LF Isolation Transformer

LF Isolation Transformer


The power supplies in the receivers and other loads will create small amounts of pulse currents with harmonics reaching into the LF band of interest. These harmonics are present regardless of how well filtered the generator power source is. The receiver chassis injects these small currents up the antenna cable shield. These currents give rise to voltages between the antenna ground and the local ground. This local voltage field is then injected into the antenna signal.

When this occurs in a LF receiving setup, power line harmonic interference is created. The placement of a 1:1 isolation transformer in the antenna line significantly reduces this source of interference. Of course, a coupler circuit providing power to the active antenna should be placed on the antenna side of the isolation transformer. Better yet, the transformer should be built into the coupler.

During the LF listening test at Nags Head NC, N4ICK had such a transformer and when it was tried the benefits were obvious. A transformer was constructed for this purpose on site and the resulting interference reduction was excellent. Follow up measurements confirms that the ad hoc design was very near optimum for this purpose.



The transformer was constructed by winding two 20 turn windings of 30 gauge wire wrap wire on an Amidon FT 37-75 or FT 37-J toroid core. An Amidon FT 50-75 or Ft 50-J core with 18 turns may also be used. The two windings were wound on opposite sides of the toroid core to minimize capacitive coupling. The transformer was mounted in a plastic insulated box and BNC RF connectors were placed on either end. Each BNC connector was connected across one of the transformer windings.



The open circuit impedance of a transformer winding at 10 kHz was 44 ohms at +87 degrees. On a 50-ohm system 10 kHz would be about the low end of the transformer operating range.

The coupling between the primary and secondary was very tight and the input impedance of one winding at 20 kHz drops to 1 ohm at +50 degrees with the other winding short-circuited.

Capacitive coupling between the primary and secondary is hard to measure but is about 5000 ohms at –90 degrees from 100 to 200 kHz.

Although simple, this transformer performs well and measurements validate the design is about optimum if a 10 kHz lower operating frequency is desired. With fewer turns on the windings the lower operating frequency could be raised but the performance in the 100-200 kHz band would not get much better. Neither would the capacitive coupling be reduced significantly. Further reduction of the coupling could be achieved by introducing a guard electrode with a shield between the two windings that could be connected to an independent ground rod. Care would have to be taken to not introduce a shorted turn in the transformer with the shield.

Frank Gentges K0BRA

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