Transmission Line Transformers

TRANSMISSION LINE TRANSFORMERS

Power Ratings

Transmission line transformers exhibit far wider bandwidths over conventional transformers because the stray inductances and interwinding capacitances are generally absorbed into the characteristic impedance of the transmission lines. With transmission lines, the flux is also effectively cancelled out in the core and extremely high efficiencies are possible over large portions of the pass band - losses of only 0.02 dB to 0.04 dB with certain core materials. A 0.02 dB loss translates to a 99.5 percent efficiency! Therefore very small transformers can safely handle surprisingly high powers. Experiments by the author have show that toroids of 1-inch OD and wound with #18 wire can handle 600 watts of continuous power without thermal runaway. Reports have also been made of transformers, of the high-power types available here, having withstood over 50KW of peak power without damage. Therefore it can sagely be said that, with properly designed transformer, the power ratings of transmission line transformers are more determined by the ability of the conductors to handle the voltages and currents than by the size of the cores.

Accurate measurement have also shown that ferrite permeabilities of 300 and less are necessary for the extremely high efficiencies these transformers are capable of. Further, the measurements showed that loss is related to impedance levels. This indicates that the losses are more of a dielectric type than those experienced by conventional transformers (hysteresis, ohmic and eddy current).

Measurements have also shown that, even with permeabilities below 300, there are tradeoffs in low-frequency response for efficiency. Here are some of the expected efficiencies for permeabilities of 250 to 300 (which are used in many of our transformers) as a function of the characteristic impedance of the transmission lines:

Characteristic Impedance Efficiency
50 ohms or less	99%
50 ohms to 100 ohms	97-98%
100 ohms to 200 ohms	95-97%

These results show that with impedance levels of 50 ohms and less (including a 50:50 ohm balun), the ferrite of choice is in the 250 to 300 permeability range. Further experiments have also shown that transformers matching 50 ohms to 300 or 400 ohms (or even 600 ohms) using Guanella's approach, can achieve 99 percent efficiencies by using permeabilities of 40. But this is at an almost 10-fold (300/40) expense in low-frequency response. Instead of 1.5MHz, the low-frequency limit would be raised to 15MHz.

Since the establishment of power ratings for transmission line transformers have not been made by any professional group (and the above results are the only available data on efficiency), we have arbitrarily used the following levels:

Experiments have shown that the power ratings for the transformers offered in this catalog are very conservative. Some of the low-power units, which use rather thick wires in order to achieve the optimum characteristic impedances, could very well be placed in the high-power category. More work has to be done in this area of power-ratings of transmission line transformers.

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