Universal antenna autotransformer for 8.27kHz, 136kHz and 472kHz


Having built a 472kHz – 10MHz antenna autotransformer able to handle 136kHz on ferrite cores, I was considering building one capable of working at 8.27kHz and above (36km wave). VLF, LF and MF air-cored coils are quite huge for 8.27kHz, which makes them difficult to operate on outdoors. This was the reason why I thought that building a small impedance matching pad based on ferrite cores was a tempting idea to pursue – all the more tempting since ferrite coils in recent years have improved.

Theoretical basis

There are several conditions which have to be met in order to have a smoothly working transformer. Most important of them are:

  • correct diameter of the winding wire
  • correct breakdown voltage
  • temperature of the isolation
  • the core has to be able to carry the required energy and electric permeability levels at the right frequency
  • the inductive reactance must equal at least tenfold of the transformed impedance at correct induction levels

For 8.27kHz the inductive reactance value at 50Ω impedance will equal tenfold of this value, i.e. xl=500Ω, therefore the value of induction should be ca. 9.62mH

The cores

The cheapest cores (AL=73) were offered in one of the Chinese online stores. Shipping costs however, were too deterrent even though the price for 10.000 items was very tempting. I finally chose the B64290-L82-X830 cores made of N30 at AL=8700 used for 0 to 400kHz converters. Their outside diameter is 52mm, inside – 29.7mm and length of 21.3mm. To get the desired reactance of 500Ω you need 10 cores and winding 10 coils

The transformer

The primary winding is made of 10 coils wound with a 2mm wire in double isolation additionally isolated with a plastic pipe. The secondary winding has 60 coils divided into 6 sections. You have to pay special attention to the isolation due to high voltage observed on the secondary winding. 200W at the end of the 1:36 winding will produce as much as 7kV!


The transformer works correctly at 1:1, 1:4, 1:9, 1:16 and 1:25. 1:36 is an experimental combination and should not be exposed to high power levels.

The transformer can be used for 8.27kHz as:

  • an audio amplifier match with impedance of 2/4/8/16Ω to an earth electrode. While matching with longwire antenna a transformer on one core with 10 primary and 50 secondary windings (1:25, i.e. 2Ω to 50Ω) has to be used
  • a broadcasting system match to longwire antennas and earth electrodes. For 136kHz and 472kHz the transformer can be used for both longwire antennas and earth electrodes

The transformer works very well at 8.27kHz to 500kHz, however, it should not be used for frequencies above 1MHz – e.g. amateur 1.8MHz. While matching audio amps to an earth electrode of 2Ω or 4Ω impedance the transformer can work at frequencies starting with 970Hz – this is already a ULF band.

The transformer can as well function as a good receiver.