Milazzo, Carol F., WB2OZA, "A Simple Bandpass Filter for 439 MHz", A5, Vol 5:3, May-June 1975, p. 30. A tunable stripline filter in a Minibox for ATV.

When I first got my commercially made preamp to pep up the sensitivity of a T44 receiver I turned my antenna towards the southwest to try to see some of the ATV activity in New Jersey. Instead, the receiver was saturated with 40 microamps of hash. I found that the problem was caused by the strong channel 47 TV signal from the Empire State Building blasting through the front end with the help of the preamp. The signal was then beating with the 24th harmonic of the crystal frequency and saturating the IF. Other ATV ops in the area have complained of the same problem.

Apparently there is not enough Q in the input circuits of these preamps to prevent the amplification of undesired signals. Having no access to fancy metal-working equipment and limited budget, I decided to build a low-cost filter with locally available parts. The result was a loop-coupled tunable reentrant cavity resonator for about $2.50. The filter is of simple construction and is quite effective.

At the local radio parts store, you can get a 5x2 ¼x2 ¼" aluminum minibox and two connectors for input and output. I used single hole RCA-type jacks. The local plumbing supply store can supply ½" O.D. copper tubing. Only 5 ½" of this is needed, but you may have to buy at least a foot, which will leave plenty for building another filter. Metal disks are needed for the variable capacitor. My local hardware store has some 1-¾" dia. disks of this steel with a shiny plating. These disks are normally used for nailing shingles to roofs. They are very easy to solder and two of them make an excellent capacitor for the filter.

The tubing should be straightened by careful rolling on a flat surface with pressure applied. Be careful not to scratch the copper. With a fine saw blade, cut a 1 ½" and a 4" length of tubing. The cut edges should be filed until even and smooth. Both ends of the 4" tubing should be brushed with steel wool, and tinned with solder. The 1 ½" piece is slit down one side and opened, and then it is pounded flat. The result is a 1 ½" x 1 ½" square copper plate. The edges are filed to remove burrs, and the plate is polished with steel wool. Four 1/16" holes are drilled in the plate on 1-1/8" centers. The burrs are removed from these holes and the center ¾" of one side of the plate is tinned with solder.

The head of a 1" 6-32 flat head machine screw is tinned and soldered to the center of one of the disks. The minibox is then drilled as shown in the diagram. The four holes in one of the small ends of the box should match up with the four holes in the copper plate. The hole in the other end is carefully threaded with a #6 self tapping screw, and then the screw with the disk is threaded into this hole from the inside of the box. Two #6 nuts are threaded on the free end of the screw. One nut is soldered to the end of the screw, the other is used as a locknut. The two connectors are mounted in their holes. One end of the 4" tubing is soldered to the center of the square plate and the other disk is soldered to the other end of the tubing. The tubing is then mounted in the box using 4 #4 self-tapping screws. Two 2 ½" lengths of bare #14 or #16 copper wire are prepared and formed as in the diagram. These are soldered from the connectors to the copper plate spaced about 1/8" on either side of the tubing. These loops are bent so their vertical portion runs parallel to the tubing. The filter in now complete.

The filter is installed between the antenna relay and the preamp. The filter is tuned for maximum received signal strength, which occurs at a capacitor spacing of about 3/8" at 439 MHz. When it is tuned correctly, the locknut is tightened just enough to prevent slippage.

The filter has reduced the interference here to less that one microamp on the second limiter with hardly any degradation of ATV signals. It also worked well in the transmitter output, and when tuned to resonance an insertion loss of about 1 dB was measured using a Bird wattmeter.