Australian Amateur Radio Station
|(TVI Filters for 2m and 70cm)
The TVI filters described below worked very well at my old QTH in suburban Melbourne, in curing
a TVI problem on 2m. that I had with a neighbour. The 432 MHz notch is an addition to the basic
144MHz Band-Stop filter.
The attenuation in the 2m band was >80dB!! This should cure ANY TVI situation caused by
"Front-End Overload". It is important that the filter be placed ahead of any preamplifiers on the TV
set. It worked very well for my neighbour's TV set.
Several units were built in diecast enclosures and "tuned" for maximum rejection with the use of a
Spectrum Analyser. 75 ohm male and female connectors were used on the input and output so
the unit could be "plugged" in-line without the use of an additional "patch lead".
|Schematic with 432MHz added
|The 432MHz Band Pass tuned circuit consists of a loop of wire as shown below that is
tuned with a Yellow "Phillips trimmer" that has a maximum capacitance of 10pF. The wire is
approximately 20mm long and sits about 3mm above the ground plane.
The 144MHz parallel and series tuned circuits are as above for the 144MHz filter.
Placing some shields as depicted in the photographs below increases attenuation because
of increased isolation from input to output.
2 techniques are shown: ABOVE: "ugly style" with components soldered
directly above a ground plane and BELOW: the main signal path is etched
on the PCB. Both methods work equally well at these frequencies. It is
important to have the PCB in good RF contact to the "ground" path.
|Pictures of assembled units.