| Transmission Line Filter |
|
A damn nice, high-power low-pass filter, made mostly
out of hardware
store parts. A good thing to have around if you happen incidentally to
be making unwanted RF power above, say, 176 MHz.
Figure 1. Finished product.
Figure 2. Interior construction.
Figure 3. Frequency response. The yellow line is measured, the blue
line
is calculated.
Loss in-band is about -0.15 dB.
The filter consists of a series of transmission lines of precise electrical length and impedance. Common to all segments is the outer conductor, which is one long piece of 1" "red" copper pipe. The inner conductor is one long piece of #12 solid copper wire with segments of 1" OD brass cylinders strung on the wire like kabobs and soldered in place (with a torch). The wire and brass cylinder assembly is fit into the copper pipe, suspended only by a rolled-up sheet of teflon. UHF connectors are mounted in standard copper pipe endcaps, which are fit on the two ends of the pipe, completely shielding the assembly.
The "red" ("type M") copper pipe has an ID of 1.055". The brass cylinders are 1" OD. The teflon sheet is specified to be 0.020", although we measure it to be about 0.021", so when it's rolled up inside the pipe, the resulting ID is just about 1.013". With some woggling, the brass just barely fits inside. We ordered the teflon from Ridout Plastics as "TEF NAT 0.020 - 1801." We probably got 3 square feet. See:
http://www.ridoutplastics.com/teflonsheet.html
The total length of #12 wire is 26.649". The brass cylinders, drilled down the center with a #44 bit, are threaded on and spaced along the wire such that the lengths are as follows:
Wire
2.071" 176.1 ps
Brass 1.000" 107.5 ps
Wire 5.848" 497.4 ps
Brass 2.000" 215.1 ps
Wire 4.811" 409.2 ps
Brass 2.000" 215.1 ps
Wire 5.848" 497.4 ps
Brass 1.000" 107.5 ps
Wire 2.071" 176.1 ps
-------------- ---------
Total 26.649" 2.402 ns
The wire sections have a characteristic impedance of about 153 ohms, and the brass sections have a characteristic impedance of about 2.53 ohms.
The UHF connectors
at the ends are
Amphenol 83-878, which are rear
mount
so they can be soldered to the #12 wire before the endcaps are slipped
on. The pipe length is trimmed so that tightening the nuts on the UHF
connectors
just puts a little tension on the wire, holding the whole arrangement
firmly
together. Once finished and tested, the endcaps should be soldered on
to the 1" pipe to keep the ground connection solid.
The filter can probably handle about 1000 W.
Figure 4. Filter topology and approximate discrete equivalents.
Figure 5. Cross sections.