Messages

691

The RF Workbench / Re: Someone had Corsair II AM TX experience?

« on: February 13, 2021, 2301 UTC »

The reason why it gets some much attention is that many have been built and used in the past. I've found out so much about the designs asking the right people. The parts are easy to get hold of and if your into home construction it can cost under 20 UK pounds to build depending on the version.

I've built Corsette (LM386), Corsair (LM386) and Commando (Transformer) that's on Dave's website and in my opinion they all work well.

Shhh. Quiet. You're destroying Stretchy's bullshit narrative that everyone MUST.THINK.LIKE.HE.DOES and no one could be happy and content without doing what he wants. There's probably been more Corsette, Corsairs (I&II) and Commandos built than he has ever sold and that gets under his skin.

692

The RF Workbench / Re: Someone had Corsair II AM TX experience?

« on: February 13, 2021, 2248 UTC »

Just depends on how you drive it, squarewave = class E, sine =C, sure there's more to it as the o/p has to be bandpass but still the quicker the switching the greater the eff% pure and simples.

Str.

Umm yeah, there is more to it.

Like, amplifier class is not defined by how it is driven. It is defined by the shape of  the drain/collector waveforms.

Also sinewave drive can also be Class A or B, in addition to C, it depends upon the associated drain waveform. Squarewave drive can associated with class D,E,F, S or J.

693

Propagation / Re: 49m good this evening....

« on: February 12, 2021, 1043 UTC »

Sorry, I was referring to the extended period above 7 MHz the day before, so the red trace on the right side of the graph.  The circa 0620 UTC blip is certainly just bad data.

Oh. OK. I misunderstood.

694

SDR - Software Defined Radio / Re: KiwiSDR, SDR-14 and possible cheaper options?

« on: February 12, 2021, 1041 UTC »

I suppose if you're on such a KiwiSDR, you could tune in CHU or WWV, see what the frequency error is, then compute the ppm error and apply that to the frequency of the station you're listening to. Note you need to treat it that way, it's not just a constant frequency offset error.

That's exactly what I have done a few times to confirm my suspicions that some receivers were a bit off frequency.

I also suspected that the offset would increase with frequency and one time I went through the trouble of confirming this at 2.5, 5 and 10 MHz. It was incrementally worse at higher frequencies (by a few Hertz, but still..). (Could not hear 15, 20 or 25 MHz at the time.)

So there is no manual trim on the frequency with the Kiwi? I guess that would have to be some sort of override to the GPS, which, in my limited understanding, seems problematic

695

The RF Workbench / Re: Someone had Corsair II AM TX experience?

« on: February 12, 2021, 1014 UTC »

Sorry, I was confusing my memory of this with something else when I wrote the above and in my case the load was probably around 3-4 Ohms but the peaks probably presented the difficulty for my Class-D amp. (My guess.) Either that or the amplifier was not what the manufacturer claimed.

In your case, 12 V/9 Ohms = 1.33 A. (12 Volts ^2)/9 Ohms = 16 Watts (average). (24 V ^2)/9 Ohms = 64 Watts (peak).
Your power was/is probably close to that of the Corsair.

696

Propagation / Re: 49m good this evening....

« on: February 11, 2021, 0620 UTC »

foF2 was up above 7 MHz for a while, this often happen just as a geomagnetic storm is starting, the K index was 4.

When I see a blip like that at ~0620 UT, my first inclination is to think that it is not authentic (i.e., bad data) and some quirk of reception happened or the algorithm that determines FoF2 from the ionosonde data was "fooled" somehow. That data point is too weirdly out of place, even though FoF2 did appear to be slightly elevated that day compared to other recent days.

697

SDR - Software Defined Radio / Re: KiwiSDR, SDR-14 and possible cheaper options?

« on: February 11, 2021, 0611 UTC »

1)  ...  Is GPS antenna really needed or can I forego that for now?

The GPS antenna is not required, but highly encouraged, that's how the KiwiSDR corrects for errors in the ADC clock sampling frequency.

Does GPS also serve as a frequency/time reference? Can you also manually trim the reference or clock to WWV/CHU/whatever in absence of (or in addition to) that?

I notice that Kiwis can be anywhere from <10 Hz to as much as 60 Hertz (maybe more?) off from a standard. While that's not a life-or-death offset in a hobby listening scenario, it would be nice to be kind of on frequency when listening to SSB, where I notice being >10 Hz offset there.

698

The RF Workbench / Re: Stretchyman 40 W TX Reliability Modifications

« on: February 11, 2021, 0546 UTC »

Chris - I think that the answer lies in the power dissipation of the crystal.

This makes sense to me because:
a) Where there is power dissipation, there is heat.
b) Where there is heat, there is a temperature change and drifting oscillators.
c) A crystal is pretty simple. What the heck else could it be?
d) I found references that say so. Here is one:

https://worldradiohistory.com/Archive-DX/Ham%20Radio/70s/Ham-Radio-197603.pdf

On page eleven, in the third full paragraph in the left column:

The permissible maximum power dissipation of crystals in the 1 to 20 MHz range, operating in the fundamental mode, is about 200 microwatts and is  similar for overtone operation. ... Operating a crystal above or near this level degrades its stability, and the circuits chosen here avoid this problem by limiting the dissipation.

Further down:

The main point to remember is  that crystal  oscillators are meant to provide a stable frequency source, not power output. Frequency stability, both short and long term, and crystal life are compromised when an oscillator is operated at an excessive power level.

I have no idea whether Stretchy's circuit goes beyond tolerable limits. I'm just putting this out there for thought.

699

The RF Workbench / Re: Stretchyman 40 W TX Reliability Modifications

« on: February 11, 2021, 0524 UTC »

Why being so negative about comments?

               Keep the Peace!    -  It's almost Xmas!


André
CoolAM Radio - Shortwave
the Netherlands

 

Hi André - I am sorry that you feel that way because on the other 364 days of the year people might come to this thread for answers, without the "burden" of being in the "Christmas spirit", and find nothing. It's now nearly 2 months later and we have no answer.

700

The RF Workbench / Re: Someone had Corsair II AM TX experience?

« on: February 11, 2021, 0518 UTC »

You could also replace the series modulator with a $11 class d audio amp board from amazon.  Efficiency would be MUCH higher, and it makes more sense to go this route since you already have a 24V supply at your disposal.

+-RH

Not disagreeing with you (you're absolutely right about the efficiency) but some day I should write up my experiences trying to use a cheap Class-D amp as a series modulator, requiring the cheap amp to supply all the bias for the RF stage.

The cheap class-D amps that I am aware of don't like to drive loads below 1-2 Ohms, which is exactly what you would be asking it to do by becoming a series modulator. It gets "unhappy" when you do this and I damaged more than one by asking it to deliver x Amps continuously as DC bias for the RF final, in addition to modulated bias. Sometimes the damage was not catastrophic, sometimes it was catastrophic. (It's a good thing that the amplifiers are "cheap".) My proposed solution (which I never got around to trying) is to add 2 Ohms in series between the class-D amp and the RF amp. That way the Class-D amp is never seeing less than 2 Ohms and is less likely to be "unhappy". Of course, this is wasteful and might eliminate the efficiency gains you made by using a Class-D amp. 

Well, I guess I have now written up my experiences on this, so today is "some day".

701

The RF Workbench / Re: Need a Temporary AM Antenna? Check out what this station did.

« on: February 10, 2021, 2101 UTC »

BTW a 1/4 wave at the top of the AM dial just happens to be 5/8 on 4 MHz 

Ahem (Cough. Cough.)   

"Speak softly and transmit with a big stick." - Teddy Roosevelt

 

702

The RF Workbench / Re: West Virginia's Green Bank Rf Telescope gets a TX upgrade!

« on: February 10, 2021, 2050 UTC »

It's in the National Radio Quiet Zone. Turn off your radios (all of them) when you enter the zone.

It would be really cool if the radar had enough resolution to see the base of the Apollo lander that was left behind and the "dune buggy" the astronauts used (for the first time in the Apollo program) while on the moon surface.

703

The RF Workbench / Re: Someone had Corsair II AM TX experience?

« on: February 10, 2021, 2043 UTC »

So I have decided to change completely the modulation type from "low level" to a more classic "high level" directly on the final mosfet, using a simple serial modulator

This is a positive step.

I may have missed it in this thread but I don't think we ever got a clear definition of what was termed "distorted audio". ("Distorted" is somewhat imprecise because it doesn't relate much about the nature of the problem.) This is why an oscilloscope image would have been very helpful.

In any case, one of the issues with the original design is that the modulated audio was probably getting into the oscillator and "pulling it". Modulated audio was driving the oscillator buffer (as the low-level modulator) and that buffer is - like it or not - part of the oscillator circuit. Its parasitics will affect the oscillator stage before it, changing the oscillator frequency, and the amount of oscillator shift will vary with the modulation. This would produce frequency or phase modulation along with the amplitude modulation and result in distorted audio. (I can't be sure that this is what is happening because I never saw oscilloscope images or heard recorded audio, but it is a safe assumption given the schematic.)

There are ways to fix this and my favorite way is to add an additional layer of oscillator buffering to better isolate the oscillator from the modulation. In effect this is what you have done by changing to a high-level modulator. That is, by modulating the final amplifier directly.

Good work. Congratulations.

704

The RF Workbench / Re: Someone had Corsair II AM TX experience?

« on: February 10, 2021, 2023 UTC »

A Scope would help?.

It's pretty infrequent but I actually find myself agreeing with Stretchy's comment from years ago. Miracles do happen.

705

North American Shortwave Pirate / Outhouse Radio 6950 USB 0445 UTC 3 Jan 2021

« on: January 03, 2021, 0458 UTC »

ID at 0457 with fun blues music. Listening via SDR in Pennsylvania. Good copy but maybe just turn down the music a smidge - sounds like you are slightly overdriving it.
0502 - better now. Thanks!
0547 - Off. Thanks for the tunes.