RF Transfer Switch

In talking with Bob, KB0**[1], 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.

A quick look online after getting home got me to HRO’s website seeing an MFJ transfer switch, their MFJ-1703B. In talking a bit further with Bob on the phone later, he also did not have a way to measure his output power or confirm that he was putting power out (sort of okay with the QRO rig as it has an TX indicator, but there is no indication with the QCX+). He also did not have a dummy load. I was actually heading to Denver the next day to take my wife up for one of specialist medical appointments. I told Bob with his permission in keeping the functionality high and costs down, that I would pick up a Transfer Switch, a Power Meter, and a Dummy Load. Bob was good with that.

I picked up the following items for Bob the next day when in the Denver area at HWO: Diamond SX-200 SWR and Power Meter (which is nice since having 5W, 20W, and 200W scales….just about perfect for him along with not being too expensive), MFJ MFJ-260C Dummy Load (was an inexpensive dummy load that was on-hand). The one thing that HRO did not have in-stock there in Denver or any other store was the MFJ-1703B Transfer Switch. So, the MFJ-1703B Transfer Switch was back-ordered (B/O) and to be shipped to me.

The next morning, I realized that I really did not like the “form factor” of the MFJ-1703B Transfer Switch as it had SO-239 connecters on each of the four sides which would be a nightmare (and quite ugly) for the cabling. I called HRO and cancelled the B/O. There was a better way – I will make and RF Transfer Switch for Bob.

I already had some good slide switches (new ones for another project, but I always get a few extra pieces when ordering). I only had a few BNC chassis mount connector and needed more. I had quite a bit of newish RG-316 in my cable bin (for some test cables and other cabling needs). What was needed was a small metal box. I did not want to use SO-239 connectors as they take up a lot of panel space which would drive the box size – my goal was to get this unit small and very usable for Bob, so I elected to use BNC connectors (which are quite good handling QRO and higher power levels). So, some BNC chassis mount connectors and several small boxes were ordered. They arrived several days later. The box chosen from the 4 ordered was a rather small silver-colored Eighthwood box measuring 4.32 x 2.82 x 1.13 inches (LWH).

[1] The name and call of “Bob” and “KB0**” are fictional and being used to protect the actual person’s identity. Yes, this is the same person as referred to in the previous page.

Now, I seemed to have everything to fabricate an RF Transfer Switch for Bob.

Parts list:

Qty P/N Description Source

1 X000SU66MT Eighthwood Aluminum Box Electronic Amplifier Amazon

1 GF-326-0000 CW Industries, Switch Slide DPDT 3A 125V DigiKey

2 unknown No. 4 hardware – screw, washer, nut junk box

1 unknown Solder Lug junk box

4 unknown QMseller BNC Chassis Panel Mount Socket Female Jack Solder Type Connector Amazon

4 RG-316 Coaxial Cable unknown

Note: could use RG-58 although bigger and probably harder to deal with in a small area

8 unknown 1/8” Heat Shrink, about 0.25 – 0.30 inches long unknown

8 unknown 20 – 22 ga. Wire, various lengths as required (coax shield returns) unknown

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 Tranfer 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:

I also wound-up fabricating some new RF cables for Bob, ones that were mainly to/from the new RF Transfer Switch, and made with RG-8 cable and crimp-on connectors (Note: I have been fabricating cables for many years transitioning to mostly crimp connectors about 20 years ago for RG-8, RG-8X, RG58, and RG-316 types of cables. I have been mostly using DX Engineering’s DXE series of crimp connectors for RG-8 and RG-8X for the past several years now. There are still times when a solder-on connector is used though.):

  • 1m cable with a BNC on one end and a BNC on the other end (connecting the RF Transfer Switch’s QRP connector from QRP Rig’s RF connector)

  • 1m cable with a BNC on one end and a PL259 on the other end (connecting the RF Transfer Switch’s QRO connector from the Tuner’s Ant connector)

  • 1m cable with a BNC on one end and a PL259 on the other end (connecting the RF Transfer Switch’s Ant connector to the SWR/Power Meter’s TX connector)

  • 2m cable with a BNC on one end and a PL259 on the other end (connecting the RF Transfer Switch’s DL connector to the Dummy Load’s (DL) RF connector). The 2m length was done to get the Dummy Load off of the operating position and down to the floor where it is out of the way

There were already two (2) cables there and used as noted:

  • 1m RG-8 cable with a PL259 on one end and a PL259 on the other end (connecting the Tuner’s TX connector from QRO Rig’s RF connector)

  • 2m RG-8 cable with a PL259 on one end and a PL259 on the other end (connecting the SWR/Power Meter’s Ant connector to the window junction connector going out to the antenna)

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.

The end result is both radios, the QRO and the QCX+, is each connected to a load. Therefore, no open connections exist for a rig to go into transmit which is critical for the QRP Labs’ QCX+ rig or any other rig not having transmit power foldback protection when seeing high VSWR.

The Switch's connections are with the following:

  • Slide in the “up” position labeled with “QRO -> Ant", and "QRP -> DL”

    • The QRO rig is connected to the Antenna

    • The QRP rig (the QRP Labs’ QCX+ in this case) is connected to the Dummy Load

  • Slide in the “down” position labeled with “QRO -> DL", and "QRP -> Ant”

    • The QRO rig is connected to the Dummy Load

    • The QRP rig is connected to the Antenna

Bob now seems to be quite happy with the new configuration - he does not have to fumble around trying to change-out a radio's cable being connected to his antenna. Whenever one radio is coupled to the antenna, the other radio is being coupled to the dummy load. Another benefit of the new configuration is he can now verify his transmit power from either rig -- whichever rig is selected for coupling to the antenna, the RF passes through the new Diamond SX-200 SWR and Power Meter.