Autotransformers

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Autotransformer Recipe Table

Ratio	Turns	Target Z	Optimal Cap	Why?	Notes
36:1	2+12	~1,800 Ω	120–150 pF (130 pF ideal)	Fewer total turns → low leakage L → needs higher C to resonate ~28–29 MHz (10m flat SWR). 100 pF → SWR >2.5:1 on 10m.	Best for very low (<15 ft) EFHW
49:1	2+14	~2,450 Ω	100 pF (±10%)	Standard leakage from 14 sec turns → resonates ~28 MHz. Proven in thousands of builds.	Gold standard for 80–10m EFHW.
56:1	2+15	~2,800 Ω	80–100 pF (90 pF ideal)	More sec turns → higher leakage L → needs lower C to hit 28 MHz resonance.	Excellent for Vertical/Monoband EFHW
64:1	3+24	~3,200 Ω	80–100 pF (90 pF ideal)	3 pri turns + 24 sec → high leakage → lower C needed. 100 pF → 10m SWR spike.	Higher elevation deployments (elevated, inverted-V, high Z).
81:1	3+27	~4,000 Ω	68–82 pF (75 pF ideal)	Max turns → highest leakage → lowest C to keep resonance <30 MHz.	Experimental — for very high Z ex: 160m EFHW.

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