RF Transfer Switch
In talking with Bob, KB0**, when returning his QRP Labs QCX+ rig (having just replaced the “blown” finals as described in the previous page entitled "QCX+ with Blown Finals"), the thought appeared that there needed to be a solution for preventing him from transmitting in to a “no load” condition in the future. I had seen him “in action” previously when there in his shack reaching behind his operating position to change out the cable going to his antenna from one radio or another. Something “told me” this cable change-out was the culprit. That is what got me to thinking of a transfer switch. Frank needed something where his QRO rig was always connected to either his antenna, or to a dummy load. Conversely, he needed the ability to have the QCX+ always connected to the dummy load or to the antenna.
The schematic for the RF Transfer Switch is with the following:
Getting the front and rear panels setup. There was still some filing to be done for the BNC chassis mount connectors (making the holes oblong to fit the flattened side of the connectors - I wanted to be able to secure each connector without having to grip on the connector end when tightening the connector's nut).
Getting the RF Transfer Switch built....
Sweep from 1.6 – 30.0 MHz. The RF Transfer Switch appears to be getting from about 45 dB port isolation at 1.6 MHz to about 32 dB port isolation at 30 MHz. I would have liked the port isolation to have been a bit higher overall; it seems to be “good enough” especially exceeding 40 dB around the 7 MHz area. Point of reference is the Spec An’s Tracking Generator being set for a -20dBm level.
Getting 42.9 dB port isolation on the 40m region.
The switch was also tested using 100 W output power for both the Antenna (Ant) and the Dummy Load (DL) ports; no issues were seen.
The interconnection diagram for Bob’s situation, one that may align with many others wanting to switch either a QRO or QRP rig to a single antenna is with the following:
Bob has been locating the RF Transfer Switch on the left side of the new Diamond SX-200 SWR and Power Meter which is almost the same height and handy there in his station for selecting what is connected to the antenna.