A 50MHz High Pass TVI Filter
January 1992Six News

Are you suffering from TVI? Then why not try this filter for size. It is a three-stage high-pass filter that blocks low-frequency 50MHz signals while letting through UHF TV.

It has been designed to be placed in the 75 Ohm coax between the aerial and television set or (masthead) pre-amp. In its current form it cannot be placed between the television and pre-amp as this cable usually carries +12V DC to power the preamp. A look at the circuit diagram will tell you that it cannot pass a DC current!

Figure 1 - The Circuit Diagram

The parts required to build the filter are as follows:

• A small piece of 0.1" pitch Veroboard as shown in Figure 2.
• Three inductors: 3 turns 26swg, 3mm dia. close wound.
• Four 3.3pf ceramic capacitors
• 30 cm of 75 Ohm TV coaxial cable
• 1 Belling Lee aluminium coax plug
• 1 Belling Lee in-line aluminium socket
• 2 3/4" tap washers
• 10cm length of 20mm plastic water pipe

Figure 2 - The Circuit Board

The circuit board is made from a small piece of 0.1" Veroboard with the centre tracks removed with a 5mm drill. The connection pads are isolated in the same way. MAKE SURE THAT

THERE ARE NO SLIVERS OF TRACK LEFT IN THE MIDDLE OR ON THE EDGES OF THE BOARD. These will act as capacitors bypassing the filter elements and thus ruining the attenuation of the filter. The component layout is shown in Figure 3. Ensure that all the leads of the components are as SHORT AS PHYSICALLY POSSIBLE.

Figure 3 - Component Layout

As far as mechanical assembly is concerned, thread the two short lengths of 75 Ohm coax through the two tap washers (middle holes enlarged) and through the inside of the plastic tube. Push the two washers into either end of the tube by 3mm and fill the ends of the tube with Araldite to seal against moisture ingression. Use tape as well if used outdoors.

Figure 4 - Mechanical Assembly

If the filter is to be used internally, connect the in-line plug and socket to the ends of the leads. If it is being used at the masthead (the best place) make the leads 1 metre in length and connect between the aerial and the pre-amp input.

The measured attenuation of 50MHz signals is 70dB on the inner core. You might need to use a braid breaker as well in some circumstances. How one of these is designed and built will be shown in a future SIX NEWS.

Good DXing while free from TVI!

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