Mod Wars — USB charger blocking Ham Radio Communication
November 9, 2017
Having just installed a new ham radio setup to get APRS, I was keen to play with Kenwood TM-D710GA and Mobilinkd to track my location. So, on a few local trips, I turned everything on and waited.
And waited and waited. Over the course of 30 minutes, I never got a single APRS beacon from another station. I knew something was wrong.
And then I realized that the difference between my initial setup and my current situation was that the truck was on. Turning the truck off resulted in almost immediate APRS packets coming through. Crap, something is causing interference — what could it be?
So I turned key to the ACC position — and boom, no more packets. Well, at least I eliminated pretty much everything in the engine bay! It had to be something in the dash.
Yeah, it had to be my Joying head unit. See, it’s an off-brand, Chinese stereo that runs Android, and has a number of radios — AM/FM, WiFi, Bluetooth, GPS, etc.
I immediately tried airplane mode, and wasn’t surprised that nothing changed — I figured that “airplane mode” in this unit might be implemented by ignoring the radio signals, rather than actually turning them off. At any rate, whenever the stereo was on, there was so much RFI that APRS didn’t work at all. In fact, even the local weather (WX) stations would become static.
In the end, I knew I was in over my head, and headed back home to ask a few ham radio experts I knew, what I should do.
Their recommendation was twofold:
- Get a handheld ham radio, tune to the APRS frequency (144.390) and move the radio around the head unit (to find where the RFI was being emitted from).
- Make sure the stereo was well grounded, and that the ham radio was well grounded and shielded.
I tackled the second recommendation first — I installed ferrite beads on the ham radio, and I made sure that both the ham radio and the stereo had a good ground. It made no difference.
So I ordered a handheld, and waited.
The day before it was to arrive, I thought I’d go open up the dash — so it was easier to get at the stereo when the HT arrived. I thought I could also verify that it was the stereo by disconnecting the wiring harness, but still having the ignition in ACC — thereby eliminating everything else on the ACC/ON circuits.
So I got the ham radio and TNC up and running, and made sure APRSDroid was parsing stations. Then I turned the ignition to ACC, expecting everything to get really noisy/static-y. It didn’t.
I confirmed that the stereo, tablet, my phone, etc. were all in the same config as previous (i.e. really noisy — BT and WiFi on, hotspot on from my phone) — they were.
Finally, I thought — “last time the tablet was charging via USB” so I plugged it in to the USB ports that I put in place of my cigarette lighter. BOOM — noise everywhere; just like before, I couldn’t even tune WX on the second band of the ham.
That was good news! To see why, and how I solved the problem, head over to Mod Wars at adventuretaco.com.
Originally published at adventuretaco.com.
