Having an interest in VHF National Field Day, I made a transverter for 23 cm. This works very well but at some sites there is a fair amount of commercial interference, plus the use of multi- bands on Field days. I decided to look at a bandpass filter I could make which would give both low insertion loss with good out of band rejection, and could be in line on transmit or receive.
The design I have used is not original. I thought it would be as well to use a tried and tested arrangement as I have limited test equipment to set up this filter at 23 cm. Ideally a network or spectrum analyser would be very useful, but my bank account will not stretch to such a piece of kit at this time.
Fig.1. The interdigital filter constructed with aluminium
The construction method I have used is in aluminium bar and plate.
Fig.2 should help with the description.
The main frame is constructed using 1 inch X 1/4 inch (25 mm X 6 mm) aluminium bar. To this rectangular frame, the two side panels which are made with 2.75 inch X 1/8 inch (70 mm X 3 mm) aluminium, and are attached with M3 set screws into holes drilled and tapped along the top and bottom edges of the frame.
Note the number of screws on each side, 16 in each side panel, to make sure of a good electrical shield at this high RF frequency, and to improve stability.
The input / output connections are made with flange mount ' N type ' connectors, which just fit the 1 inch size of the frame. They are also attached with M3 set screws into threaded holes.
The resonant elements are made with 1/4 inch (6 mm) aluminium rods 50 mm in length, tapped and bolted to the frame. The adjusting screws are M5 bolts with a lock nut to tighten up when the resonant frequency is found.
The coupling loops on the ends, are short lengths of 1.5 mm diameter silver plated copper wire, soldered to tags under the first and last inductive elements, and soldered to the centres of the ' N ' connectors.
Fig.2. Inside the Band-pass filter
As you can see from the inside view of the filter it has five tuned circuits. It has low insertion loss, less than 1 dB; and a has been designed as a broad band filter for transmitters, but it does have good rejection with low ripple throughout the 23 cm band.
Fig.3. The frequency response of the filter tuned for 23 cm
The above shot of the frequency response of the filter shown with a vertical scale 10 dB/cm. The horizontal scale is 50 MHz/cm. The attenuation at 1268 MHz is greater than 40 dB.
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