Based on the work of Jan, DJ1JAN I've build some QRP-1:49 UNUNs.
My first experiences with this board on 20m using a FT82-44 core are, that the core gets hot at 5W on 20m / 14MHz.
Based on this experience I consulted the mini Ringkernrechner and and did some calculations. As far as I understand, the primary coil with 3 turns limits the maximum usable power.
14MHz / 20m Band
The voltage of 19Veff equals to 7W HF and that about 1W is burned inside the core!
7 MHz / 40m Band
If we switch to 7MHz the core with 7W input is in saturation:
On 7MHz you can use only 3.5 W HF (=13Veff) from which 0.77W is converted to heat.
3.5MHz / 80m Band
Switching to 3.5MHz reduces the maximum power down to 1.6 W (=9Veff):
Now I've build another balun using an FT82A-43 core. This core has the same diameter but is nearly twice in height. This gives an AL of 850 instead 470 for the FT82-44 core.
This gives an increase of nearly factor x2 for the usable power:
| Core / frequency | 3.5MHz | 7MHz | 14MHz |
| FT82-43 | 1.6W | 3.5W | 7.0W |
| FT82A-43 | 3.2W | 7.0W | 14.0W |