December 06, 2003
Some radios are rated for continuous duty at maximum output. Examples include the Icom M710, Icom M700Pro, and Icom M802. If your radio IS rated this way, it will be prominently noted on the brochure and on the specifications because the manufacturer will be proud of the fact. The brochure will say something like "designed for full power SITOR/RTTY service", or "continuous duty for digital applications", or "100% duty cycle" etc.
Other radios are designed for voice usage, and while they can still be used for data, they can not be run at maximum power output or they will overheat. Many radios will automatically detect that they are overheating and reduce their output to avoid total failure, but you will shorten their lifetimes by abusing them in this way. Examples of radios designed primarily for voice usage are the Icom 700, the SGC SG-2000, the Furuno FS-1502, the SEA 235 and most older radios.
Setting the drive levels on a continuous duty radio is much less critical than for a voice rated radio, because the radio has sufficient cooling so that it will not be damaged by overheating. You just want to ensure that the drive levels are not too high because the ALC (automatic level control) in the radio might not be able to avoid distorting the output signal, which would slow down your SailMail message transfers, and would cause interference on adjacent channels.
FSK (frequency shift keying) is the transmission mode (used in Pactor-I) that the KAM+ uses all of the time, and that the SCS PTC-II uses briefly before it automatically switches to Pactor-II mode which uses PSK (phase shift keying). FSK mode has the potential to operate your radio continuously at maximum output. For engineers, its crest factor is 1.0 (i.e. the peak and average voltages of the modulating signal are the same).
PSK is the transmission mode used in Pactor-II and Pactor-III that is used by the SCS PTC-II after it connects with the SailMail station. When your radio is running at its peak output transmitting Pactor-II, its average power output will be half of its peak output. This is due to the properties of the PSK modulation, not due to any shortcoming of the radio. View this as an advantage of Pactor-II; it uses less power than does the FSK modulation used by Pactor-I (KAM+), and works over longer ranges at faster rates. For engineers, the voltage crest factor of the PSK used in Pactor-II is 1.45 (i.e. the peak to average voltage ratio is 1.45, which means that the peak to average power ratio is 1.45 squared, which is 2). A big advantage of Pactor-II, with its lower average power output, is that radios designed for voice duty can perform nearly as well as continuous-duty radios.
The crest factor of Pactor-III varies depending on how many tones the modem decides to use. Depending on the number of tones, the voltage crest factor for Pactor-III ranges from 1.24 to 1.93 (which corresponds to power crest factor of 1.9 dB to 3.7 dB). Remember by comparison that FSK has a voltage crest factor of 1.0 and Pactor-II had a voltage crest factor of 1.45. So in terrific conditions, Pactor-III, will use all 18 tones and will be very easy on your radio and only run your (150 watt peak power) radio at an average power of 40 watts. In lousy conditions Pactor-III will only use two tones, and with a 1.24 voltage crest factor will run your (150 watt peak power) radio at an average power of 97 watts.
So enough theory for the engineers, here is what to do.
The first thing to do is reconsider whether you want to do this yourself. The (sensible) alternative is to hire a marine radio dealer who knows exactly what he or she is doing, and has the appropriate test equipment. If you feel compelled to (or have to) do it yourself, read on.
Unless you have access to a RF power meter, measure the DC current drawn by the radio from your boat's 12 vdc electrical system as a measure of the radio’s power level. Be sure that your batteries are fully charged (or on charge). Pick an open SailMail channel on which you are unlikely to interfere with anyone (5 MHz in the afternoon), and work quickly. Listen to make sure that the frequency is clear before transmitting. Better yet, use a dummy load if you know a ham who can lend you one.
Follow these instructions to set your FSK and PSK levels, and the resulting levels will be just right if you upgrade your modem and use Pactor-III.
For a voice rated radio, adjust the drive levels (FSK and PSK) so that the radio draws 10-12 amps from your boat's 12 vdc electrical system.
For a continuous rated radio, increase the drive level so that the radio just draws maximum current (probably 25 amps for FSK, and 15 amps for PSK) and then reduce the drive level just a little so that the radio draws about 1 amp less than the maximum. If the radio has an ALC display, there should be very little deflection of the ALC "meter".
Under AirMail's Terminal Window, under the Control Tab, select the "Xmit Unproto" function. This will cause the radio to continuously transmit in FSK mode. Select the "Set PTC-II Amplitude" function. Move the FSK tab back and forth and monitor the DC current drawn by the radio. Set the tab so that the radio is drawing the appropriate amount of current (see above) from your boat’s 12 VDC system. When done, press the stop button (looks like a stop sign).
Next select the "PSK Unproto" function. This will cause the radio to transmit continuously in PSK mode. Select the "Set PTC-II Amplitude" function. Move the PSK tab back and forth and monitor the DC current drawn by the radio. Set the tab so that the radio is drawing the appropriate amount of current (see above) from your boat’s 12 VDC system. When done, press the stop button (looks like a stop sign) to quit.
For a voice rated radio (where you are adjusting both FSK and PSK for 10-12 amps draw), the level that you set for PSK will be about 1.45X the level for FSK. For a continuous rated radio (where you are adjusting both FSK and PSK for an amp below maximum current draw), the level that you set for PSK will be about the same as for FSK. Many users choose to not set the FSK levels at the absolute maximum, even for a radio that is designed for it. Instead they set FSK levels so that the radio draws about 5-8 amps less than the maximum.
For a voice rated radio, during subsequent operation of your radio, on various SailMail frequencies, watch to make sure that the radio does not draw more than 12 amps from your boat's 12vdc system. If it does you can adjust the levels in the "Audio Tones Amplitudes" box in AirMail's Message Index Window, under Tools/Options/Connections.
If your radio ever displays a continuous over-temperature warning or shuts down due to over-temperature, check your antenna connections and tuner, and if there is no problem there, reduce the drive level still further.
Read the instructions that came with your KAM+ to work out how to open the KAM and find the adjustment. There is both a jumper (K9), and a trimpot (R-28) that you can adjust. You will want the jumper to be in the lower output position (i.e. on one post only, which is the factory default).
Under AirMail, in the Terminal Window, under the Control Tab, select the "Xmit Unproto" function. This will cause the radio to continuously transmit in the KAM's only mode, FSK. Adjust the trimpot in the KAM+ until the radio is drawing the appropriate current (see above) from your boat's 12 vdc system. Press stop (the stop sign) to quit.
For voice rated radios, during subsequent operation of your radio, on various SailMail frequencies, watch to make sure that the radio does not draw more than 12 amps from your boat's 12 vdc system. If it does you can further adjust the trimpot. If your radio ever displays a continuous over-temperature warning or shuts down due to over-temperature, check your antenna connections and tuner, and if there is no problem there, reduce the drive level still further.
Please send any suggestions or clarifications for this Application Note to Stan Honey, sysop@sailmail.com