Updated 10/11/2010
Some folks get a bit confused about how to set their radio dials for proper digital operations. Navy MARS stations are given an Assigned frequency and must figure out their dial frequency. For some services such as Winlink, you might wonder why the Airmail software or RMS Express sets your dial to a frequency that is different from the frequency listed by the RMS.
When you are using digital modes, more specifially Frequency Shift Keying modes such as AMTOR or PACTOR, if you don't have the transmitting station's signal properly centered in your modem filters, you won't copy much of anything.
This Dial Frequency Calculator is intended to assist you in calculating your dial setting when you know your assigned frequency and some information about your particular modem.
This Calculator assumes that you are feeding your transmit audio to the mike input or aux input on your transmitter, and getting your receive audio from the normal audio output jacks (speaker or headphone). This is called Audio Frequency Shift Keying (AFSK). If you are using a transceiver with built-in FSK, then this calculator won't tell you where your dial should be (Hint: use the assigned frequency). The Calculator will work with software/sound card modems.
In order to use the Dial Frequency Calculator, you must know several pieces of information.
In the form below, enter the Assigned Frequency. If your modem documentation lists the Center Frequency of the modem, enter it in the next block and click "Compute Dial Frequency". Your correct dial setting for both USB and LSB will be displayed.
If your modem documentation gives you the frequencies of your Mark and Space filters, enter them instead of the Center Frequency, and the program will calculate the Center Frequency, the Shift, and the Dial Frequency.
For example, Suppose your Assigned Frequency is 4040.000 kHz. If Mark = 2125 Hz and Space = 2295 Hz, enter those values in the appropriate boxes. On clicking the button, the program will show your center frequency (2210 Hz), shift (170 Hz), and USB and LSB dial frequencies (4037.79 and 4042.21 kHz, respectively).
This also works for dial setting when you are using Voice (Sideband) modes. You don't use a modem, but your Center Frequency is the standard offset of your BFO. Enter your Assigned Frequency, say 4040.000 kHz, and your Center Frequency of 1500 Hz. Hitting the button should give you 4038.5 kHz for your USB dial setting and 4041.5 kHz for your LSB setting.
The last calculation above also shows that if your hardware or software modem permits you to tune your filters, you could set your filter frequencies so that the center frequency was 1500 Hz or whatever offset you normally use for Sideband voice. Because the dial frequency would be the same as for sideband voice, you would never have to change your dial setting when changing from sideband to digital modes and back.
To continue with the example above, first click the Reset button and then enter an Assigned Frequency of 4040.00 kHz. If you are using a 170 Hz shift FSK mode such as Amtor, enter a Mark Frequency of 1415 Hz and a Space Frequency of 1585 Hz into the calculator. Then hit the Compute button. The USB dial frequency will be exactly the same as for SSB. Try it. It really works!