Continuing the discussion from 1090 MHz interference from HDMI adapter?:
With home use of USB C-HDMI adapter on the rise, I’d like to share my findings on this subject. (Very) long story short, I ended up buying two adapters (C2 and C3) in attempts to replace the old one my office IT furnished (C1). All adapters are not created equal.
Now, I get some verifiable results. The tests are conducted during day time when “natural” noise is high. Each adapter is turned on for 20 minutes (as marked in colored time band), then off for 10 minutes before testing the next. They all connect to the same monitor and the same USB C port. The adapter is placed 1 m lower (39") than the magnet-base wire antenna and 0.5 m off its axis (19"). The antenna is connected into FA blue dongle without filter.
As the above charts show, on-cycles of C1 and C2 each raises noise floor by 6 dB, but that of C3 shows no measurable difference from background noise. (Aircraft count appears to have a corresponding counter move.) So, the last purchase finally landed a device that does a good job controlling RF emission.
Now, to the selections. After I verified that the old unit (C1) may have a float metal case, I tried many ways to “ground” it. After failing to make repeatable improvement, I bought the first replacement, C2. In a way it feels sturdier (and verifiably heavier). My first test resulted in a measurable, but smaller noise increase. But later tests showed it to be just as noisy as the old one. Needless to say I was disappointed and disillusioned - or misdisillusioned, abandoning the hope that less noisy adapters exist.
If all adapters are equally bad, I reason, I ought to move it away from the antenna. Although I cannot move my workstation or antenna, I can add 2 m (6.5’) to axial distance by connecting adapter input via the USB C charge cable - in addition to the built-in 5-cm (2") USB C pigtail. This will nearly double the distance between the adapter and the antenna, reducing noise intensity by 6 dB. This, however, requires a USB C extension, not just the male-to-male charge cable.
I was almost ready to order a USB C male-to-female adapter when the unique design of this TNP adapter (C3) popped into my eyes: It has no pigtail-style USB C input cable.
Instead, it sports a female USB C port for input (marked as “TO HOST” on the body). It also comes with a 5-cm detachable (and concealable!) male-to-male USB C cable (as shown stacked on top of the body in the above picture). In product description, the included stub is depicted as the input cable, like the following:
But this adapter’s design allows alternative cabling: Use the longer charge cable as input, and use the stub to connect to charger, as illustrated below.
In other words, even if C3 doesn’t offer better RF suppression, I have a plan to reduce noise received by the antenna. In other words, no-pigtail design is still more desirable. it is super sweet that this unit seems to solve the interference without using alternative cabling.
- The laptop used in this test is a 13" Macbook Pro, not the 15" Macbook Pro from work used in previous testing. In previous tests, noise floor was raised by 10 dB, not 6 dB.
- The test interval is merely 20 minutes, and performed during high noise time. There is a non-zero chance that the result is a fluke.
All should become clearer when work returns after New Year.