The BCC Joker Switch

Why this reprint? You may get some goals here:

Download PC boards ready to foil print
Download complete data sheet of the relay used
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This reprint is made under kindly permission of the author, Thomas Moliere, DL7AV

[reprinted from the CQ Contest November 1998, p. 8-13]

The BCC Joker Switch
Here is another weekend project sponsored by the Bavarian Contest Club
By Thomas Moliere DL7AV, AL7IB e-mail: DL7AV@online.de

This universal switch is simple but usable up to 450 MHz, and it can do several things to transceivers such as inserting a preselector or a low-noise amplifier during receive, switching between different antennas, or inserting special filters during transmit. It has an incredibly low insertion loss of only 0.05 dB at 28 MHz and a TX/RX isolation of 45 dB. The relay used can stand several kilowatts at HF.

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.

Most high-end transceivers have the connectors to allow the insertion of a filter between the T/R relay and the RX input. But the state-of-art compact transceivers don’t. They don’t even have space for additional connectors. This construction article opens a way to insert accessory boxes such as preselectors (CQ Contest, November 1996, pages 10-14) into the receive line of any transceiver without disturbing the transmit function, or into the transmit line without disturbing the receive function, both without modifying the transceiver. The principle is an external relay switch driven by the T/R signal which is identical to the PTT signal accessible on the transceiver. s rear cover. This is a nice weekend project.

Circuit Description

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.

Supply voltage	0	+12V	0	0	+12V	+12V
PTT input	open	open	0	+12	0	+12V
Resulting relay position	TX	TX	TX	TX	TX	RX

Table 1. The TX/RX truth table.

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.

Applications

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 2. Adding a preselector to the receiver.

Figure 3. Adding a preamplifier to the receiver.

Figure 4. Adding a second receive antenna input.

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.

For the transmit application as shown in Figure 7, the inverter option has to be used. The maximum power figures shown in Table 2 were calculated assuming 3 watts total dissipation (relay, printed circuit board [PCB] line, and connector losses) with a 25% duty cycle of the transmit signal. The S2 Matsushita relays can certainly stand more power than the epoxy board. The PCB is the critical part. Overheating the PCB during soldering might leave small, carbonized dark spots which could turn into dangerous hot spots during high-power RF exposure. Therefore, do not use the PCB version in Figure 10 for 1500 Watt RTTY applications. In the high-power cable version using two parallel-connected relays as shown in Figure 11, the PCB is only used for the logic circuitry. Sophisticated soldering is necessary for this one, though. The relay contacts are connected by copper band. Although a VHF version was built using A and BNC connectors, the insertion-loss figures at 144 and 440 MHz were measured using the PL connector version. Amazingly, the VHF version was not so much better than the PL version; insertion loss at 144 MHz came out as 0.13 dB compared to 0.18 dB.

The Relay

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.

Mechanical Design

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 13. The ground metal sheet. DC Connectors

The back view shown in Figure 14 (sorry, not included here) also shows the DC connectors. A 3.5 mm open mono jack chassis socket is used for the TX/RX signal input and two 2.5 mm power sockets for the power supply. The spare power connector can be used to connect additional hardware.

Figure 15. Printed circuit board (component side)

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.

Figure 17. Component placement drawing.

Availability

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).

Figures not included: 10, 11, 12 and 14 – photographs showing construction details of the PC board and cabinet. Apologies, not scannable from my copy of the original. I believe they are not so important.

Appendix – Matsushita S2 Relay