Doppler 101 for 1296 MHz EME (...and other bands)


If you just want the answer, here it is:
Assuming you are using "Listening on Echo" mode:
Calling CQ (on the "Sked" frequency) and listening on your own echo (the normal mode on 1296 EME):
  • TX on Sked
  • Rx on Sked plus self Doppler

Answering station who announced they are calling CQ and listening on Echo:
  • Tx on Sked + (Self Doppler - Dx Doppler)
  • Rx on Sked + Dx Doppler

Reply to a station you hear on "Rx Frequency" (and you assume is listening on their own echo frequency)
  • Tx on "Rx frequency" - self Doppler
  • Rx on "Rx frequency"

Caveat before we start. All the numbers here assume that both you and the Dx station you are trying to work are operating on the frequency you think you are. If you (or the Dx station) don't have accurate frequency calibration, all bets are off! If you say you are calling CQ on 1296.070 but you are on 1296.068, your signal will be outside the audio passband of anyone looking for your echo with WSJT(x)

EME operation involves bouncing a signal off the moon. Since the moon is always moving with respect the any point on earth, the signal reflected back from the moon is shifted in frequency. This is Doppler shift (see https://en.wikipedia.org/wiki/Doppler_effect if you want to know more about the Doppler effect). The amount of frequency shift you see depends on where on the earth you are located and the velocity of the moon towards you or away from you relative to that location, as well as the frequency in use. The amount of shift you see on your own signal will be different from the amount seen by a DX station in a different location. This can (and does) lead to some confusion about where you should transmit and where you should listen when working a DX station.

On 50MHz the shift is small, maybe up to ~120Hz, on 1296Mhz the shift is significant, maybe up to ~3000Hz and on 10Ghz the shift is large, maybe up to ~24000Hz. WSJT and WSJT-X operate with a waterfall which is the width of the receiver audio passband. Typically this might be from 200Hz to 2200Hz. If you are expecting a signal to show up at an audio frequency of 1200Hz, it doesn't really matter on 50MHz if it's shifted somewhere between 0 and 120Hz. You'll still see it on the waterfall. On 144MHz the shift can be from 0Hz to around 340Hz. Again you'll still see it on the waterfall even with maximum Doppler shift. On 432 the Doppler shift can be up to 1000Hz, so there's a good chance part of the signal you are looking for will be out the the Rx passband if you don't compensate for Doppler.

However on 1296MHz the shift can be up to 3000Hz, which will move it right out of the audio passband of the receiver and way off the end of the waterfall disply, so you'll never even see it, much less decode it. On 10Ghz the signal can be shifted by 24KHz, so you certainly won't see it!

So the bottom line here is that unless you are operating on 50 or 144MHz, you need to compensate for Doppler shift.

How do you compensate for Doppler shift?

By far the easiest way to do this is to use WSJT-X and allow it to set the Tx and Rx frequencies via CAT control of your radio's VFO(s). If you have a radio that was made in the last 20 years or so, it's probably capable of setting the frequency via a serial CAT connection. See http://physics.princeton.edu/pulsar/k1jt/wsjtx-doc/wsjtx-main-2.0.0.html#RADIO

However, if you have a radio that won't accept CAT control, or you can't figure out how to use CAT control, then you are going to have to get Tx and Rx frequencies yourself. You can do this several ways. If you have two VFOs and split control, you can set one to the Tx frequency and one to the Rx frequency. If you only have one VFO you can use RIT to offset the Rx frequency from the Tx frequency.

On 1296 you can get away with manual control because the change is Doppler shifted frequency during a 60 second (actually 48 second Tx/Rx) period will be small, less than 10Hz. On the higher bands you'd have a difficult time tracking Doppler shift manually when it can change by up to 80Hz (on 10Ghz) during a singe Tx or Rx period. For the WSJT or WSJT-x decoder to work properly you really can't let the received signal drift by more than about 10Hz. Under some conditions (low libration and narrow submode) it can be advantageous to be able to track the signal to within 1Hz.

Thinking about Doppler - SFOM (Same Frequency On Moon)

It can sometimes be difficult to get a mental picture of what's happening with Doppler and how to correct for it. Perhaps the best way to look at things is to think about what a station located on the moon would see. There is a Doppler mode called CFOM (Constant Frequency On Moon) and I explain this in some detail on another page (see CFOM). The most common Doppler mode on 1296 (and some other bands) is one in which stations listen on their own echo. This mode could also be called "SFOM" or Same Frequency On moon. Here's why and how it works:

Let's say you are working a DX station and you are "transmitting on 1296.070 and listening on your own echo". You know from WSJT(x) that your local Doppler shift is 1200Hz. That means you will hear your echo on 1296.070.000 MHz + 1200Hz = 1297.071.200 MHz. But what does a local Doppler of 1200Hz mean? Well, half the Doppler shift occurs as the signal travels to the moon and half happens on the way back from the moon. Your one way Doppler shift is 600Hz. That means that a station on the moon would hear you at 1296.070.600 MHz and since the signal reflects off the moon, the signal "transmitted" by the moon is on the same frequency, 1296.070.600 MHz.

Now let's say the self Doppler of the DX station is 2000Hz, so that the one way Doppler is 1000Hz. The DX station has to transmit on a frequency so that their echo arrives at your station on 1296.071.200 MHz since that's where you will be listening (see previous paragraph). You know (again from the previous paragraph) that for this to happen, the signal "transmitted" (reflected) by the moon has to be at 1296.070.600 MHz. Since the DX station's one way Doppler is 1000Hz, in order for their signal be be "heard" and "retransmitted" from the moon's surface at 1296.070.600 they would have to transmit 1000Hz lower in frequency, i.e. 1296.069.600 MHz.

Note that the DX (or mutual) Doppler is given by (your Self Doppler + DX self Doppler)/2, or in this case (1200+2000)/2 = 1600Hz. The normal formula for where to transmit back to a station listening on their own echo is the calling frequency (1296.070.000) + (Self Doppler - Dx (mutual) Doppler)[1200-1600] = 1296.069.069.600 Mhz, the same number we derived in the previous paragraph.

The point to take away here is that both you and the DX station are setting your Tx frequency so as to keep a common (but not constant) frequency on the moon. It's not a constant frequency (that would be CFOM), but at any given time it's the same frequency for both stations. It changes with time because the calling station's frequency is fixed but their Doppler changes with time. Because both stations put same frequency on the moon, any other station listening to the QSO will see both stations on the same frequency (but that frequency will be different for all three stations, depending on their individual self Doppler shift)).

Some Numbers

Both WSJT and WSJT-x have an "astronomical window" which shows you a lot of data, including the two parameters you may need to calculate the frequency you should transmit and receive on when making and EME contact with a DX station. These windows are shown below, WSJT on the left and WSJT-x on the right:

The two numbers you are looking for are "Self Doppler" and "DX Doppler". The two programs show very slightly different numbers. The WSJT-x numbers are the more accurate of the two. So we have:

  • Self Doppler = -2487 Hz
  • DX Doppler = - 2302 Hz

When using JT65(A, B or C), the "pilot (sync) tone" is at 1270 Hz, so in all cases the following should result in the receiving station seeing a waterfall trace at (or close to) 1270Hz. Let's say you want to call CQ and you are seeing the data in the WSJT-x window above. On 1296 Mhz the default mode for CQ announcements on the HB9Q logger is "Calling CQ on .070, listening on my echo". That means that your Tx frequency is 1296.070 MHz and you will be listening on your own echo frequency, which is given by your Tx frequency (1296.070) + your self Doppler at the time (which is this case is -2487), so:

Tx on 1296.070.000 MHz; Rx on (1296.070MHz - 2497Hz) = 1296.067.503

But suppose you are not the one transmitting. You want to reply to that CQ. Where do you transmit and where do you listen? The DX station announces they are calling CQ on 1296.070 and they are listening on their echo frequency. Where do you tune? First you enter the callsign (and locator if it's not already in the database) in WSJT(x), or you double click on the decoded CQ message and that is done for you. The calculated values for Dx Doppler in the Astronomical window will then be correct for this QSO.

You Rx on the sked frequency [in this case 1296.070] plus the Dx Doppler value shown on the window. That means you Rx on 1296.070MHz - 2302Hz which is 1296.067698MHz. The Tx frequency would be the sked frequency [again 1296.070] plus (self Doppler - Dx Doppler). This is 1296.070000 + (-2487 minus -2302) = 1296.069815 MHz. Note the signs here. Adding a negative number means subtracting its absolute value. Subtracting a negative number means adding its absolute value.

A third possibility is that while tuning around you hear a station calling CQ on a particular frequency (call it the "Rx frequency" and it appears as a trace on the waterfall at 1270Hz (the standard for JT65 mode). You assume that they are listening for replies on their own echo frequency. Where do you transmit? The answer is simple, it's just the "Rx frequency" frequency minus your self doppler. So if you hear the station at 1296.065, you transmit on 1296.065 Mhz - 2487 Hz = 1296.067.486 MHz

Summary


Calling CQ (on the "sked" frequency) and listening on your own echo (the normal mode on 1296 EME):
  • TX on Sked
  • Rx on Sked plus self Doppler value from Astronomical window

Answering station who announced they are calling CQ (on "sked" frequency):
  • Tx on Sked + (Self Doppler - Dx Doppler)
  • Rx on Sked + Dx Doppler

Reply to a station you hear on an "Rx Frequency" (and you assume is listening on their own echo frequency)
  • Tx on "Rx frequency" - self doppler
  • Rx on "Rx frequency"

If you want to do this faster and without pain you can download and run my EME Doppler Calculator. It reads the values of self Doppler and Dx Doppler from the AZEL.DAT file which WSJT(x) created and does the calculation for you. The "Own Echo" mode is for when you call CQ (or another station) and listen on your own echo. The "On Dx Echo" mode is when someone is calling you (or CQ), they are using "On Echo" mode and you want to reply. The "Call Dx" mode is when you reply to a station you hear calling CQ while tuning around (send you see their waterfall trace at 1270Hz if they are using JT65)