Blocking mains power noise.
Without measures, noises at the mains power lines, and the safety ground lin enter the radio room,
Safety ground noises can be blocked by disconnecting the safety ground, and connect all equipment in the radio room to a separate noise free, and low inductance ground source.
One solution for blocking mains power line noises, is the use of an "Ultra Isolation Transformer". Such a transformer has a negligible small parasitic coupling capacitance between primary and secondary windings, and/or a screen in between. The noise coupling between primary and secondary transformer windings is therefore very weak. The transformer iron core also damps high frequency noise. The output is isolated from the input, reducing chances of electrical shocks.
A cheaper solution is blocking mains power
line noise by a
good multi stage mains filter, and a noise free RF
Coax cable noises.
Antenna cables can run near mains cables and noisy equipment. These noise sources can induce noise currents into the common mode circuit of the antenna feed cable (the outer skin of coax cable screening). Good Common Mode Chokes ("CMCs") will block these common mode noise currents. Noise currents then cannot run towards the antenna nor the radio, and will not be heard.
But CMCs can also prevent harmful common mode
Such resonance will amplify common mode
currents, causing :
- Distortion of the antenna radiation pattern ( antenna not anymore radiating maximaly in the wanted direction).
- RF in the radio room ("feed back" and "hot" radios),
- Amplified noise currents, stronger noise at certain frequencies. Read details
Installing common mode chokes.
This way, all cable pieces and common mode chokes can be fabricated in advance, and later on easily installed, or exchanged for repair.
At the top : a 100 Ohms bifilar (bi-wire) choke (wound with two PTFE
plated wires in parallel).
Both CMCs are put over
short pieces of PPC pipe with N-female
Start installing CMCs at both antenna and transmitter. REM : In the matchbox at the top of the loop, a CMC is already installed.
For this antenna extra chokes should be installed in between at intervals of maximal 10m.
IN GENERAL : At feeders with lengths
for the highest working
frequency of 3/8
lambda or more,
CMCs should be installed at the coax feeder at maximal intervals of 1/4 lambda.
My grounding system.
Our large atrium steel construction is connected to the earth via steel reinforcements in walls and pillars.
It showed to be the best "ground" in my location.
The separate box contains a second noise filter for the tuning motor and relay lines, protected by high speed bi-polar over voltage diodes, which short circuit these lines in case of lightning nearby.Then both HF and VHF coax cables, and the command line, enter into the radio room through an insulating wall.
The black box contains a first relay and motor line filter.
The ends of the coax cables are again via surge arrestors connected to
an aluminum L-profile, which serves as a
central ground point for the radio room.
A thick cable connects the ground to the ground rail on the back of the radio table.
mains power supply in the power cabinet contains ground leak switches. In an
advent of only a small harmless current difference between live and neutral of more than 30 mA,
power is quickly switched-off.
Mains ground can therefore be exchanged by a separate noise-free RFground (that is the Atrium roof).
To prevent noises on the mains safety ground wire to enter the radios,
the "mains safety ground" for the radio room is disconnected. All
equipment is only connected to the noise free system ground via the ground rail.
In the radio room, all ground connections of radio and PC equipment are only connected to the steel roof construction in the atrium.
The mains safety ground is not connected to prevent import of mains related noises.
At the back of the radio table is an aluminum strip which functions ar grounding rail. It is connected to the grounding point of the incoming coaxial cables.
The length of the connection between the rail and the grounding point at the roof is as short as possible (a few meters). Both measures are effective to achieve "cold" (RF-free) equipment housings