Nuts and Volts - September 2015

RF interference — is it interference to you? Is it interference by you?

Possibly both! What does this interference consist of? And how can you tell what type is present? A topic that starts off with so many questions is bound to cover a lot of ground, so let’s get started.

The phenomenon known as RF interference — RFI to its many friends and acquaintances — is a sub-type of electromagnetic interference, or EMI, which is itself a topic that is part of the general subject of electromagnetic compatibility, or EMC. Specifically, RFI involves interference caused by signals propagated wirelessly as radio waves, but even this sub-type of a sub-type of a subject still has a surprisingly subtle and broad reach.

Is RFI Really a Problem?

Undoubtedly. It’s a growing problem beginning to have an impact on the broadcasting and wireless data/telephony industries. Electromagnetic “smog” is pervasive in urban and suburban areas across the entire frequency range, from long-wave signals through microwaves.

The most widespread effect is higher random noise levels all across the spectrum, just as atmospheric smog affects an entire region. This raises the noise floor (the noise present at any frequency), meaning weak signals are increasingly obscured and stronger signals degraded by bits and pieces.

Why would, say, a mobile telephony company care about analog noise since the phones communicate digitally? Because, at some level, just about everything is analog. Inside that phone is a radio receiver looking for analog radio signals from the local base station.

If I mix in a little more noise — let’s say, I raise the noise level by a measly 3 dB at the receiver input — the distance at which the phone’s digital protocol breaks down will decrease by about 30%. That means the coverage area for the base station has decreased by half!

The service provider then has to either build more towers or you have to transmit at a higher power, or both. That’s money out of your pocket, one way or the other. The same is true for stations broadcasting programming, or public safety agency communications.

On a personal basis, most people don’t know (or care) about RFI until their garage door opener won’t work or an audio system starts speaking in tongues. As readers of Nuts & Volts, however, you should become aware of the problem, understand its causes, and know how to solve it. As day-to-day users of the spectrum, ham radio folks are intimately familiar with the symptoms and cures for RFI.

RFI Basics

The definition of RF itself (radio frequency)

spans electromagnetic signals over a range of just above audio (about 20 kHz) through infrared light at several hundred THz. To keep this column from occupying the entire magazine, we’ll stay in the range of 500 kHz through the upper end of UHF (ultra-high frequency) around 3 GHz — still a 2,000-fold range.

RFI is discussed as having sources and victims with a path between them. The relationship should be pretty obvious. The goal — paraphrasing Poor Richard’s Almanac — is to “neither a source or victim be.” To deal with RFI effectively, you must be able to identify all three:

1. What system or device is creating the interfering signals and how — the source?

2. What system or device is being interfered with and how — the victim?

3. What is the path by which the RF is radiating from the source and getting to the victim?

Even across the many octaves, the basic techniques to achieve the goal are the same: Keep RF out and keep RF in. The techniques for implementing those two seemingly simple tactics are themselves deceptively simple sounding: Avoid and block.

hwardsil@gmail.com

■ BY WARD SILVER N0AX THE HAM’S WIRELESS WORKBENCH

RF Interference

It's everywhere! It's everywhere! Fortunately, you can take a bite out of RFI.

What is the Pin 1 Problem?

In the pro audio world, the “pin 1 problem” was identified by Neil Muncie in 1994. It is caused by the shield pin of the ubiquitous XLR connector (pin 1) being attached directly to circuit common. This connection provides an RF highway for signals being picked up on the cable’s shield directly into the equipment where they can cause interference. Simultaneously, any noise or spurious signals present on the circuit common are given a terrific antenna to radiate from.

The Mackie Company — a professional audio equipment manufacturer — has addressed this in the application note, “Grounds, Shields, Hums, and Buzzes” at www.mackie.com/pdf/CMRefGuide/Tips_Ch6.pdf. More information is available in several papers and tutorials by Jim Brown K9YC (a retired professional audio engineer) on his website at http://audiosystemsgroup.com/ publish.htm.