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QRP EME on 23cm
Why 1296?
On 1296MHz, terrestrial noise levels are usually low, it's not difficult to build a preamp (LNA) with a noise figure of 0.25dB, dish antennas are reasonably easy to find (old TVRO dishes, surplus commercial dishes, direct broadcast TV dishes) and not all that hard to build since the 23cm wavelength is long enough that ultra high precision is not required (1/10 wave accuracyy would be about 1"). Power is relatively easy to come by and not that expensive at the sub 100W level.
So how small and low power can a system be and still work most of the stations on the band. I'm not talking about working the handful of stations with dishes 10m or more in diameter, I'm talking about working the fairly well engineered stations with 2m to 3m dishes. The calculations are easy enough to make a theoretical estimate.
There are 4 Q65 modes currently available for operation on 23cm; Q65-30B, Q65-60C, Q65-120D and Q65-300E. Since most activity uses Q65-60C let's look at the sensitivity that mode will give you. From my simulation testing, in AP (A Priori) operation (i.e. then the DX station's call and grid are known and entered into WSJT-X), you should get reliable (close to 100%) single period decoding fot a signal with 15Hz spreading at an signal to noise ratio of about -26dB (in a 2500Hz bandwidth). According to Joe, K1JT, for a 50% decode probability the numbers should be -27.6 dB for non-AP decoding and -30.2dB for AP decoding (Q65 quick start guide, but these numbers are for signals without spreading (i.e. terrestrial). To be safe, lets look at what you need to get a signal with an SNR of -25dB. All else being equal, that should be a signal strength which is enough for a reasonable probability of decoding.
EMECalc is your friend when it comes to calculating signal strength, though it's a pretty critical friend in that it assumes you are doing everything right. It assumes your dish has the right profile to give you full gain, you know your noise figure, you have no terrestrial QRM, your dish is pointed accurately at the moon etc.
Lets look at three dish sizes, 2m, 2.4m and 3m. These are small to moderate sized dishes on 23cm. Let's assume a system noise figure of 0.3dB. This is pretty reasonable assuming you don't have much loss ahead of the preamp )connector and relay losses). I'll also assume a mesh dish using mesh with 10mm square holes and using 1mm diameter wire. Solid dishes will perform slightly better. Moon distance is near apogee, i.e. maximum DGRD loss.
| Dish #1 Self |
Dish #2 DX |
Power W |
SNR |
|---|---|---|---|
| 2.4m | 2.4m | 25 | -22.7 |
| 2.4m | 3m | 25 | -20.8 |
| 3m | 3m | 25 | -18.9 |
| 3m | 3m | 10 | -22.9 |
| 3m | 2m | 10 | -26.4 |
| 3m | 2.4m | 10 | -24.8 |
| 2.4m | 2.4m | 10 | -26.7 |
As you can see, in all cases, with a power of 10W, these contacts are possible. The worst case is a 2m dish to a 3m dish or 2.4m dish to 2.4m dish at a power of just 10W at the feed. The theoretical SNR is less than the chosen threshold of -25dB, but is still above the theoretical decode limit. How good is the theoretical limit? Well Using 10W at the feed of my 3.1m dish (Septum feed with choke and flare) I made contacts with Bill, KB2SA, who was using a 1.9m dish) Septum feed with choke and flare, using both Q65-60C AND Q65-30B. We also made a QSO with me just running 5W at the feed and using Q65-120D. Reports SNR was in the -26/-27dB range. Now Bill has a very good, very well optimized system, and my system isn't bad, so this is a contact between two stations who are "well sorted", but it does show what you can do. Further, DGRD was around -2dB at the time of the contacts, so conditions were not optimal. Of course if you have a dish that's not well profiles, a feed that's less than optimally matched to the dish, a noisy preamp or lossy relay and you don't accurately track the moon, your received SNR will be lower and decoding will be more difficult. I've also made contacts using 10W or less with larger stations of course.
If you use Q65-120D you gain (theoretically) something like 2-3dB sensitivity over Q65-60C, and if you then go to 300E you may gain another 2-3dB , though 300E qSOs can take 25-30 minutes, so are unlikely to become very popular! When spreading is at a minimum (as low as 5Hz on 1296) the use of 60B rather than 60C (or 120C rather than 120D) will allow decodes at slightly lower SNRs. For example, with a spreading of 5Hz, 60A may provide about 1dB more sensitivity than 60C
QRP Power
What go you need to generate 20W or so at 1296? Well, most transverters put out a least a hundred mW, so you need an amplifier with about 23dB gain. A single Mitsubishi RA18H1213G amplifier module (specs) can put out 20W (maybe more)for a hundred mW of drive at 1296. The official rating is ">18W" out at 12.5v, but by all accounts the can be run at a slightly higher voltage, and with correct drive and bias setting can deliver more power then the sec sheet numbers indicates. See for example https://www.oz1bxm.dk/1296/PA-Mitsubishi-RA18H1213G.html and https://www.w6pql.com/1296/index.htm. However anytime you exceed the manufacturer's recommendations, you do so at your own risk. A good heatsink is required. You can probably build an amp lifer using a RA18H1213G for something like #100-$150. Circuit boards are available, so it's just a matter of putting a few components in a box with a heatsink. See https://www.w6pql.com/1296/index.htm for some construction tips
Bottom line
With a decent antenna (e.g. 2.4m dish) and a properly setup station, you can work a lot of similar stations with only 25W at the feed, and in fact 10W may often be enough power for a Q65-60C contact on 23cm. This suggests that EME operation is quite possible from Austria, which currently has a 10W power limit on 23cm. A 3m to 3m dish digital contact using Q65-60C should not be too difficult at that power level.