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
- 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
- 10 PC Board
- Double sided, 4 inches by 4 inches. Solder 2
- 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
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
Adapted from The Radio Amateur's Handbook, 1980.
Original drawing by AB1P. Additional comments by Howard Sodja, W6SHP.
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