Why this reprint? You may get some goals here:
- Download PC boards ready to foil print
- Download complete data sheet of the relay used
- Get more construction details
[reprinted from the CQ Contest November 1998, p. 8-13]
Here is another weekend project sponsored by the Bavarian Contest Club
By Thomas Moliere DL7AV, AL7IB e-mail: DL7AV@online.de
To Connect Additional Circuitry to Transceivers
Some transceivers require additional circuitry. A low-loss amplifier might be added to an insensitive transceiver, the receiver might need a preselector to cope with strong broadcast interferrers, a low-pass filter can be added to the transceiver path in case of multi-band multi-operator contest effort.
Circuitry is quite simple, as shown in Figure 1. A DPDT relay is used to feed through the transmit signal and to open up the antenna line during receive. The circuit had to be designed carefully, though. It has to make sure that the transmit path will never be interrupted under two conditions: (1) no supply voltage and (2) no PTT connection. The “truth table”, Table 1, confirm this.
|Resulting relay position||TX||TX||TX||TX||TX||RX|
Figure 1. Circuit diagram of the switch.
The two transistors used are low-power switching MOSFETs which are capable of withstanding the maximum supply voltage on their gates. The ON resistance has to stay below 3 ohms to drive the S2 relay used. The BSS100 transistors used here is a Siemens transistor with R DSON =1 ohm. A second stage is used to allow the inverter option for TX line switching by putting the wire jumper in the other position. The circuit input is connected to the transceiver’s PTT driving signal. Using an IC-706, it would have to be connected to pin 3 of its ACC1 interface connector.. Polarity is 0V for transmitting and >+6V for receiving. This seems to be standardized for all RF transceivers known to this author.
An incredibly large number of applications are possible (therefore it is really a joker switch!). They are:
- Preselector (Figure 2)
- Preamplifier (Figure 3)
- Second receive antenna input ( Figure 4 )
- Second antenna remotely controllable input for transceiver (Figure 5)
- Remotely controllable switch for three different antennas (Figure 6)
- Power amplifier (Figure 7) in case you have a PA without T/R relay
- Stopband filter switch (Figure 8), e.g. for M/M stations
- T/R switch for separate receivers and transmitters (Figure 9)
Figure 3. Adding a preamplifier to the receiver.
Figure 5. Remotely controllable antenna switch for two different antennas.
Figure 6. Remotely controllable antenna switch for three different antennas.
Figure 7. Adding a power amplifier to the transceiver.
Figure 8. Stopband filter switch for three different transmit filters.
Figure 9. T/R switch for separate receivers and transmitters.
Almost every 12V relay can be used with the circuit shown, of course. But the relay is the most important component of this project. The S2 relay is recommended here because it has excellent receiving and transmitting switching capability, and its switching speed of 7 ms is comparable to a vaccuum relay. It costs about 7 US $. Its maximum hot switching current is 5A while the continuous current allowed is much higher, so 1500 Watts is no problem for a single contact. At the same time, its properties for very low-level switching are excellent also. This is not self-evident, by the way. Some high-current relays exhibit occasional high insertion loss during receive. The S2 relay has been designed to handle an extremely large RF dynamic range.
The data sheet – click here.
Search for the S2-12V relay. This is a German language site. The data sheet should be easy to understand, though, English language sites should also exist.
As can be seen from Figure 12 (sorry, not included here), the board is directly soldered to the RF connectors. This minimizes RF losses. Fifty ohm stripline connections are used. Because of that, a double sided epoxy board has to be used. A disadvantage must be mentioned. The acoustic connection to the cabinet is also quite good. You can hear the relay clicking quite clearly. That is no problem for a remote switch. Soldering including the ground metal sheet should only be done after fixing the connectors to the cabinet. Two thin copper metal sheets as shown in Figure 13 are fixed under both TX connectors, and then bent and soldered to the boars. s bottom ground layer.
Figure 16. Printed circuit board (soldering side).
Download complete drawings of the PC board in TIF format here.
Details of the board layout are shown in Figures 15 through 17. The feedthrough holes are only shown in Figure 17.
In case of strong interest, the Bavarian Contest Club might offer a kit of the BCC Joker Switch. The cabinet used as shown in Figure 12 or Figure 14 (sorry, not included here) is of the same type as the BCC preselector, but half as wide. Parts of the BCC preselector are still available, by the way (requests to the author).
Appendix – Matsushita S2 Relay
Download complete English data sheet here.