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Michael J, Geier KB1UM c o 73 Magazine Route 202 North Peterborough NH 03453

Ground Loops and Other Bugaboos

Nol long ago- I was on my tocal packet radio BBS and I noticed a bulletin requesting information about ground loops. I answered and it occurred to me that \ ought to discuss the topic here in the column, since il can be a source of hard-to-find prob' lems in your stalton. So, let s Lake a look ai ground toops and other things related to ground problems.

Around and Around

What the heck is a ground loop anyway? Isn't everything mat's connected to ground "grounded"? II seems reasonable to think sof but it just ain't so in a perfect universe it would be. bul we live here on Earth, and on this planet and every other place we know of, no male rial is perfect In particular, every conductor has some resistance. and any long piece of wire will have some inductance, too. 0u1 what is "long"?

It's All Relative

Thai depends on the frequency you re trying to pass through it! Let s say you have a ground wire for a station transmitting on 80 meters. That wire is 15 feet long. Well, 80 meters is about 240 feet, so 15/240 is 0.0635 That's tho fraction of a wavelength the ground wire represents, ft isn't much. If you imagine an 80-meteMong sine wave drawn over that wire, you can see that Ihe potential difference between what's at the beginning and wnars at the end will be quite small, Consequently, your 15-foot wire is a good ground connection at that frequency. Now fet's say you switch to the 10 meter band. Hmmm, 10 meters is about 30 feet, and 15/30 is 0.5, and ... uh oh, sounds like a problem! Yup, your ground wire is a half wavelength long. So what? Weti, draw a 30-foot sine wave over it. As you can see, the voltage potential between the ends is tremendous. So how, exactly, does that ' ground'1 your equipment when the ground point's voltage won't be anywhere near the voltage on the radio's chassis at any given moment?

The answer is, it doesn't At 10 meters. this ground wire will actually make things worse try resonating and building up voltage; it's a tuned circuit, You're gonna have one hot rig, and RF feedback into your microphone circutt-ry is pretty likely. But what's this got to do with ground loops?

A Smaller Scale

The basic idea is the same Let's say you have several pieces of gear connected together, all with nice, shielded cable. After you connect lots oí stuff, the cable length all the way around from the first box to the last can add up. If it happens to hit a 1/8 wavelength, 1/4 wavelength or 1/2 wavelength, watch out when you transmit, cause here comes trouble.

The same thing can happen fight on a radio's chassis, especialfy when

Ihe frequency of operation goes way up. At microwaves, even a 1-inch spacing between where two "ground" po^ts are soldered can be a very significant part of a wavelength, or even several wavelengths, resulting in their being at different voltage potentials, and thus ungrounded! That's one reason microwave gear is harder to build; you can't even take anything as simple as ground for granted! Thai kind of problem can occur even in VHF and UHF radios, but it usually doesn't in HF rigs because the wavelengths are so much larger, Still, Tve seen some squirrely 10 meter setups; now and then It happens.

Passive Aggressive

But what about receive problems? Ground loops can get you Into ail kinds of problems even when you're not putting out any energy: ask any recording studio engineer. How? Consider this: There is no such thing as true ground, unless you mean the earth we walk on. Each ¡piece of equtpmem has a common poini it calls ground, but what does that mean in relation to other equipment? Not much. If they're all using the same power supply or at least have their common "grounds' connected to each other, ihey all should have these common points at the same voltage potential. In theory, at least. In the real world, the length of the cable between the power supply and the rig induces some resistance and inductance, so the radio's chassis may be at an ever-so-slightly-different voltage than the power supply's common point. For most circuits, the millivolts of difference are meaningless, bul for low-level amplifiers like microphone input

"What the heck is a ground loop anyway? Isn't everything that's connected to ground 'grounded'?"

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