There is much ambiguity about the maximum allowable power on an Endfed antenna. It depends on many factors.

• Format ferrite core
• The Mode (SSB, FT8, CW, RTTY etc…)
• The band
• SWR
• Ambient temperature
• Length of broadcast

In an “average” 1:49 Endfed transformer about 1 dB of loss occurs. This is because a compromise has to be made on many fronts. This loss means heat. If the ferrite core gets too hot, it becomes saturated and loses its function. After cooling down, the transformer works normally again.

The efficiency of the transformer is not the same on all bands, hence this plays a role.

The larger the core, the more heat you can put into the core. Also, a larger core has more surface area to get rid of the heat again.

The SWR of the antenna is directly related to the efficiency of the transformer. If the SWR is worse then the efficiency also decreases increasing the losses.

Obviously, the critical temperature of the ferrite core is reached faster when it is 30 degrees outside or when the sun is burning directly on the enclosure. The size of the enclosure also plays a role.

Modes such as FT8, RTTY, FM have a 100% duty cycle. This means that 100% power is output continuously. In SSB, the output power depends on the LF input. So the power varies continuously. in CW, no power is output between the stripes and points. There is some discussion about exact numbers, but the following is a good starting point: SSB 25%, CW 50%, Digi 100%, FM 100%.

Of course, the length of your transmissions also has a big influence. Do you continuously give CQ in a contest, or are you only sporadically on the air.

So it is quite difficult to give rock-hard specifications in terms of power output. You also see that various manufacturers specify varying power outputs.

The following list is a nice guide.