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Messages - Charlie_Dont_Surf

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616

The RF Workbench / Re: Hellschreiber beacon - with some oomph!

« on: February 20, 2022, 0215 UTC »

Quote from: Radiotech on February 19, 2022, 1359 UTC

It turns out that the inductor(8T FT50-43) between drain and VCC impacts the power and efficiency quite a lot! The monofilar had the highest efficiency at 61%, but the trifillar had the highest output in RF-power.

It does, but only if it is too small.

If it is large enough that XL is sufficiently high then any inductance beyond that has little effect. What is "sufficiently high" you ask? I think that Stretchy's Rule Of Thumb (ROT) of 5x the drain impedance makes sense to me, but I've learned after doing enough of this that at 43 meters for low and moderate power transistors that 2 uH is the minimum sufficient value to avoid any possible issues and get well beyond the range of values where there is a strong effect of L upon the output. I came to this conclusion via simulation and experimentation with real circuits. It is a very clear to see in simulation.

617

The RF Workbench / Re: Hellschreiber beacon - with some oomph!

« on: February 17, 2022, 1809 UTC »

The efficiency is low (if the circuit is actually biased for Class C, I would expect something more like 50-60%) and I have to say that some of that is probably coming from the construction technique. Please don't be offended by this; you've constructed it in a very clean manner. Nice work. It's just that at 7 MHz, there's going to be a fair amount of loss going through all those through-hole components with relatively long leads. A PCB would allow for a more compact circuit with shorter connections between components. Going to SMD would be even better.

618

The RF Workbench / Re: Hellschreiber beacon - with some oomph!

« on: February 17, 2022, 1734 UTC »

For the original circuit, with an IRF510 and 7V gate drive, you could modify the existing circuit in the paper to get a bit more out of it. Since you said you got nothing before, maybe you can get something now.

Change the 139 pF to 124 pF (the exact value matters quite a lot here - a few pF either way makes a large difference.)
Change the 652 pF to 740 pF.
The RFC must be 1 uH or more, I would use 2 uH.

I continued with the 4.81 uH for the above simply because modifying it is more of a pain than changing a capacitor. If we pick better inductor and capacitor combination, the other two capacitors may have to change to suit.

619

The RF Workbench / Re: Hellschreiber beacon - with some oomph!

« on: February 17, 2022, 1643 UTC »

Quote from: redhat on February 17, 2022, 0727 UTC

The design process is as simple as tuning the tank to resonance and going about your day. In theory you could precisely calculate the tank values taking into account the Cds and so forth, but I found it much easier to spitball it.

OK, I'll take your word for it.

For what it is worth, I haven't built any yet but I have done some simulations with a few different power transistors in CMCD configurations for 43 meters and the optimized results have ended up with very different tank circuits; the same network (parallel RLC between the drains) but with very different component values depending upon the transistor.

Now, to be clear, my design process in the simulator is to do a bunch of sweeps of the RLC values to pick values that basically work then I put the optimizer to work to peak up the output power and efficiency, minimize power dissipation, etc. From there I usually end up tweaking it for one reason or another. The reason I mention this is because the optimizer can spend minutes trying to squeeze out every last milliwatt and that can move the design to a very different place. (I'm not watching everything that goes on at this stage - I'm usually asleep or doing something else while it does the drudgery for me.)

The end effect of this is that if you don't care about the difference between 120 and 125 Watts (picking numbers out of the air) then, yeah, it's probably fine to just swag at it. For better or worse and potentially overdoing it, the optimizer sweats the small details for me and that may be why I end up with very different tank circuits for different transistors at the same frequency.

What I do find interesting about CMCD is it doesn't have the high voltage peak on the drain at resonance like Class E; it's a current-operated mode (duh) and that seemingly permits the use of a lot of dirt cheap power MOSFETs with 100 V or lower BVdss that are plentiful with power inverters, switching supplies and motor control everywhere now.

620

The RF Workbench / Re: Hellschreiber beacon - with some oomph!

« on: February 17, 2022, 0303 UTC »

Quote from: Stretchyman on February 12, 2022, 1638 UTC

The ripples on your O/P waveform are due to the lengthy earth lead of your 'scope probe.

Yes, certainly good practice but also I was going to say that attention has to be paid to stray inductance in the circuit in general. It's hard to say where the problems lies since he's provided 'scope images of what appears to be different locations in the circuit and I'm not sure which is which.

621

The RF Workbench / Re: Hellschreiber beacon - with some oomph!

« on: February 17, 2022, 0253 UTC »

Quote from: Radiotech on February 15, 2022, 1347 UTC

Also tried this circuit from https://www.researchgate.net/publication/320623200_Notes_on_designing_Class-E_RF_power_amplifiers But i got no usable power from it.

My biggest problem with this paper and this circuit is that to specify a Class-E output network as he has without specifying a transistor is somewhat misleading; it fools the user into thinking that they can plug any old transistor (even a MOSFET) into it and expect it to work. Maybe this would work at 50 KHz with old power MOSFET transistors (which tend to have humongous output capacitance that swamps any other performance differences between models of transistors) but not at 6.9 MHz (as in the paper). The reality is more complicated and the component values of a Class-E network absolutely have to be tuned and optimized for each transistor at each frequency.

Also, it is humorous to me that this author does mention that he performed SPICE simulation but again without specifying what transistor was used and without providing any actual measured results to go with it. Had he substituted another transistor into his SPICE model, he would have seen exactly what I wrote above - the matching network has to be tuned and optimized for a particular model of transistor, which would make it obvious that specifying a network needs to be accompanied by naming the actual transistor used.

This paper is one of these garbage undergrad papers that people put out to pad their resume/CV; there's nothing new in there and it's lacking aspects that would make it useful to others. He references Sokol's book on Class E but then goes through a bunch of derivations that Sokol already did in his QEX article from 6 years before the book. I don't see the point.

I'm sorry that you wasted your time expecting this kid's navel-gazing project to work. That's a few hours of your life that you won't get back.

622

North American Shortwave Pirate / Re: Outhouse Radio 6930 USB 0010 UTC 12 February 2022

« on: February 12, 2022, 0054 UTC »

623

North American Shortwave Pirate / Re: Radio Pushka 6920 AM 0715 UTC 10 FEB 2022

« on: February 12, 2022, 0015 UTC »

Quote from: RadioPushka on February 11, 2022, 0238 UTC

The 1/4 vertical has really helped over loaded antennas and antennas with top hats.
time to build a linear amplifier though. 15 watts is doing good.

In terms of transmitting efficiency: a full 1/4 vertical is incrementally better than a shortened vertical with a top hat (capacitive hat) and in turn both of those two are incrementally better than a shortened coil-loaded vertical.

Put another way: 1/4 wave > top-hat vertical > coil-loaded vertical, all other things being equal, like the ground system, etc.

Source: ON4UN Low-band DXing.

624

North American Shortwave Pirate / Re: Radio Pushka 6920 AM 0058 UTC 11 FEB 2022

« on: February 12, 2022, 0007 UTC »

Quote from: ErikMattson on February 11, 2022, 0058 UTC

And thanks for the ID Charlie_Dont_Surf

Happy to help.

625

North American Shortwave Pirate / Re: UnId 6930 USB 0125 11 February 2022

« on: February 11, 2022, 0135 UTC »

ELO at 0135 UTC.
SSTV at 0137 UTC.
ELO, "Telephone Line" at 0142 UTC.
0146 - Recognize the electric guitar tune but can't place the name and artist.
0154 - SSTV but signal was too weak for decode. TX off after the image?

626