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Wolf DL4YLF,

In response to your questions on what type of balanced feeder I use to my active antenna, it was originally a "data cable" that I obtained second hand at very low cost. It is a plastic sheathed cable that has 4 twisted pairs of separately shielded wires. My active antenna is in the back corner of my property, and the shack is in the house, so mainly for cosmetic reasons the cable has been buried a few centimetres in the lawn and garden. Burying the cable may also have benefits in minimising "antenna effect" of the feeder. I use one of the pairs as a balanced RF feeder and another pair for DC. I earth all cable braids at the shack end and leave braids floating at the antenna end. The active antenna has a separate local earth. Two pairs in the cable have been left floating, but could be used in the future for the likes of connecting to a loop antenna and varicap diode tuning voltage. For RF transformers, I use FT-50A-75 ferrite cores (permeability 5000) and 20 turns for primary and secondary, wound on opposite sides of the toroid, and these respond from a few kHz to MHz. I did not use bifilar windings as these could have more capacitive feedthrough compared to windings that rely on magnetic flux coupling, and the incidental leakage reactance is no problem at any part of the LF band.

I also run my entire shack via a 5 kVA isolation transformer, so I have a combination of factors that help to keep mains conducted noise from getting into my receivers. However, I am aware of successful results where the mains power supplies are double insulated and care is taken to minimise "inadverent earthing" via LF antenna feeders.

The effectiveness of isolation at LF can be checked if gear can be run from 12 volt batteries. Testing for local QRM with no mains power connected (actually unplugged) from the receiver is the first acid test. Then "antenna effect" of feeders to the active antenna can be checked by having no power applied to the active antenna. Hopefully when all is working well, the receiver noise floor will be dominated by external QRN being received by the active antenna.

I also need to "float" my LF transmitting vertical while receiving, otherwise local QRM that is "seen" by the much larger antenna (and much larger near field) dominates the signal received by the separate active antenna.

73, Bob ZL2CA