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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
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