TVI Filter
Here is a design for a simple television interference
filter. It is a 5 pole High Pass filter with an optional braid break to reduce common mode
transmission. Attenuation in the VHF TV band (50 MHz to over 200MHz) is typically between
0.5 and 1.5dB. Attenuation at 14 MHz is typically over 50dB. It can be used to fight TVI
from any HF transmitter system into a VHF TV.
IT WILL NOT STOP HARMONICS FROM THE TRANSMITTER OR ANY
INTERFERENCE AT THE TV FREQUENCY.
I have not been able to measure common mode (braid
breaking) attenuation. The braid is the most common route for TVI from short-wave
transmissions ( HF )
The Board is made of single sided PCB material, and either
etched or cut to leave copper as shown. You can omit C4 and R1, and link them out for a
High Pass only.
Typically the loss is 0.3 to 0.8 dB and very flat. You
will have no braid break capability, so ferrite rings can be added if needed. With no C4 /
R1 a metal enclosure is OK
Layout / board (actual size 35 x 25 mm approx.)
Circuit Diagram 
Alternate Diecast Housing
Component values:- C1=C3= 3.5 to 20pF C2=5 to 30pF C4=5 to 30pF Trimmer
Capacitors
The components I use are available from Altronics stock no R 4005 for the
20pF trimmer and R 4007 for the 30pf trimmer, F1 + F2 are L 4820A - Ferrite
Sleeve Cores OD 14.3mm ID 7.3mm Length 28.8mm Permeability 1,000 Ui
R1 is 1MOhm 1/4 W..not critical.
L 1 & 2 are 5 turns 0.5mm Cu wire 5mm OD, approx. 15m long (i.e. wide
spaced turns) They are mounted at 90 degrees to each other. To reduce stray coupling.
The coaxial cable is soldered directly to the board, and the lead to
the TV should be less than 150mm long, the shorter the better.
The filter MUST NOT be placed in a metal housing if you use the braid
break, else the braid breaking ability will be lost. You can use 25mm white plastic
tubular conduit, It is OK to trim the board a bit to make it fit, and you can squeeze the
tube to allow the board too sit in tightly.
Or If you use a Diecast boxe then cut Double Sided PCB to make an
Isolating section for each section of the filter F1, L1/C1,C2,L2/C3,F2 so you will end up
with 5 seperate isolated boxes inside the Diecast Box like a VHF reciever IF filter
section, you can add an RF ground to the Case.
The leads from the filter make one 150mm and the other 1500mm test
which end works best connected to the TV the shortest lead should give best isolation.
For extra mains protection add a ferrite ring to the mains cable with
the cable wound around the ferrite and entering and exiting opposite, this may require
replacing the cable plug to allow the cable to be inserted through the ferrite core.
With the component values as shown the loss in the TV band will be
minimal and you can adjust the trimmers for best Channel 2 reception. You can expect 50dB
attenuation at 14MHz. The filter effectively stops anything below 30MHz from coming down
the coax.. It may ever completely cure the interference problem.. If you are lucky.
If a second filter is required in series with the first, it is likely
that the problem is common mode. That is to say the unwanted signal is coming down the
outer of the coax. Ferrite rings are a good addition to the braid breaking capability of a
system. Wrap a few turns of the coax around a torroid , ensuring that the cable in and out
of the ring are kept separate.