Another Sad Story of Common Mode
12 Jul 2021
I have been running a KiwiSDR for a number of years since I arrived on Orcas Island. When it was first set up with a nice Beverage antenna running East-West (thanks Teresia and Kyle!) many people commented on how useful it was and how well they could hear things on it.
The years have gone by, and for various reasons, the antenna no longer qualifies as a Beverage, just a random wire, and it seemed like the SDR wasn’t working too well anymore. Doing a quick check seemed to indicate that it only seemed to be receiving on the lower bands and MW/LW.
One of the fascinating things about the KiwiSDR is that it seems to be half test equipment and half radio. The built-in SNR measurement capability seems to be taylor-made to drive people crazy trying to improve their “scores” and move up the list (although I’d tend to remove any sites that do not filter out the MW band, because all of those nation-wide advertising emissions just drive up the “S” in “SNR”).
Since I’m planning to do a more “permanent” installation of a new Beverage antenna this fall, and I also have a brand new, never installed Wellbrook active loop antenna, on this past weekend I decided it was time to bring in the feedline and see what shape it was in.
Basic continuity tests showed that things were in order. Shield resistance of the ~400 feet of RG-6 equivalent was ~0.1 ohm. Center conductor to center conductor was ~1.3 ohms.
This is where we use the receiver to check the cable. The waterfall display for the KiwiSDR is amazingly useful for this. We set the “span” to 0-4 MHz and then check if we see anything. Considering we have a feedline connected to nothing, we shouldn’t see anything. Reality looks a bit different:
|Open feedline, common mode choke|
|Terminated feedline, common mode choke|
As usual, this probably boils down to common mode noise getting introduced into the system. As the feedline is also very long, and our ground here on “the rock” is very poor, it’s acting as an antenna too.
When all cables are removed (“No input” above), we don’t see any signals, so that rules out ingress via power, Ethernet, or GPS Antenna lines.
The biggest difference seems to come from adding the common mode choke, which increases the impedance along the shield of the coax to non-differential (i.e. common mode) signals. This greatly reduces the endless amounts of EMI/RFI “cruft” from stuff in the shack getting out anywhere near the antenna end of the feedline. Even so, a few of the local MW broadcasters and a lot of LW “hash” can still be seen. Really serious persons would introduce a second common mode choke at the antenna end of the feedline and perhaps add a separate ground connection there too.
The next test will involve replacing the ~400 foot feedline with one that is no more than ~200 feet long, as this is the limit for powering the Wellbrook loop via the coax. Also, it would probably be good to get some serious bonding and grounding going on in the shack too.