All TNCs and soundcard programs have to have some means of tuning in a RTTY signal. If you have that hearing ability known as “perfect pitch” you might get along with ears alone, but the rest of us need something to look at. For now, use what your equipment provides, but if you get really hooked on RTTY, you will probably want to invest in an oscilloscope. A scope works by having the MARK and SPACE tones from the receiver drive the X and Y channels, without any internal sweep. The pattern on the scope will be a pair of ellipses (flattened circles) at right angles to each other. When properly tuned, the ellipses are exactly vertical and horizontal. When the receiver is mis-tuned, they lay over at an angle. Properly set up, a scope provides an instant indicator of any tuning error. Fortunately, even a very inexpensive scope will do just fine. High speed and lots of bells and whistles are not needed.

BANDWIDTH and FILTERS

When the bands are nearly empty, you can use practically any receiver bandwidth with good success. Your SSB filters are probably between 2.1 and 3.0 kHz wide and as long as no other stations are nearby, copy will be fine. For optimum performance however, less bandwidth is better, in fact MUCH better. 170 Hz shift RTTY only needs about 250 Hz for proper copy. If you don’t have a 250 Hz filter, 500 Hz will do pretty well, but anything wider than that will not be satisfactory in the long run.

You may wonder why, for 170 Hz shift, you need a 250 Hz filter? Why not 170? The reason is that shifting the frequency generates sidebands adjacent to the actual signal and if the sidebands are attenuated, the signal will be degraded. RTTY (as used on HF) is actually a form of FM, and FM does produce sidebands, believe it or not. If you’d like to understand FM a little better, the ARRL handbook is a good reference.

Depending on your transceiver, you may or may not be able to use a narrow filter for RTTY. Some of the less expensive transceivers do not have a true FSK mode, and also are unable to select a narrow filter while in the LSB mode. Using an outboard audio filter between the speaker output and the TNC or soundcard input can make some improvement, but unfortunately, that will not prevent a strong adjacent signal from causing the receiver’s AGC circuit to reduce gain, often to the point where the desired signal disappears. The best solution is to upgrade to a transceiver which has FSK mode built in, AND which allows you to select a narrow filter while in that mode.

BAND PLANS

It’s easy to remember the HF band plans for RTTY. Most activity will be found between 80 and 100 kHz up from the bottom edge of the band, except for 80 meters which goes an additional 40 or 50 kHz higher, and 160 meters. 160 meter RTTY activity is rare, but when found, it is usually between 1800 and 1820. Avoid the CW DX window between 1830 – 1840. At present, there is not much activity on the WARC bands, although 30 meters can be active at times.
Here is where you will find most of the RTTY activity:
80 meters: 3580 – 3650 (3520 – 3525 in Japan)
40 meters: 7080 – 7100 in the US (see note below)
30 meters: 10110 to top of band
20 meters: 14080 – 14099 (avoid the NCDXF beacons at 14100)
15 meters: 21080 – 21100
10 meters: 28080 – 28100

RTTY allocations for 40 meters vary greatly all over the world. In the US, RTTY is permitted between 7000 and 7150, although most US activity is between 7080 and 7100. DX activity is often found between 7020 and 7045. The ARRL promotes 7040 as the RTTY DX calling frequency, but the CW QRP’ers use it as their calling frequency too. Be a gentleman!

For US operators, remember that RTTY is not allowed in the phone portions of the HF bands except on 160 meters, where it is legal anywhere in the band.

RTTY DX

Chasing DX on RTTY is highly popular with the RTTY crowd. As you might guess, 20 meters is the premier DX band for RTTY, and most rare DX stations and especially DXPedetions operate on 14080. Just as with CW or phone, if the DX is calling CQ and getting no answers, you can feel safe in calling him right on his frequency. If things are busy however, he will often work split, which means you should call him on a different frequency, usually 2-10 kHz higher. He will say “up 2-10” or something similar at the end of his transmission, and that’s your clue. Your transceiver owner’s manual will explain how to do “split”.

RTTY CONTESTS

RTTY contesting is a passion with a lot of hams. There are more than two dozen major RTTY contests each year and when they are on, the bands will be full! Even if you don’t care to compete, it’s a great way to pick up new states or countries. Many of the rare DX stations are serious contest operators. A list of RTTY contest times and rules can be found on the web at: