50MHZ TVI Filter
The second and higher harmonics of six-meters transmitter fall directly into the FM and UHF and VHF television bands. An effective lowpass filter is required to adequately suppresss unwanted emissions falling in these bands.Described in this section is a six meter TVI filter rated at the two-kilowatt level which provides better than 75 decibeles suppression of the second harmonic and better than 60 decibels suppression of higher harmonics of six-meter transmitter. The unit is composed of a half-wave filter with added end sections which are tuned to 100mhz and 200mhz.An auxiliary filter element in series with the input is tuned to 200mhz to provide additional protection to television channels.
C1-50_pf Centra 1/lab 7505. Resonates with L1 to 200mhz
C2 C7 4pf Piston capacitors
C3 C4 C5 C6-60pf. Three 20pf capacitors in parallel.
L1-Copper strap, .5" wide 2,25" long 17/8" brtween mounting holes. approximately 0.01" thick. Strap is bent in U-shape around capacitor and bolted to capacitor terminals
L2-11 turns #18 enam wire .25" diameter, .75"long, airwound. Resonates to 100mhz with capacitor C2
L3 L4-3 turns 3/16" tubing, 1/4"i.d., spaced to occupy about 2.5".Turns are adjusted to resonate each section at 50mhz
L5-6 turns #18 enam wire 1/4" diameter, 5/8"long, airwound. Resonates to 200mhz with capacitor C7
Six-Meter TVI Filter
The filter is built in an aluminum box measuring 4"x4"x10" and uses type-n coaxial fittings.The half-wave filter coils are wound of 3/16" diameter copper tubing and have large copper lugs soldered to the end.The 60-pf capacitors are made up of three 20-pf, 5kV ceramic units in parrallel.A small sheet of copper is cut in triangular shape and joins the capacitor terminals and coil lug is attached to the canter of the triangle with heavy brass bolts.
The input compartment of the filter is at the left.The series coil is wound of copper tubing, and the connection to the output section (right) is made by a length of tubing which passes through a hole in the center shield.Series elements carry less current and employ wirewound coils.At right is antenna relay, with power leads bypassed as they leave the filter compartment.Filter is set to correct frequency by adjusting the inductance of the tubing coils.
The parallel-tuned 200mhz series filter element at the input terminal is made of a length of copper strap shunted across a 50-pf, 5kV ceramic capacitor.In this particular filter, the parallel circuit was affixed to the output capacitor of the pi-network tank circuit of the transmitter and does not show in the photograph.
The filter is adjusted by removing the connections from the end of the half-wave sections and adjusting each section to 50mhz by spreading the turns of the coil with a screwdriver while monitoring the resonant frequency with a dip oscillator.The next step is to ground the top end of each series-tuned section (C2,L2 and C7, L5) with a heavy strap.the input section is tuned to 100mhz and the output section to 200mhz.When tuning adjustments are compleated,the straps are removed and the top of the filter box is held in place with sheet metal screws.
Home