Chasing RFI and Lost Transmit Power

You hear a lot in passing as a new ham about RFI noise, or common mode current interfering with your signals, both outgoing and incoming, but it’s seldom explained how that can affect the power going out of the antenna in a real way.

I come to this subject after chasing around an issue I’ve mentioned in other posts with my SignaLink “fluttering” on and off during transmit sometimes (but not all the time) and my signals dropping off PSK Reporter like a rock when that starts happening.

So in the process of trying to figure out how to fix that issue I’ve come back to the concept of RFI in the shack. I’ve never been actually “bitten” by touching my radio, or mic, as I’m working digital from a laptop like some hams have, and so not handling things that might “bite” me, so I didn’t think it was a serious problem for me– maybe more of a nuisance. 20watts is all I have on the G90, so how bad can it be, right?

Then one morning my trackpad mouse on the laptop quit working every time the radio started to transmit. And that was my aha to take it seriously.

Studying articles and discussions around the web, and on YouTube about chokes, and controlling RFI, I have learned a few things interesting enough to put together in one place.

Electricity is a very interesting beast. It seems attracted to itself, and one current going out, prefers to return as close to it’s original path as possible. It’s almost like there’s “suction” drawing the two together, even if they can never meet because of the insulating barrier.

People who design printed circuit boards will often run traces and grounds on both sides of the board in roughly the same path because that’s where electricity WANTS to go. Close to its original self.

You also see this concept in some of the MOSFET transistors where the gate is opened on the switch by electrical “pressure” on the other side of a wall, even though the electrons remain separated from each other and never touch.

This means that a coax cable, which has a center conductor, and an outer braid wrapped around the center concentrically, will conduct the signal one way along the center conductor, and the other way back down the braid along the INSIDE of the braid, because it’s closest to the original path– the center conductor.

This leaves the OUTSIDE of the braid free to pick up other noise and frequencies from nearby electronics, the atmosphere, etc. (Electricity likes to travel on the “skin” of things, and not at the center, so there being two paths on the braid, the outside skin and inside skin makes perfect sense. )

Now here’s where it gets interesting.

This noise on the OUTSIDE of the braided shield can actually SUCK THE POWER AWAY from the power running on the inside!

So instead of feeding your outgoing power on transmit, the current can be drained up to 60+ percent away from going out the antenna, into just radiating inside your room.

And in the process interfere with your other electronics.

I saw this problem with my SignaLink fluttering and the laptop trackpad completely disappear when I went on a mission to install ferrites everywhere.

I now have ferrites on the power cable to the laptop, the USB cable from SignaLink to Laptop, the cable coming in to the shack, at the transciever, and a few other places.

No more fluttering. No more dropping signals in the morning. Lots more watts going out the antenna. Isn’t that amazing?

There is probably still more to do– I have the feeling I need a proper choke at the feedpoint.

From what I read, the so called Ugly Balun, or air wound 1:1 UnUn choke can sometimes make the issue worse, depending on the other characteristics of the feedline, such as length.

Extremely long runs of coax may need a line isolator every 1/4 wave or so to keep them quiet.

But for now at least, I’ve made enough of a difference correcting it that I’m getting the signal out with a lot better signal strength. Before, I was often only showing up with -17 to -24dB on digital modes. Now the reception reports are more likely to be in the -10dB range.