Transmitter Harmonic Filters

1. Low Pass Filters for 70 and 144MHz

The following 50ohm impedance filters are straight-forward to make and do not contain any high precision components. They may be constructed in die cast boxes (approx 4 x 2 x 1 inch) with internal screening sections, tin plate boxes (approx 4 x 1 x 1 inch) or boxes made from fibre-glass PCB material. The latter two options will require removable lids retained with tapped spacers and screws or right angle brackets and self tapping screws.

The design uses a four section constant-k structure with one section being 'm' derived to provide a steeper transition into the stop band. The capacitors shown are all 10% tolerance disc ceramic (not plate ceramics) or small wire ended or concentric silver mica types and have calculated values which may be made up of two (or three) similar value capacitors in parallel to reduce the lead inductance, increase the UHF attenuation and ease the problems of obtaining the final values. For example, 20pF may be made up of two 10pF or three 6.8pF units in parallel.

The tables below show the values and measured performance for each band. Values may be scaled for other bands of interest.

Values:
Band	C1 pF	C2 pF	C3 pF	C4 pF	L1	L2
70MHz	40	64	20	80	5t 0.375inch ID	3.5t 0.375inch ID
144MHz	20	32	10	40	4t 0.25inch ID	3t 0.25inch ID

All inductors are wound with 18swg tinned copper wire with turns spaced approximately one half the wire diameter. ID = Inner Diameter.

Attenuation Results:

Band	<0.2dB	-3dB	-50dB	-65dB Notch	-55dB stopband
70MHz	75	80	100	103.5	To 500MHz min
144MHz	150	160	192	205	To 500MHz min

All frequencies in MHz

The notch figures are typical of a production run and are determined by the adjustment of L2.

Each filter should be adjusted for minimum passband ripple and minimum loss at the frequency of interest by small variations of the turns spacing of each inductor. L2 may be adjusted to obtain the -50dB frequency performance.

Conventional 500v disk ceramic capacitors but not miniature or plate types should be good for at least 50 watts transmitter power output. Concentric feed-through capacitors mounted in the screens would be even better. Smaller values used in parallel to achieve the final value will increase the maximum power handling capability e.g. 4 x 10pF to obtain 40pF.