Transmitting QRSS.
(within the H.F. amateur bands.)
Introduction.
Introduction.
The text that follows assumes that you are familiar with the meaning of
the term "QRSS", if this is not the case then please have a look at the
"What is QRSS"
page. The following information is based on my own
experience of QRSS operation within the amateur H.F. bands. I am very grateful to
the many fellow QRSS enthusiasts (Knights) who took the time to give me help and advice in order to successfully build and operate my first QRSS transmitter (M.E.P.T.) for the 30 Mtr QRSS sub-band.
QRSS terminology.
A couple of terms which crop-up frequently in the world of QRSS (and on these web pages) are "Knights" and "MEPT" so perhaps its a good idea to clarify those terms here. Many QRSS enthusiasts and operators are called "QRSS Knights", this is simply a term which is used on the QRSS mailing list to describe a fellow QRSS enthusiast.
"MEPT" is a term used in QRSS circles to describe our transmitting equipment, MEPT
stands for Manned Experimental Propagation Transmitter and the term was
adopted in order to clarify the goals associated with QRSS experiments.
A more "in-depth" explanation of the term MEPT can be found here.....
So what's a M.E.P.T. ??
A starting point.
Transmitting QRSS signals can be achieved very quickly and simply but
if you are new to QRSS then it is a good idea to follow a few basic
guidelines before going on the air for the first time. The
guidelines listed below are based on my own experience, depending on
your level of expertise or previous experience you may be able to skip
many of the steps listed but I would recommend that you still follow
steps 7 an 8 listed below.
1) Read all the information you can find on QRSS operation, frequencies used, modes etc.
2) Become familiar with receiving QRSS signals and identifying QRSS modes before you start transmitting your own signals.
3) Establish an accurate and reliable method of frequency calibration for both transmitting and receiving.
4) Subscribe to the QRSS mailing list and read the postings for a few weeks to become familiar with current QRSS activity.
5) Decide on a frequency and mode(s) for your first M.E.P.T.
6) With respect to 5 above you can now decide if it is better to
home-brew a dedicated QRSS TX or to use an existing piece of ham gear.
7) When you are ready to begin test transmissions be sure to announce your intentions on the QRSS mailing list and the QRSS Clipboard.
8) When transmitting be sure to check your transmitter frequency from
time-to-time to ensure your TX is not "drifting" out of the QRSS
sub-band.
9) Be patient! Even if everything is working perfectly your QRSS
transmission is still at the mercy of the prevailing propagation. It
may take a little time before you get any confirmed signal reports or
screen captures.
Requirements of a QRSS transmitter.
The
prime requirement for a QRSS transmitter is to ensure good short and
long term frequency stability. As an example, the center of QRSS
activity is in the 30 Mtr band between 10.140000 MHz and 10.140100 MHz
(100 Hz wide sub-band) so any small frequency error or "drift" will
result in the signal possibly falling outside of the QRSS sub-band
which reduces the chances of receiving confirmed reception reports of
your signals. At the very least your intended QRSS transmitter should
be crystal controlled or referenced to a stable crystal oscillator.
Additional stability can be achieved by carefully regulating the
temperature of the crystal oscilator or by using a reference signal
that is known to be inherantly stable such as a GPS time reference
signal.
Sending a stable
carrier signal in itself does not convey much information so some form
of modulation is required in order to convey some useful information
such as your call sign or a unique identifier which others will
recognise and associate with your call sign. Severel methods of
modulating the carrier are possible but two of the simplest are ON-OFF
keying and Frequency Shift Keying (FSK) both of which are used
extensively in the world of QRSS.
ON-OFF keying.
ON-OFF keying is perhaps the simplest form of modulation to implement,
indeed any simple QRP CW TX could be addapted for QRSS providing it
covers the required frequncy and providing its frequency stability is
deemed to be adequit for QRSS. It is also important that the TX does
not "chirp" when keyed. "Chirping" is a small shift in transmit
frequency when keying, this sometimes happens in badly designed
transmitters due to changes in loading on the oscilator/buffer circuits
or fluctuations in the power supply. Chirping can also be caused by
feedback from the transmitter output stages getting back to the
oscillator stages. The main drawback with on-off keying is that the
receiving station may have difficulty in decoding the transmission in
the pressance of QSB. The carrier may appear to be off when in fact the
signal level has dropped below the noise floor due to QSB. With aural
CW the operator can normaly tell the difference between "key up" and
QSB because QSB is generally happens more slowly than the keying of the
sender. With QRSS the much slower sending speed used results in the key
up/key down times being comparable to the QSB so decoding a message can
be difficult.
Frequency Shift Keying (FSK)
Frequency shift keying can make the decoding process much easier
since two (or more) descrete frequencies can be used to indicate the
difference between a dot or a dash of the Morse code. The shift in
frequency does not need to be very large, 4 or 5 Hz is a typical value.
Providing the frequency stability of the transmitter remains fairly
constant between the frequency shifts then the "step" from one
frequency to another will be clearly detectable by the receiving
station providing that receiving station has similarly good frequency
stability. In practice frequency shifts of between 4 to 5 Hz have
proved entirely successful. A very s-l-o-w drift in frequency (much
slower than the FSK intervals) is less of a problem because individual
elements of the message are effected less.
Using commercial equipment.
More to follow shortly...
Well, that’s about it, thank you for reading this and please
send any questions, comments or "heckles" etc to the e-mail address linked below.
Des (M0AYF)
