EFHW autotransformer two ways

EFHW articles

I’ve been focusing a great deal on end-fed antenna systems over the years including the last several posts. Therefore it’s time to take a moment and put some focus on the transformer that makes antennas fed at their current loop work.

The humble autotransformer

I first learned about the application of the autotransformer in my first article reviewing and testing the LNR Precision EF-10/20/40 end-fed HF antenna. The step-up transformer looks like this…

Schematic of LNR Precision 10/20/40 Portable Antenna Matchbox
Figure 1 – LNR Matchbox Schematic

Later I noticed VHF and UHF products with a slightly different configuration of the autotransformer. One difference is the VHF+ units typically do not use a ferrite as transformer core while the HF examples do.

EFHW autotransformer two ways

Let’s review key differences between the HF and VHF+ autotransformer configurations.

Difference between autotransformer configurations
Figure 2 – Difference between autotransformer configurations

Highlights in figure 2 include:

  • E1 and E2 are both aerials with a length such that a current loop (Hi Z) connects to the top of the autotransformer. This could be a half-wave aerial or multiple thereof.
  • Transformer T1 on the left wraps its windings around a suitable ferrite material¬†usually in toroid form.
  • Transformer T2 on the right has no core.
  • Shunt capacitor C1 across the low Z side provides reactance compensation for the T1-E1 end-fed system.
  • Series capacitor C2 on the low Z side provides reactance compensation for the T2-E2 end-fed system.
  • The left schematic represents the actual autotransformer in the LNR Precision EF-10/20/40 antenna.
  • The right schematic represents the NEC design I used to simulate and analyze the end-fed antenna concept.


This web site is as much my technical journal as well as article collection. Hence please accept this post as merely a quick reminder the key notion that capacitor position correlates with the core material (or lack thereof).

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