RF Overload vs. RF in the Shack by Al W6LX

Mike-VA3MW

I wanted to thank Al W6LX for this excellent post from another reflector

----------------------------------------------------

Here's a statement: "RF in the shack" does not equal "simple RF overload". Let me define my terms.

"RF in the shack", which many people have experienced when using "unbalanced" antennas like verticals, off-center fed wires, and others, is caused by radiation from the outside shield of the coax, or by unequal currents on either side of a balanced transmission line. As you know, this causes everything from interfering with household devices to RF burns. Many folks have cured this kind of problem with various ferrite chokes and grounding.

But there's another kind of interference: that caused by "simple RF overload", which occurs when your beautifully balanced antenna, with lots of ferrite all over the place, judicious routing of the feedline, and good grounding, is radiating so well that the RF field around the antenna-- and thus, inside your home-- is very high. Paradoxically, this is what you want to happen; your highly efficient antenna system is turning your RF power into a nice, strong radiated field.

So it's important to understand the difference between these two mechanisms. Maybe a story will help you further understand the distinction I'm making. I attended a high school not far from 50 kW radio station KRLA. KRLA used to get into everything. It was so loud on a crystal radio that you could hear it easily with the earphones laying on the benchtop. If you left an oscilloscope probe disconnected the scope would show an enormous 10V peak-to-peak signal on the screen.

Was KRLA guilty of RF on its feedline, putting "RF in the shack" into our electronics lab? Would ferrites and additional grounding at KRLA's antenna have improved anything? No, because it was a case of "simple RF overload"... caused by a very strong, very close transmitter. KRLA was doing exactly what it should have done: put a bodaciously loud signal everywhere. We just happened to be 'in the way' of that signal.

This is what can happen when you have a perfectly good and RFI-proofed antenna system. The radiated signal is simply too strong for various electronics in your house to handle. And, as you can imagine, running the legal limit makes the problem worse.

I had essentially asked, "Given that your properly functioning antenna is putting a bodaciously loud signal everywhere, why are some devices immune to the signal, and yet others fall apart? Why does one ham's garage door opener go crazy when he transmits, but another ham's is unaffected?" That's what I was wondering.

A number of respondents to my other post told me that yes, in fact, there are devices that act naughtily when they operate high power, but at least as many said that they've *never* had any such problems. Isn't that interesting? Is it just luck? I've purchased faulty devices while they lucked in and purchased all good ones? The engineer in me hates this arbitrarity and drives me to understand why.

What are the unaffected hams doing differently than the rest of us, if anything, to prevent not "RF in the shack", but the dreaded Simple RF Overload?

Thanks!

Al w6lx

Barry Isseks

Great Post on RF in the shack !!

Sometimes the "luck" is determined by the orientation/placement of the antenna.

This is proven out by my current experience with a 4 element 20m yagi at about 50 ft. When the yagi is pointed in most directions there is no issue with RF into any device. When the yagi is pointed South, RF intrusion into the network causes connection to the radio to fail almost 100% of the time with high power. The flex goes from full bars to yellow then disconnect almost immediately. Turn the antenna a few degrees and there is again no rf issue. I am grappling with how to filter out this very specific intrusion point but so far there is no joy after $150 worth of torroids.

K6ZA