I had a few minutes to experiment this week and wanted to try something involving my 100 Mhz O’scope and a hank of wire. Impetus for this came from the fact that my wife informed me that the HDTV had its’ menu appearing intermittently while I was transmitting. I knew of course, what the issue was- I recently hooked up a new HDTV Blu Ray DVD player and failed to choke both power line to the player and its HDTV HDMI connector. The HDMI connector was but 1.5 meters long and so (I thought) it should not be coupled sufficiently to carry a lot of RF into the HDMI port. Not so fast.
My experiment entailed hooking a one meter long bare wire to channel one of the O’scope and hanging said wire as vertically as possible about 20 feet from the rig and by shortest distance from the antenna of about 45 feet. After setting up the O’scope I went into the radio and began sending dits or dahs (I forget which) and running out to the O’scope to measure my received RF Envelope before my keyer timed out. I did this on 18 Mhz.
After doing this several times I averaged my readings on the scope and found that while transmitting 500 W output to an antenna 27 feet up and 45 feet away from the one meter sensing wire I could measure 3.5 volts RF peak to peak. If you consider it takes but 1.4 volts to turn on a transistor junction, you can readily see why RFI is so prevalent. Most devices are connected via wires much longer than one meter.
And so, after I measured the RF Envelope, I returned to the new DVD player and choked off both power and HDMI with binocular Mix 31 snap its and as if by magic no more intermittent menu’s popping up on the TV set.
There is never enough ferrite.
Thanks for reading my Blog. Best, Chas W5PG