Inner Resistance of Air Core Transformer at VHF/UHF with Closely Coupled Windings

The discussion addresses the unexpected inductance values calculated for a straight wire versus a two-turn air core coil of similar wire length at VHF/UHF frequencies. Calculators indicate that a straight 1mm diameter, 16cm wire yields higher inductance (~182 nH) than a 25mm diameter coil with two windings (~118 nH), which contradicts the expectation that inductance increases with the square of the number of turns. The discrepancy is attributed to the assumptions and empirical nature of inductance formulas and measurement challenges at low inductance values. It is noted that inductance arises from magnetic flux linkage between coil turns, and a single straight wire's inductance depends heavily on the return current path, which is often ill-defined in calculations. The high inner resistance of closely coupled air core transformers at VHF/UHF is also questioned, with references to power electronics concepts where short circuit current is effectively zero. The discussion highlights the complexity of accurately modeling and measuring inductance and resistance in air core coils and the limitations of common online calculators.
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