Fig. 1.
For satellite QSOs, it is convenient to have full duplex mode. Duplex makes it easier to make QSOs. Especially the most difficult and interesting QSOs: when the satellite is low above the horizon and in DX pile-up.
But duplex mode requires very good decoupling between the transmitting and receiving antennas.
The decoupling problem becomes more complicated if the satellite uses a 145 MHz uplink and a 435 MHz downlink. Such satellites include the RS-44, SO-50 and similar satellites. The third harmonic of the 145 MHz transmitter falls into the 435 MHz receiver band. If the RX and TX antennas are located on the same mast (which is most often the case), the transmitter's third harmonic can interfere with or even block the receiver. Especially if the latter has a small dynamic range, e.g. RTL USB.
Therefore, for full duplexing in the feeder path of the 145 MHz antenna, a filter significantly suppressing the 435 MHz band is required.
Fig. 1 shows a schematic of such a filter. It will require three small sections of coaxial cables. Two from 50 ohm cable and one from 75 ohm cable. Fig. 1 shows the physical lengths of the cables with a velocity factor of 0.667. If you use cables with a different velocity factor, their lengths must be recalculated accordingly. The filter model file is in the RFSim99 program format.
A 75 Ohm section of cable is included between the filter input and output. Short sections of 50 Ohm cables are connected in parallel to the input and output connectors of the filter. The lower (in the diagram of fig. 1) ends of the 50 ohm cables are left unterminated. Note that the 10 kOhm resistors shown in fig. 1 are only used to simulate open end cable losses, but in reality these resistors are not present.
The characteristics of the filter are shown in fig. 2 and fig. 3.
The data of fig. 2 show that the reflection coefficient S11 in the 145 MHz band < 0.005, i.e. VSWR = (1+S11)/(1-S11) =1.01. In other words, the addition of the filter to the feeder path of the 145 MHz antenna does not degrade the VSWR of the 145 MHz antenna system.
Data fig.3 shows that in the band 435 MHz provides suppression of more than 70 dB, which completely solves the problem with interference to the receiver 435 MHz.
Constructionally, the filter is housed in a fully shielded and sealed brass box. No tuning is required, but be careful with the cable. The cable length includes its stripped leads.
I have such a filter was enough to use RTL USB + SDRConsole program as a duplex receiver 435 MHz, when using two Uda-Yaga antennas (145 MHz 7 el. H-polarization TX 100 W, 435 MHz 14 el. V-polarization RX), separated in height by 1 m.
The decoupling problem rarely happens with satellites with uplink 435 MHz and downlink 145 MHz (AO-7 and similar). However, if the 145 MHz receiver has a small dynamic range (e.g. RTL USB), the signal of its TX 435 MHz may still interfere with the receiving.
Fig. 4 shows a schematic of such a filter included in the feeder path of a 435 MHz antenna. The filter model file is in the RFSim99 program format. The filter will require three standard SMD capacitors and one small coil.
The filter characteristics are shown in fig. 5 and fig. 6.
The data of fig. 5 shows that the reflection coefficient S11 in the 145 MHz band < 0.02, i.e. VSWR = (1+S11)/(1-S11) = 1.05. Including the filter in the feeder path of the 435 MHz antenna does not practically decrease the VSWR of the 435 MHz antenna system.
Fig.6 shows that in the band 435 MHz provides more than 60 dB suppression. This solves the problem of interference to its 145 MHz receiver.
The coil is winded with 1 mm lacquered wire on a 6 mm diameter carcass and contains 3 turns. The length of the coil is 8 mm. SMD capacitors I used 0805 size. it was enough for 60 W TX 435 MHz power.
Constructively the filter is made in a small fully shielded and sealed box made of brass. Adjustment of the filter is made by carefully extending and / or pressing the coil to obtain a minimum in the band 145 MHz. Be careful not to tear off the metallization of the 51 pF capacitor during this procedure.
I have such a filter was enough to use RTL USB + SDRConsole program as a 145 MHz duplex receiver, when using two Uda-Yaga antennas (145 MHz el. H-polarization RX, 435 MHz 13 el. V-polarization and 14 elements TX 60 W), separated in height by 1 m.
