10 July, 2021

Coordinated WSPR band hopping with the QRPLabs U3S

There is a recommended sequence for multi-band transmission on WSPR. It is given in the K1JT WSPR documentation. If enough receivers follow this sequence, then presumably letting WSPR transmissions also follow it, will increase the chance of detection. 

The recommended band hopping sequence cycles through 10 bands over a 20 minute period according to this table:

Band (m)   160  80  60  40  30  20  17  15  12  10
UTC Minute  00  02  04  06  08  10  12  14  16  18
            20  22  24  26  28  30  32  34  36  38
            40  42  44  46  48  50  52  54  56  58

The multi-band U3S is capable of adhering to such a sequence. There are however three considerations that need to be taken into account. 

First, for frequency stability reasons, it has been recommended to start with transmission at high frequencies and move to lower frequencies, i.e. the opposite of the recommended band hopping sequence. See recommendation #5d in QRPLabs Application Note AN001. My experience with a U3S with a 25 MHz TCXO driving the Si5351A is that it makes no difference for stability if the order is reversed according to the table above. The same applies to my other U3S which has the standard crystal with the Si5351A, but enclosed inside the QRPLabs OCXO box. Despite the oven being unpowered, just the thermal shielding seems to be enough to ensure stability no matter which order the frequencies come in.

Second, the U3S needs time for GPS calibration of the frequency from time to time. Typically the calibration interval is 60 seconds. There is no time in the above sequence for that. A calibration interval can however be squeezed in in these ways:

  • Skipping bands. By skipping some bands, initially or at the end. I typically transmit on the higher bands, from 17 m and up, in order to detect when there are openings. Then there is time from 00 to 12 for calibration every 20 minute. The 'frame start' in that case is 12. 
  • Longer frame. If transmission on all 10 bands is desired, one can transmit every 40 or 60 minutes instead of every 20 minutes, leaving a 20 or 40 minute break. This is done by transmitting with a 'frame length' of 40 or 60 rather than 20 minutes, and with a 'frame start' of either 00, 20, or 40.
Third, if one should desire to transmit on, say, the 17 m band, and then on the 12 m band, there isn't a direct way to put a 2 minute break between them. There will be a new transmission in the 12 m band immediately following the 17 m transmission, thus losing the timing of the band-hopping sequence. There are however some work-arounds: 
  • RX mode. If the mode is changed from WSPR to RX for a particular frequency, a 2 minute receive cycle is inserted between transmissions. In the example above, this would be the 15 m band. In this way a 2 minute break will be inserted between the 17 m and the 12 m transmissions. This requires firmware 3.11 and up (U3S only, not U3). A receive module is not required.
  • Aux bit. There is an indirect way if you have two antenna outputs as I have on my U3S. Assume a dummy load is connected to the second antenna output. By setting up a 17 m transmission for antenna 1, then a transmission on any band to antenna 2 into the dummy load, and then finally transmission on the 12 m band on antenna 1, a two minute break without transmissions can be achieved. The antenna is selected using the Aux configuration parameter documented in QRPLabs AN003. This feature is present even for the U3.
Some of the ideas here come from a discussion of band-hopping on the Groups.io list with contributions from G4APB. Taking the three considerations listed here into account, the U3S will be able follow the recommended WSPR band hopping sequence.

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