The question: “what power rating do I need for the choke or impedance transformer for my station? is asked many times and the answer is usually the same: “It depend on you transmission mode, frequency, impedance ratio, duty cycle, and load impedance. (Note: FT8 mode update – see below).
This page attempts to clarify some of the issues to be considered in selecting a power rating that meets your needs. Higher power ratings are always more robust, but physical size, cost and topology (shape) of the choke/transformer will also dictate your choice.
Feed Line Chokes (1:1)
Coax feed line chokes are by necessity designed to the coax impedance, usually 50 or 75 ohms depending on model. Common mode chokes are used to “choke” off the common mode currents which are in general very much smaller than the RF current flowing in the center conductor and the inside of the coaxial braid. As long as the common mode current is small (higher choking impedance causes smaller common mode current), the power dissipated in the ferrite core will also be small. If the choking impedance is too small and the power in the cable is high, the common mode current will be large and the heat dissipated in the ferrite core may be excessive and actually crack the core rendering the choke inoperative. The choke may also fail if too much power is run through an undersized choke (e.g. 3Kw RTTY/digital mode run through a 1500 watt PEP rated choke).
Another factor is the SWR on the the load side of the choke. For example feeding an 80 meter dipole with a CB-1-1500 balun and switching to 40 meters by mistake and driving 1000 watts into the balun will probably damage the balun (now seeing 4000 ohms impedance at the load rather than 50-100 ohms at 80 meters). The SWR is now 4000/50 = 80:1 and the voltage across the balun is square root of 80 * original SWR = 9 times greater than on 80 meters thus causing likely excessive voltage breakdown on multiple winding toroidal transformers. Sleeve chokes and Super Chokers™ generally don’t have high impedance problems if power is kept within the coax cable limits, but toroidal based chokes usually have higher choking impedance and if used properly are more effective in reducing common mode current on feed lines.
For additional information see : Choosing a Feed line Choke
Impedance Transformers (< 1:1 and > 1:1)
Impedance transformers < 1:1 and > 1:1 are specifically designed for a fixed load (e.g. 200:50 (4:1) is designed for a 200 ohm resistive load and a 50 ohm resistive input which requires a transmission line (on the transformer) of 100 ohms). If the transformer is presented with a load of 450 ohms, the transformer may or may not transform the load impedance by a factor of 4 and there may be excessive (due to higher SWR in the transformer) heat buildup or voltage breakdown and the transformer may fail. If the load has high reactance due to high capacitance or inductance, the SWR also may be high, the impedance ratio may change or the transformer may fail. For most effective use, choose the transformer with a load impedance, that is well behaved, in the ratio and power required in the frequency range specified.
Impedance transformers for OCF Antennas and for Antenna Tuner/Ladder Line use have been specifically designed for large impedance ranges and can be chosen by power levels based upon the transmission mode ratings (see table below). If these transformers are driven by excessive power outside of their specified frequency ranges or impedance ranges or heavy duty cycles they still may fail so choose the largest power rating within your budget requirements.
As a general rule, the higher the impedance ratio the narrower the frequency range and the lower the power rating due to the characteristics of ferrite materials and transformer construction techniques. For example, running a 500 watt PEP rated 9:1 unun for an end fed antenna at high power or without a feed line choke at a low frequency (where the antenna radiation resistance may be very low), will likely cause the unun to prematurely fail due to excess rf current. Use of any impedance transformer at the limits of the frequency range or impedance levels or power levels is likely to result in reduced performance or permanent damage to the transformer. If in doubt, choose a higher rated model and feel confident with a hassle-free, higher safety margin.
Choke & Impedance Transformer Power Rating Table
The table below attempts to generalize the power rating of various chokes and impedance transformers manufactured by Palomar Engineers. These rating do not apply to other manufacturers. (We have replaced many failures of other manufacturers who rate impedance transformers for 1500 watts PEP but fail at 250 watts CW in a high duty (contest operation or digital mode) use. This failure is quite common for OCF 4:1 baluns used for contesting or high duty digital modes at high power as the transformer is under engineered for the wide range of impedances encountered over the antenna range. (Note: a simple test to determine the quality of the impedance transformer or choke is to weigh it – most high power (>1500 watts PEP) devices will weigh between 1-3 pounds as that is the weight of typical ferrite materials that is required for proper operation over a wide frequency range). Don’t expect a wimpy 1/2 pound OCF balun to take a 1500 watt PEP signal for long! When your OCF balun fails, check out our OCF Baluns.
The ratings in the table assume a resistive load (SWR = 1:1) within the frequency range specified for the choke or transformer. Check the product specifications for each model as certain models have specific frequency ranges, impedance min/max, and power ratings. For SWR other than 1:1, divide the rating shown in the table below by (square root of swr). E.g. if SWR is 5:1, the square root of 5 = 2.24 so a 5KW PEP rated transformer should be limited to 5000/2.24 = 2,232 watts max PEP.
FT8 Update (2/2019)
With the popularity of the digital mode, FT8 we have tested our chokes and transformers (and coax noise filters) and determined that you can safely use them at full power rating under FT8 conditions (< 50% duty cycle) when used with a load matched to the transformer output impedance. So a 1.5KW PEP rated 4:1 balun will work at 1.5KW FT8 mode into a 200 ohm load (SWR =1:1). For other load values derate as enumerated above.