Q65 - Choice of mode for 1296 EME

A Q65 mode is characterized by two parameters, transmission length (15, 30, 60 120 or 300s) and submode (A, B, C, D, E, F, G, H). Not all modes and submode combinations are allowed and the 15s mode isn't enables for EME. A mode/submode combination is characterized by the two parameters. The transmission length which depends on the duration of the individual tones. The frequency spacing between the tones is a function of T/R period and chosen submode. The following table shows the relationship between T/R period length and tone spacing:

The important thing to notice here is that the submode letter isn't an indicator of tone spacing. For example (above cells in orange). to get a constant tone spacing of 6.67Hz you need submode B for the 30s mode, but submode C for the 60s mode and submode D for the 120s mode. The tone spacing which is required for best performance depends on the Doppler Spreading of the echo. On 1296 this is typically around 20Hz, but can range from a few Hz to more than 30Hz. The tone spacing required is a function of the spreading width. Maybe 20 or 25% of the spreading width is good. That means that for 1296 EME, a safe choice would be 6.67Hz, which in turn would be Q65-30B, Q65-60C or Q65-120D. For 300 seconds Q65-300E is probably good. Using 30C or 60D would never be the best choice for 1296 MHz (though they would work), but at times could be a good choice on higher frequency bands (e.g. 10GHz), where Doppler spreading is wider. The consequence of choosing the wrong submode would be a (usually small) reduction in best possible decode sensitivity.

Now in a little more detail:

The choise of which mode/submode to use for EME on 1296 depends on a number of factors These include:

• Doppler Spreading. On 1296 this can typically range from about 5Hz to about 30Hz. Slightly lower and slightly higher values are possible, but rarely occur. The range also depends on the locations of the stations involved. A typical value is around 20Hz. For more info see https://physics.princeton.edu//pulsar/k1jt/EME2010_K1JT.pdf and Lunar Libration and Doppler Spreading
• Rate of change of Doppler shift. This will typically range from around 4Hz/min to around 13Hz/min under extreme conditions, but is more typically in the 6-9 Hz/min range. If you are tracking perfectly, this doesn't matter. If you are tracking in 10Hz steps, or not tracking at all, it can be a factor.
• Frequency stability and tracking accuracy. Highly stable signals which track Doppler shift in 1Hz steps may have a different optimum submode from less stable signals tracking in 10Hz steps.

Which modes?

After extensive simulation testing of modes, submodes and tracking step sizes (1Hz or 10Hz), plus some on air testing of modes and tracking, I would make the following recommendations for the "standard" mode choice on 1296 EME, i.e. the choice which will give best results for a typical station under average conditions. Stations with GPS locked radios and 1 Hz tracking could probably operate most of the time at one step narrower submodes and gain a slight sensitivity advantage, but we are only talking about fractions of a dB most of the time. The B/C/D/E submodes allow a little extra room for unusually wide spreading, 10Hz steps and somewhat less than GPS locked frequency stability.

1. Q65-30B
2. Q65-60C
3. Q65-120D
4. Q65-300E

The B/C/D modes share a common tone spacing of 6.667 Hz. 300E would have a tone spacing of 4.688Hz. Having made 300s mode contacts, I'm guessing it will not be used very often! Though it works very well for the ultimate weak signal work, it is tediously slow, especially if any any repeats or averaging which need to be done. A QSO will take at least 25 minutes if everything goes well and only one station sends 73. That can easily extend to 45 minutes or mode with averaging and repeats. Not all PAs may be happy with 5 minute key down operation either.

If both stations are tracking with 1Hz steps (or Full Doppler to DX Grid is used by one station with 1Hz steps), and/or Doppler spreading is 10Hz or less, then a small reduction of the decode threshold can be gained by the use of a a lower submode with smaller tone spacing, which would be 30A, 60b and 120C. The typical gain in decode sensitivity would be expected to be around 0.5dB. The use of these submodes during times of wider (>20Hz) spreading or with 10Hz steps and/or lower stability transmissions might be of the order of a 0.5dB sensitivity loss. With a stable system using 1Hz steps using the optimum mode for a given spreading, going from C to B or B to A submodes will result in a sensitivity gain of about 1dB. To the use of, for example, 60A when at a libration minimum of 3Hz spreading, might produce a sensitivity gain of 2dB over the optimum submode (60C) if spreading was >30Hz.

Here's just one example of a small data set on a small selection of modes at two different Doppler spread values. The numbers are % of "MyCall DxCall DxGrid" messages which decode (Q3) at two values of Doppler spreading for the 60s Q65 submodes.

mode - spread	-25	-26	-27
60b - 33hz	98%	0%	0%
60c - 33hz	98%	6%	0%
60d - 33hz	98%	10%	0%
60a - 5hz	100%	100%	76%
60b - 5hz	100%	100%	56%
60c - 5hz	100%	76%	18%

At the widest spreading there's really not much difference between the submodes. Differences of a few % aren't statistically significant. At very narrow spreading you can see that A, B and C submodes all produce better decoding, probably by about 1.5dB. 60C is the least sensitive mode, but A and B are quite similar. These are all based on "perfect" signals, i.e. perfect tracking of a very stable signal. Tracking in 10Hz steps and/or frequency instability will move the results in the favor of the slightly wider submodes. After taking everything into account, for an average station and average spreading conditions, 60C would probably be the safest compromise. If everyone was GPS locked and used 1Hz steps, 60B would probably be slightly better, but only by a fraction of a dB.

Q65 does show higher decode sensitivity then JT65 when both are using AP assisted decoding. Given the capabilities of a typical station on 1296 and the average signal strengths that I see with a 3m dish, Q65-60C offers a small, but real, advantage over Jt65C. However the Q65-30(B) mode is almost as sensitive and offers the perhaps more significant advantage of reducing QSO time from 5 or 6 minutes (depending on how many 73 messages are exchanged) to 2.5 to 3 minutes. This could be very useful in contest operation and contests will probably use whatever the "standard" mode is on the band to minimize confusion.

I would suggest that the Q65-30B mode might be a reasonable choice for a "default" mode between average stations on 1296. A 30s mode might also help the average user in identification of an unknown signal. If it's a 30s transmission cycle it can't be JT65. It must be Q65.

More knowledgeable users who are looking for weak signals can then select a mode and submode which is optimal based on the capabilities of the stations involved and the current values of Doppler spreading and Doppler rate of change.

Bob - KA1GT [last edit 1/9/21]