Background
AMTOR is an acronym for AMateur
Teleprinting Over Radio and was mostly
used on HF. The mode is based on the
marine SITOR code that was introduced to
amateur radio by Peter, G3PLX. The mode is
an enhancement of RTTY with active error
correction that copes reasonably well with
interference and fading.
There is a good write up of AMTOR in June
1992 QST, written by the late Bill Henry,
K9GWT, who was president of HAL
Communications. It is available to view or
download from the ARRL web site if you are
a member. There is a full engineering
specification on an ITU web page, it
describes the commercial SITOR system in
great detail, this link has the document.
ITU SITOR Specification
The data throughput of AMTOR is fairly
slow under most band conditions, but is
generally error free. For normal "ragchew"
contacts the data flow will match the
typing speed of a reasonable to good
typist. The error correction is fairly
crude but adequate for most purposes.
Pactor is a development of AMTOR and improves both the data rate and the error correction. There are legacy modems with Pactor 1 from Kantronics, Timewave and SCS. More recent versions of Pactor (2, 3 and 4) are only available on SCS modems.
Suitable modems can sometimes be acquired
for little cost from fellow hams who may
have them lying gathering dust in garages
and on shelves. eBay is a source for old
modems, but beware that prices seem a bit
steep for items that are often several
years old and may be hard to repair. Available modems
Probably the most popular AMTOR modem was
the PK232, in addition there were modems
from Kantronics (KAM), ICS (AMT), HAL ,
SCS and an MFJ offering. Older Pactor
modems can also operate on AMTOR. Most of
these modems are quite long in the tooth
and were often only supplied with DOS
software. Currently, as at January 2021,
the following new modems can also operate
AMTOR:
PK232 DSP from Timewave
KAM-XL from Kantronics
Sadly, SCS only offer the Dragon series of Pactor modems these days. The Dragon modems include Pactor 1 to 4, but not Amtor. In addition, they are very expensive. Older SCS modems are sometimes available on the surplus market, the PTC 2 series are popular and may include a Pactor 3 licence key.
Repairing a KAM Plus
My KAM Plus developed a fault that has
also been seen by others and was
straightforward to repair. The issue was
the lack of a -7V from pin 5 of U15, an
LT1054. First I checked the 10uF 50V
aluminium electrolytic capacitors
associated with the power supply (C48, C49
and C51), these all had a higher ESR than
20 Ohms and were replaced. That didn't
cure the problem. The circuit of the KAM
Plus doesn't show a connection to pin 1 of
U15, this pin is a shutdown pin which
stops the output if the pin goes low, the
measured Voltage on that pin fluctuated
when measured with a 10 M Ohm input Fluke
DVM. One modification I saw elsewhere
linked pin 8 (the input) to pin 1
(shutdown), so I tried a 12K resistor
between pins 1 and 8 of U15 and it did the
trick of producing a -Ve supply every time
on switch on, however the fault turned out
to be either leakage under the socket or a
faulty LT1054 (U15). After removing the
socket and thoroughly cleaning the print
board, replacing the socket with a new one
and fitting a new LT1054, the problem was
resolved without needing the 12K resistor.
Several months later the KAM is still
working fine.
Software
Many of the older modems used DOS
software which can be a problem for modern
PCs which often don't even have
conventional RS232 serial ports. Some of
the modems can be operated from a simple
terminal program such as PuTTY, however
most users would probably find a dedicated
program easier to use, unfortunately,
Windows 11 terminal programs written
specifically to run a KAM or PK232 do not
appear to exist. SCS modem users are able
to use the free Alpha program which works
very well.
PuTTY is a free
terminal program, it is very comprehensive
but doesn't specifically cater for amateur
radio comms. You can manually enter
commands but it becomes tedious to have to
remember them.
Terminal BPP is a
Windows 11 "donation-ware" program with
macros, it is similar to PuTTY but with
the benefit of being able to program
scripts and macro keys. It doesn't need
installing, copy the file into a
directory and create a short cut, eg.
C:\terminal. It works very well with a KAM
Plus. I have included a set of macros for
a KAM that can be edited in Notepad to
replace the dummy callsign of X1ABC and
selcall XABC, with your own
callsign/selcall. Note it needs to be used
in conjunction with a read of the KAM
manual, for example you need to manually
set ECHO OFF and to call a station on
Amtor you need to be back at the Command
Prompt and enter their selcall after the
word Amtor, eg. amtor WABC. The macro keys
are easily edited and the commands are
described in the KAM manual
This is my KAM Plus Macro file for
Terminal BPP, unzip it, edit the callsign,
etc. and save it in the same directory as
the terminal program, then load it using
the "set Macros" button in Terminal. Note
the macro buttons for Amtor set the TX
delay to 20mS and for GTor set it to 80mS.
Edit those settings to suit your radio,
these timings have been tested with an
Icom IC-7300 over a 200 mile path. The
"50bd WX" macro sets the KAM to 425Hz
shift, 50 baud and inverted with unshift
on space off, this is to work with the
commercial RTTY weather transmissions.
Specific settings needed for the KAM Plus
are Echo off, XMitecho on and FLow off.
Otherwise anything you type in Terminal
BPP (or PuTTY) will stop the incoming text
stream, or mix your typing with that
stream. Note in order to use the XMitecho
command you will need to change the
INtface to Terminal from Newuser. If you
want to use 1400/1600 tones, the shift
needs setting to modem (command shift
modem), this makes setting the receiver
frequency a little easier. In version 8.2
firmware Amtor standby will ofen result in
spurious messages similar to <FREESIG
20 GWxxx>, to stop this set FREequal to
0.
For a screen shot of Terminal BPP
decoding commercial RTTY from Germany, click
here.
Alpha for SCS modems is free from SCS.
Simple32 Gold is shareware for SCS modems
from Seaglemail.
The log program Logger32 supports
KAM, PK - 232 and MFJ 1278.
In addition, there is one Linux ( KPTC
) offering but unfortunately it only
works with SCS modems - and needs
compiling for the particular version of
Linux used.
Suitable radios
The requirements are fast
transmit/receive change over and a
reasonable level of frequency stability. A
dedicated modem socket (or line socket) is
desirable to avoid needing to change
microphone and speaker leads. Most recent
analogue HF transceivers are suitable for
use on Pactor and some will also work on AMTOR, however it is worth checking
the specification and ARRL reviews for
"suitable for AMTOR" or similar wording.
More recently SDR transceivers have
appeared and some of these have issues
with latency, which is the time delay
through the radio, something that was not
an issue with analogue radios and can be a
cause of significant problems for longer
range QSOs on AMTOR.
One of the very best data radios is the Icom IC-7300, these work out of the box on Pactor and Amtor. My current Elecraft
K4D needs a hold on circuit to prevent the
PTT line dropping before data has left the
transmitter, hopefully that will be
resolved with a firmware modification by
Elecraft, who are aware of the issue. Some
other DSP radios may suffer from the same
problem. I have built a simple "hold on" circuit and the diagram is available from my Radio Mods web page. In the KAM Plus settings there is a Post Key parameter to cater for this additional delay, unfortunately SCS modems do not have such a parameter.
AMTOR has a maximum range of around
11,000 miles when using an analogue radio
with fast change over, however every
millisecond of delay through the radio
reduces the range by 186 miles. Measured
latency through an Elecraft K4 was 23 mS
on transmit and 32 mS on receive, by
comparison my Icom IC-7300 has 4.2 mS on
transmit and 3.3 mS on receive. These
latency measurements do not include the
additional time taken to change from TX to
RX, or back again. Radios such as the K4
will work fine on short haul Pactor QSOs, and
will probably work OK on longer distances
for Pactor with long path connect, but are
far from being a good data radio.
I previously used a Kenwood TS-480sat and
that was fine too. Some much earlier
transceivers may need modifying in order
to change over quickly enough and some
later transceivers, especially SDR types,
may not switch fast enough, nor be easily
modified. There is a slight advantage in
using transceivers with diode antenna
change over to avoid the endless "click
click" of the antenna relay when operating
AMTOR, although it is doubtful that relay
wear will be significant unless you want
to operate AMTOR 24 hours a day.
Mostly SSB is used with audio tones on
AMTOR, although FSK can be used with some
modems as AMTOR is based on RTTY. Note
that modems which include Pactor will only
use audio tones as FSK (direct frequency
shift keying) is not suitable for use on
Pactor.
Hooking it up and getting going
Wiring modems to radios and computers is
covered in the modem user manual, all that I
have either wired or seen, have been
comprehensively covered in the manual. In
some cases running a ground wire from the
radio to the modem or passing the
connecting cable through a large ferrite
ring is useful to avoid RF hanging up the
modem, it depends on how much RF you have
in the shack as to how much of a problem
it might be, again refer to the user
manual for any grounding or filtering
information.
Beware of how you connect the screening
and ground connections between the modem
and the radio, in general you should use
multicore screened cable with the
braid/shield connected to the shell of the
plugs, the ground pins should connect to a
wire within the screened cable and not to
the shell of the plug. These images show
the connection to an IC-7300 13 pin
accessory plug (below left) and to a KAM
Plus (below right). Note that I have not
used the moulded plastic cover for the KAM
end DIN plug as with the older KAM Plus
the DIN socket is recessed in the rear
panel and the plastic cover of most plugs
prevents the plug fitting properly. Using
a piece of heatshrink works well and
allows a normal sized DIN plug to fit into
the KAM Plus. Also note that I have cut
off the unwanted pins in the 13 pin DIN
plug on the IC-7300 end of the cable to to
make soldering the plug much easier.
Tuning
Depending on the tone pairs used, you
will have to pick USB or LSB, working out
the actual carrier frequency for the mark
and space tones may require a little
mental arithmatic in order to meet up with
someone on a certain frequency.
AMTOR uses 170 Hz shift, the same as for
RTTY. Some modems will only accommodate a
200 Hz shift, which is near enough for all
practical purposes. Some modems include a
feature to specify the mark and space
frequencies, traditionally for RTTY these
were 2125 and 2295 Hz. The more modern
modems, which include Pactor, often
default to 1400 and 1600 Hz tones, which
are the initial tones for a Pactor call.
In any case, your transceiver needs to be
able to work with whatever tone pairs your
modem is set for. With an Elecraft K3
these tones are best set around a centre
frequency of 1500 Hz as this is fixed at
that frequency when operating in
synchronous data modes such as AMTOR and
Pactor.
Pactor modems by SCS tend to use 1400/1600 Hz tone pairs as the initial Pactor 1 tones. On USB the dial frequency will be 1.5 KHz below the centre frequency of operation. It is common practice to refer to the centre frequency, for example on 20m a common frequency used is 14.109 (CF), which is likely to be 14.1075 USB dial frequency.
The basic parameters necessary on both
the radio and modem are as follows, these
are based on using an SCS PTC-IIex modem with an
Elecraft K4, other modems and radios will
have similar parameters - although not all
radios have a specific data mode:
Radio settings
Mode - DATA
Bandwidth - 2.5 KHz
AGC - fast
TX Line In - Line in jack
If you haven't a PTT "hold on" circuit, consider using VOX with a delay of 0.02 (20 mS)
Modem settings
ARX 1 (enables AMTOR, ARX 0 would prevent
AMTOR operation)
BC 1 (AMTOR FEC enabled)
Plus these from the Alpha software "HF Port" settings file (includes the above). Set the amplitudes to suit your radio.
There are dozens of modems commands,
these are just the most basic ones.
Note the settings for a KAM include a TX
to RX delay setting, POStkey sets a time
delay to allow sufficient time for data to
exit the radio before returning to RX.
If your radio covers 490 and 518 KHz, you
can test your Amtor receive by listening
to Navtex, which is basically the same as
Amtor broadcasts. There are many
navigation and weather bulletins broadcast
on these frequencies in Europe, although
there are none in Australia.
Finding other stations to work
BEACON or Call CQ on 14109 CF
(14107.50 USB DIAL), 10146.50 CF (10145
USB DIAL), 7047.90 CF (7046.40 USB DIAL)
or 3609.50 CF (3608 USB DIAL). There are
active Pactor and Amtor users on the
Groups.io Pactor group, it is well worth
joining that group. See https://groups.io/g/pactor
Format for contacts
Normally a station would call CQ in AMTOR
mode B (FEC) in a similar manner to this, Pactor calls would omit the selcall in brackets:
CQ CQ CQ DE G4AON G4AON (GAON)
CQ CQ CQ DE G4AON G4AON (GAON)
PLEASE CALL IN ARQ K
The four letters in brackets are my
selcall, it is helpful if stations list
their selcall during a CQ as a few do not
follow the normal arrangement for creating
a selcall and confuse those of us who try
and guess what they used! Normally for two
letter suffix calls you would repeat the
first letter, W1AB would use WWAB. Equally
WA1BCD would use WBCD to make a 4
character selcall. You can use what you
like provided that if you deviate from the
expected selcall you make it clear in your
CQ.
The caller would selectively call the
station calling CQ and give their
callsigns as the AMTOR exchange (unlike
Pactor) does not include a callsign. It is
useful if the modem is configured to auto
reply, when called, with your callsign and
name. The change over command is +? and
can be manually typed or done via one of
the pre-configured buttons in most
terminal software.
CW ID may be necessary depending on your
licence conditions, again most modems are
configurable to auto send CW IDs
periodically and at the end of a QSO.
All the above is usually covered in the
user manual for the modem.
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