Since installing the 12v LED lighting in both the bathroom and the kitchen, i've been aware that the circuits cause interference at around 220MHz, sufficient to render DAB reception impractical. As VHF FM is not affected by this, I havent been particularly quick in getting around to solving it, but Radio DSO is approaching, and I prefer several DAB stations for my normal listening anyway, I thought it time to have a go at it.
My circuits use 12v MR16 lamps fed with one switch mode LED driver per circuit. It was my conviction that the drivers were the most likely source of the interference. Today, I spent some time adding a 10nF bypass capacitor and several ferrite snap-on beads to the wiring close to the driver transformer of the bathroom circuit.
This has made no difference to the level of interference at all.
Further online research leads me to suspect that, contrary to engineering reason that LEDs are passive and cannot cause interference if DC fed, the lamps themselves may be the problem.
It seems that some of these, and quite likely the ones I use, contain a boost regulator to step the 12v supply up a bit, usually to around 18v. These being a class of switch mode supply, will produce pulses in the area of a few tens of kHz, but these being square waves will produce all manner of interfering harmonics.
What I need to do now, is take one of the lamps and feed it with pure DC (easily done from one of my large SLAB backup batteries), to see what the level of interference is from a single lamp. It should then be possible for me to develop a filtering regime per lamp that should cut the level of radiated and conducted interference.
I will also report the problem to the supplier.
The ADS-B collinear is now in its tube, just awaiting some end caps and sealant. Performance at ground level inside seems poor, but of course that isnt its intended operating position! Once sealed up, that will mark the endpoint of my experiments into ADS-B antennas.