It's all about the Software

jdow wrote: Fri Nov 20, 2020 12:16 pm E^2 for 2 kW at 50 ohms is 100,000V^2 (316 volts RMS). E^2/R is 5.5555W for each of the 18k resistors. You are at the "official" edge at 727 watts. If course, that poor thing WILL get HOT if it is "RF small".

{o.o}

Your first equation is correct in that it takes 316 volts to produce 2KW at 50 ohms. That's the only relevant number we need to concern ourselves with going forward.

Your next equation is where the escape occurs. 316 volts into an 18k load produces 5.55 watts of total heat, not 5.55 watts at each resistor. Per my earlier post I have 9 resistors in a 3x3 parallel-series arrangement, so each resistor is sharing an equal part of the load. 5.55 divided by 9 is .616, so each resistor is dealing with .616 watts of heat load at 2KW. My little AL-80B is only capable of half that power so each resistor is only subject to .308 watts max heat load. Those are all large 2 watt resistors, so the hardest my amp is able to push them is effectively less than 1/6 of their rating.

Regardless, as a sanity check after fab I subjected the coupler to several hours of full-power operation using various transmission modes (SSB, AM, digital) on multiple bands, and never once did the resistor pack feel anything other than stone cold to my touch.

That coupler is way overbuilt.

OK, that series parallel part is what I missed. They are going to get hot. Key down CW, xFM, or a digital mode that is constant envelope are the acid tests. All the others average well below maximum. Nonetheless each individual resistor is getting about 0.6 watt, it seems. (Quickly divides 5.5 by ten and adds a skosh and it agrees with your number.)

{^_^}

Hi Simon and Colleagues,

Now that quite a few SDRC users have HL2, Red Pitaya, TRX-Duo and a variety of Anan radios, I wonder if you may consider developing pre-distortion within SDRC? I believe that it would be a fantastic feature, though I do not know the architecture of your software or how difficult the implementation may be.

I see that the ICOM world (IC7610) will have pre-distortion from later this year - a long time after Anan/PowerSDR/Thetis, but good news nonetheless. Presumably, they will build the feature into their hardware/firmware going forward, and I guess this would spur interest in the amateur community.

Thank you again for the great software,

Regards,
Charles
VK1CM