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Doppler shifts |
| Guessing where your partner station can be found, coming off the moon is not always easy. Below is a description extracted from the 432MHz-UP NL, published in March 2000. |
| Here are some practical examples to help you out. |
| Sked frequency = 432.020 MHz = 432020 kHz.
Your echo doppler as indicated in various programs: + 0.800 kHz and his as -0.471khz. If both of you are transmitting exactly on the sked frequency, you can hear your own echo's +0.800 kHz, or on 432020.800 kHz. You will hear his echo's on 432020 + (-0.471 / 2) + (+0.800 / 2) = 432020.165 kHz.. the / 2 operations are because his signal experiences half of his doppler on the way to the moon and half your doppler on the way back. He will hear his own echo's on 432020 + (-0.471) = 432.019.529 kHz, and he will hear your echo's on 432020 + (+0.800 / 2) + (-0.471 / 2) = 432020.165. Note that both your receivers will be tuned to the same frequency to hear the other guys echo's (so long as you are both transmitting on the same frequency, the sked frequency). |
| The situation gets a little trickier if you are running random, that is, without a sked. Lets say you are calling CQ on 432.020 MHz = 432020 kHz. You will hear your own echo on 432020.080 kHz (see above). He will hear your echo's on 432020.165. If he replies to you by transmitting on the frequency where he hears your echo's, you will hear him on 432020.165 +(-0.471 / 2) + (+0.800 / 2) = 432020.330 kHz. |
| However, if he can hear his own echo's and
adjusts his transmit frequency so that he hears his echo's on top of your echo, what
frequency will you hear on? Keeping the above example, in which you are calling CQ on
432020 kHz, he will hear your echo's on 432020.165 kHz. In order for him to hear his signal on that frequency, he will have to transmit on 432020.165 + 0.471 = 432020.636 kHz. If he transmits there, you will hear his echo's on 432020.636 + (-0.471/2) + (+0.800/2) = 432020.080. In other words, you will hear his echo's on that same frequency where you hear your own echo's. What a deal! |
| That's all from the newsletter.
(Thanks to Yoshiro JA4BLC for this information) |
| To help you out in an easy way, download this little excel file and change indicated doppler's accordingly to find your -off the moon- signals: Dopplershift |