A Mode J Desense Filter

This type of filter can be used in a Mode J satellite station (uplink on 2m, downlink on 70cm) to reduce the problem of the uplink signal desensing the receiver. This filter has low insertion loss (typically less than 0.5 dB) in the 70cm band and good rejection (typically over 50 dB) in the 2m band. Ideally, this filter should be inserted between the 70cm antenna and the 70cm preamp. Transmitting through this filter is not recommended, so it should be removed or bypassed before transmitting on 70cm.

1 Pipe, Copper
3 inch diameter, 5 inches long. Cut ends square. Drill or punch for connectors 3.75 inches from the bottom.
2 Pipe, Copper
0.75 inch diameter, 4 inches long. Solder to the center of 10.
3, 4 Disc, Copper
0.75 inch diameter, 0.0625 to 0.125 inch thick. Drill through center. Solder solid hookup wire between disc and connector to space disc 0.1875 inches from 2.
5, 6 Connector, Coax
BNC, SMA, or N type. Solder to prevent turning. For large connector use a chassis punch. Mark one connector In or Antenna and the other Out or Preamp.
7, 8 Nut, Brass
1/4"-20 hex
9 PC Board
Double sided, 4 inches by 4 inches. Drill hole in center to clear 1/4"-20 bolt. Solder 7 and 8 on each side of the hole. Use bolt 11 to hold nuts in place while soldering.
10 PC Board
Double sided, 4 inches by 4 inches. Solder 2 in center.
11 Bolt, Brass
1/4"-20, 3 inches long. Insert through 12 then through 7 and 8. A 4 inch brass threaded rod may be used for a wider tuning range.
12 Locking Nut, Brass
1/4"-20 hex. To hold 11 after resonance adjustment

Mark the filter's antenna and receiver coax connectors, as they must always be connected the same way once the filter is tuned. Upper frequency roll-off will be near the rejection tuning peak. Check the rejection bandpass and adjust the 2 meter frequency used for tuning if this bandpass does not cover the entire 70cm satellite downlink band segment.

Adapted from The Radio Amateur's Handbook, 1980. Original drawing by AB1P. Additional comments by Howard Sodja, W6SHP. HTML conversion by and feedback to KB5MU.