Winding an Un-Un

[joeqsmith]

That amplifier is limited to about 14MHz.   I drug out another that will get us to 30MHz.   SWR is much worse as expected.   I took the power up higher and am using both fans to now cool the load but they are still well over 100C.   

[joeqsmith]

2:1 even with our poor mans load seems really high.   So back to the VNA and it looks like it should be closer to 1.5:1.   Of course, I have no idea how well my vintage Mars VSWR meter works.  With it being an UnUn,  I used a 100X probe and attached my scope across the load to have a look. 

I started with 19.4V or roughly 800mW at our load.   The waveform seemed fine so I bumped it up to 48.9V or about 5.1 Watts.  Again, the waveform seems fine and the VSWR looks like we would expect based on the LiteVNA.   

The resistors are rated for 2Watts each so the load should easily handle 30Watts.  With it getting so hot with the two fans, I was putting out a lot more power.  So I bumped it to 152.8V or roughly 50Watts.   The VSWR goes up and now we see the problem. 

[T3sl4co1l]

Poor cores! How hot were they getting?

Or is that the amp's problem?

Tim

[joeqsmith]

To answer this,  I have other PAs, however I keep this vintage radio in the lab.  It seemed like a good use for it.   A bit on the scary side  267.9V or 152Watts on our poor little resistor load.  I only keyed it long enough to capture the waveform with the DSO,  set the VSWR meter and take a picture.        Those resistors were not made for this kind of abuse.

So, the UnUn is fine but I was pushing my little amplifier.    Good to see the old Drake's tubes light up. 

[joeqsmith]

I should say poor little VSWR meter too.  I don't have the manual for it but doubt it was rated for that much power.   

[T3sl4co1l]

TOOB! 

Aha, well that does it.

Are the cores getting hot, warm at all even, in any of these--?

Tim

[joeqsmith]

Well, nothing has unsoldered and the Kapton tape is still in place.  Without a better load the best I could do is run it around 40W or so and see what the heat rise is.  I would guess most of it would be conducted from the load.    Now if we want to talk heat, that old radio is throwing off a fair amount.   

[cdev]

A while back I asked about favorite wire to use making ununs. I still have not found the ideal hi temp wire, so over the years several times Ive use a medium size of enameled solid copper wire. Thats worked okay for receiving. But I would kind of like to acquire something better. For a full power capable ham unun, what gauge of teflon wire would you use? I'm probably going to use two toroid cores. #43 ferrite. IS that a good choice or would y-all. (spoken in my best Memphis drawl) prefer something else?

I like using different colors of wire too. To make pretty, easy to understand pictures. I have teflon plumbers tape (which also makes a good ulta white card for photographic purposes, should I wrap the toroids with that stuff first, do you think, so they dont/can't arc over?

Otherwise I'll probably just use my 18 gauge solid copper enameled wire again. The insulation seems to work well, when ive tested it. Maybe I can find some thin, translucent teflon tubing for it?

[joeqsmith]

I placed a layer of Pyrople (Polyimide fiber board) between the two cores and load.  I placed TCs on each core and blocked the majority of air flow around them.   I brought the power up to roughly 40Watts and let it stabilize.   The load is sitting over 100C but the cores remain well below 50.   Hard to say how much of this is conduction.   

Maybe one of the CB or Hams will jump in with some better data. 

[cdev]

Orange Skotchbrite Kind of,  That's a new one for me.. Have you used it for anything else? I guess its originally for cleaning stuff while its hot? Yup, Google is helpful here.. Definitely looks useful for holding or juggling hot and spicy ununs, quite possibly. They are sold as griddle pads.. for cleaning hot griddles in-situ.

Just a wild guess.. Presumably it works at that? Ive never made a balun like that but I do like using binocular cores for baluns (and ununs, which also works, )and my experience is that that works really well. Enclosing the winding lets it be shorter, so it works better at higher frequencies as well as low. My switchable imitation elecraft BL-2 balun works at both all of HF and at 6 meters.

[joeqsmith]

https://pyropelinc.com/
No it's not a Scotch-Brite pad.  Yes, I have shown using Pyropel a few times on this site.   

https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/msg648003/#msg648003

[joeqsmith]

Touching the PA made a rapid change in the BM869s temperature readings.      Added some ferrite to the TC leads which has seemed to tamed the problem.   The singles didn't help.  The common mode one did the trick.    Readings are now believable.   

[joeqsmith]

Showing the whole setup, no camera flash so you can watch the fans.  Note 2:1 SWR from over driving the small solid-state amp.  Cores remaining nice and cool.

[T3sl4co1l]

Nice.

Tim

[joeqsmith]

I let it run for a few hours like this and nothing changed.  I removed the two single clamp-on cores beads with no effect. 

I was curious about the temperature and tried two BM869s meters rather than one.  The inputs are not isolated and with the probes attached to the cores, there may be some common mode.   It had no effect.   As soon as I remove the common mode core, the both meters display very high temperatures...

One reason I bought the BM869s was because it has two TC inputs.   The one other meter I have that has this feature is the UNI-T UT181A.  While mine works fine without the common mode bead attached, it is no longer a stock meter.  This meter, like pretty much all UNI-T products I have looked at, was very sensitive to ESD which damaged the meter.  After repairs, I modified the meter which would certainly improve the high frequency rejection.   The UNI-T fanboys would need to check a stock one. 

The new Brymen BM78x supports two TC inputs but sadly, it is a single display only so you have to select between the two.   I tried it out and as you can see, it is also way off.  Adding the bead corrects the measurement.   

[T3sl4co1l]

Have also noticed poor CMRR on the BM235, mainly just an issue when probing a heatsink / transistor tab / pin and the TC ball touches a switching node and the reading goes kablooie.  Seems fine as long as it's not touching.  In your case, probably it's similar immunity (or lack thereof), but being nestled down inside a network at even higher voltages*, it has similar effect.

*In my case, switching nodes, so, 100s kHz at 100s of V, but harmonics in the 10s MHz range then are only some ~V.  So your case is probably the more aggressive -- give or take if it's more sensitive to impulsive or CW noise, or what effect those have on the acquisition, etc..

Tim

[joeqsmith]

Are you also using beads to combat the problem with your BM235? 

[T3sl4co1l]

I forget if beads helped.  That was just a matter of, better choice of pin or package spot to probe, so, I didn't look into it.  Likely it would.

Tim

[joeqsmith]

My 189 with the same setup.  Over driving that amplifier is making some harmonics. 

[joeqsmith]

With no goals and constraints, here is a coaxial wound 9:1 UnUn.    Performance is very poor.   

[T3sl4co1l]

Probably too low shield impedance; looser spacing on larger cores, or fewer turns, might do better.

Considering 450 ohms needs ladder line, the multi-filar (parallel laid) design is pretty good.  Would need pretty gross geometry coax to improve on it at all.

Tim

[A.Z.]

With no goals and constraints, here is a coaxial wound 9:1 UnUn.    Performance is very poor.

I'll refrain

https://cromwell-intl.com/radio/9-1-unun/

[joeqsmith]

It would provide decent results down to about 500kHz, so yes, too many turns.  After removing a third, it was still poor.   

Shown after removing the turns with a 220, 450 and 900 ohm load.   I'm using my substitution box which of course is going to skew the results but at least we can see the transformer is doing something.   Also shown is the substitution box compared with a 1/4W TH. 

[joeqsmith]

Shown with 4 turns with the substitution box set to 220, 450 & 900.   Sweeping from 1.8-21.6MHz.   It's not quite as bad as it looks as that substitution box is adding a lot of error.  Still, not very good.    Maybe pull one more turn off and see how bad it hurts the low end.   

[joeqsmith]

Brings the high end down to around 2:1 VSWR but the low end really suffers.