Baluns, Ununs and Power Ratings

Unfortunately there is no industry standard for measurement of power ratings for baluns and ununs.  Consequently, comparison of the same type of balun/unun from different vendors can have very different outcomes when the unit is put in to service.  With this in mind, we have set our own company standards and what follows is provided for a better understanding of the installation and best use of our units.

Research and testing by Jerry Sevick (W2FMI SK) determined the following:

• Baluns lose approximately 40% of their power rating at a 2:1 SWR and 60-70% at 3:1 SWR.  This means that when one of our 3kW rated baluns is used at 2:1 SWR, the rating of the unit is reduced to 1.8kW and at 3:1 reduced to a conservative 1kW.  A 5kW rating is reduced to 3kW and 1.7kW respectively

• Ununs do not develop a choking impedance so the effect of elevated SWR is reduced to 30% at 2:1 and 50% at 3:1 SWR.

• Power ratings are impacted even more by high duty cycle digital modes such as FT8.  While it can be said that FT8 is a 50% duty cycle i.e. 15 seconds on and then 15 off, it is also at 100% carrier for the full transmit time. The frequent on/off cycle of FT8 mode when used with an antenna with elevated standing wave, does not allow adequate time for a balun to dissipate heat build up.  This heat can cause a slowly increasing SWR and ultimately thermal breakdown if the unit is used for extended periods.

• Consequently station power used in the FT8 mode should be multiplied by at least a factor of 3 when calculating the power de-rating of your unit.  Example, use of 1kW in FT8 mode should be considered 3kW for the balun which means at 2:1 SWR, you would be at the edge of the balun power rating. (5kW rating is reduced to 3kW at 2:1 and the 3x multiplier for FT8 brings the power up to the same 3kW reduced limit)

Stations with power levels of 800 watts and greater, need to be aware of both the 2:1 and 3:1 bandwidths for each antenna (and band) in use to minimize possible damage to the balun or unun. It is also important to take SWR readings at or near the antenna to eliminate changes that feedline length can create in these readings.  While many will say "I know that", it is also important to note that a tuner does not reduce the SWR (mismatch) at the balun, only the impedance from the shack end of your  feedline is matched to your transmitter or amp which takes place after the balun or unun in the feedline.