EFHW Power Rating Myths Part 2

The actual power rating of the 49:1 transformer core is limited by the core temperature. Core cooling is actually more important than core losses.
Please watch part 1 first. Here is a link.
• EFHW power rating myths
There is a link in part 1 to get you back here to part 2.
These videos show how hot ferrites cause signal distortion which then generates harmonics "spurious emissions" .
• EFHW emissions violate...
• EFHW Spurious Emission...

[@rq6884]

Clearest explanation I have heard of the effect of heat on the efficiency of EFHW antenna transformers. Thanks!!

[@Jimscoolstuff]

Thanks for your nice c0omment.

[@NikoSan713]

Toutes les EFHW n'ont pas ce problème seulement celle utilisant des ferrites.

[@Jimscoolstuff]

@@NikoSan713 Thank you for your comment.

[@KK7JMH]

Excellent demonstration! I can’t wait to try some of your kinds of measurements on my build. Thank you for your work to demonstrate harmonic interference as well as good temperature measuring.

[@Jimscoolstuff]

thank you four your kind comment. Good luck.

[@turinskiboy]

Thanks Jim. I've been operating an E.F.H.W. with the unun mounted in a plastic box about 15 feet off the ground at the feed end. I mainly operate SSB on 20 metres for short QSO's. I'm wondering just how hot the 140- 52 toroid gets up there. It's very rare for me to use more than 50 watts. I'll be keeping an eye on the SWR when operating for longer durations. Your presentations are always informative, and a pleasure to watch.

[@Jimscoolstuff]

Thanks for your kind comment. You could attach the thermistor to the core and let wires hang down where you can reach them and determine the temperature with an ohmmeter.

[@dogastus]

That's quite an eye opener! Maybe ununs should be fitted with heatsinks.

[@Jimscoolstuff]

Thanks for your comment.

[@ve3cwq47]

And/or a nice big quiet fan.

[@neilbchilton]

Nice work Jim, that's a very clear explanation. Nice to see some science in this topic! I only use QRP and an EFHW and even with FT8 I don't think it'd affect mine too much but I might add temperature monitoring to see. Just wondering why there's not been more use of (for example) water cooling for QRO users. Should be fairly simple to implement I would think...depending on the location of the transformer of course!

[@Jimscoolstuff]

Thanks for your comment.

[@Stingmni]

Great information. Thank you.
Very clear explanation. And I do not like math, but was able to follow everything.

[@Jimscoolstuff]

Thanks for your kind comment.

[@moozoowizard]

I think the message is watch your swr meter and stop if it starts going up. If it does consider a different toroid or stacked toroids.
The watts going into the cores can also be worked out via back to back efhw toroids and two power meters, one each side
Now do toroid baluns, ununs and chokes.

[@bobsac10]

Thanks Jim for your efforts and great information.

[@Jimscoolstuff]

Thanks for your comment.

[@johnarcher9480]

Now we need to test with more efficient cores and windings.
70-75% efficiency is pretty bad for a proper EFHW.

[@Jimscoolstuff]

Thanks for your comment.

[@philb5289]

Several of the EFHW's I use have the UNUN mounted in a black box. Here in Arizona the temps easily get to 110 degrees in the summer. This would mean my UNUN is already performing poorly even before I make a transmission. Or does it? Thanks for this great 2-part video lesson.

[@Jimscoolstuff]

All the temperatures shown in the video are Degrees C. If you look at a data sheet for a ferrite material, all the data is in degrees C. Your 110 degrees F is only 43 degrees C.

[@philb5289]

@@Jimscoolstuff Thank you Jim. Keep up the good work! 73

[@danielhorvat4340]

Nice effort but why use the lossy material, not enough core cross-section for 3.5MHz, and what myth is busted? I made hundreds of EFHWe transformers and pointed out hundreds of times that a single FT-240-43 is the worst possible design. There are real myths present and perpetuated by amateurs when it comes to designing RF transformers. The main reason why the cores are heating is lossy material and bad design just like the one used in the test with not enough primary turns for the 80m band.