Introduction |
New transmitters don't seem to need a filter on the output, but it is often needed with older equipment. There has also been a surge of hams building new/old equipment which might need a filter on the output. So here are a few designs that could provide you with the needed filtering.
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HF Low-Pass Filter - Radio Amateur Handbook, 31st Edition, 1954, pp504-507 |
As the heading indicates, this Low-Pass Filter is included in the Radio Amateur Handbook, 1954, 31st, edition. It is a three section filter that contains two full Constant-k sections and two m-Derived terminating half-sections. There are five configurations included, three for 52 Ω and two for 75 Ω. In the schematic on the right, there are buttons for each configuration. Selecting one of the buttons will adjust the annotation on the schematic.
The inductors are marked with a Referenence Designator, number of turns, and calculated inductance. I used my Air-Core Inductance Calculator to calculate the inductances. All inductors are air wound with #12 or #14 Wire, 1/2" Diameter, and 8 TPI. The wire can be bare or enameled. It is usually easier to first wind the inductor tightly on a form (1/2") and then spread the windings until the proper length is reached.
For some of the configurations, the capacitor values listed are made up of two capacitors in parallel. For example, the 170 pF capacitors are made up of a 100 pF and 70 pF capacitor in parallel. The 120 pF capacitors are made from a 70 pF and 50 pF capacitor in parallel. The 150 pF capacitors are made from a 100 pF and 50 pF capacitor in parallel. As mentioned later, the capacitor leads need to be kept a short as possible.
The 154 pF and 106 pF capacitors can be variable capacitors that are adjusted to the specified value.
The filters are designed to use mica capacitors of readily-available capacitance values. I redrew the filter schematic and included shielding that should be placed between the sections. The filter should be built in a metal enclosure, complete with a bottom and top cover. An example of an enclosure and layout is on the left. The enclosure is sectioned into three compartments. In the example, the section dividers are angled, which minimizes any wasted space. J1, L1, C1, and L2 are mounted in the first compartment. L1 and L2 must be mounted perpendicular (90°) to each other, with L2 connected to a feed-through on the first divider. C2, L3, and C3 are mounted in the second compartment. L3 should be attached to a feed-through on the first and second section dividers. The third compartment contains L4, L5, C5, and J2. As with the first compartment, L4 and L5 must be mounted perpendicular (90°) to each other, with L4 connected to the feed-through on the second divider. The capacitors, C1, C2, C3, C4, and C5 can be mounted on the dividers and grounded very close to the capacitors and feed-throughs. The inductors should be mounted on ceramic stand-offs, about 1" high. When the bottom cover is attached, the entire enclosure should be RF tight.
The input combination, L1/C1, and output combination, L5/C4, form traps that produce a very deep attenuation spike that starts around the FC listed and is a maximum, <-85 db, near 50 MHz. Beyone 50 MHz, the attenuation rises, but never rises above -50 dB. The attenuation in the 54 - 88 MHz range ( TV, VHF-Lo, Ch 2-6) varies from over -50 dB to nearly -70 dB, depending on the frequency and the particular set of values used. Above 174 MHz ( TV, VHF-Hi and UHF) the theoretical attenuation is better than -85 dB, but will depend somewhat on internal resonant conditions associated principally with the lead lengths to the condensers. These leads should be kept as short as possible.
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VHF Low-Pass Filter - Radio Amateur Handbook, 31st Edition, 1954, pp504-507 |
The schematic for the VHF Low-Pass Filter looks just like the HF Low-Pass Filter. The only real difference is with the L/C values and the physical construction.
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