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Author Topic: RFI/Grounding  (Read 999 times)
AA1UZ
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Posts: 12


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« on: June 27, 2001, 08:26:51 PM »

I have a very bad RFI Problem.  When I transmit on HF using around 100w, maybe a little less, I get a terrible hum on a speaker system(that has nothing to do with the ham stuff)I have set up on the other side of the room.  Also, at the same time, my computer freezes up.  I know something is really wrong.  My antennas are in the attic(two di-poles right above the room) and the coax is run along the wall(right next to the speakers).  Will grounding the radio do anything to help?  What are your suggestions?  This is about the most annoying problem I have run into so far while running HF.

Drew,
AA1UZ  
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AC5UP
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« Reply #1 on: June 27, 2001, 11:26:25 PM »

Forget the RFI, running that much power within a few feet of yourself is probably not a good idea. Yeah, I know there are differing opinions on the effects of RF on body tissues, but I'd advise against more than 25 watts or so.

You don't have a ground problem, it's proximity...
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KL7IPV
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« Reply #2 on: June 27, 2001, 11:54:59 PM »

Well the first answer is right, you should limit the RF around you. Having agreed with that, you can do other things too. Probably the antenna is running parallel with the speaker leads. That is really the worse thing to have since the induction is greatest then. Move the speaker leads or use a bypass capacitor on the leads. The computer is susseptible to RF as well and is probably in the same space as the speakers. That puts the RF right into the computer space where you dont want it. I'd move the computer, or the best, reduce the power output to the point the problem goes away. If the RF is cooking the speakers and computer, imagine your brain on RF. Not a good thing.
73,
Frank
KL7IPV
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KA2QFX
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« Reply #3 on: June 28, 2001, 01:51:31 PM »

Here's a lengthy answer to your question.  It's simpler just to send you this article from my web site than to address your specific question, although I think you'll find what you need here. Best of luck. If you have any specific questions just email me. Or visit my web site at http://www.bridgeport.edu/~msedutto


Grounds and Grounding for RF Operations
By KA2QFX
   
   There are two reasons why you need, or should have, good grounds in a station installation. First, to provide a good RF ground plane for unbalanced antennas or a ground reference for your stations antennae to work against. Second, a good DC, or power ground is required in any installation of electrical equipment to protect against mains power being present on operating equipment. The latter can occur from line bypass capacitors leaking to the chassis (common on older tube equipment, power supply failures or open neutrals.  Note I distinguish between RF and DC grounds. Not all grounds serve the same purpose nor should they be applied equally.
   The idea that a ground will pull any RF in the station down to zero is false. While you may be able to reduce the RF voltage present by tying the station to a good ground it will have the opposite effect for RF current, as it will provide a completed path for the current to flow.  RF current flow = RF radiation which in your station is undesirable.
   There are primarily two reasons for RF to be present in areas where there should be none.  The first would be antenna currents coming back on the feedline.  This is to be expected where any unbalance occurs. Should you be using dipoles, or similarly "balanced" antennae, no such imbalance should be present. Since few if any stations operate a dipole in free space, unencumbered by buildings with conductive elements, power lines, telephone wires, etc. it is unlikely to expect a perfectly balanced load. The solution, or at the very least a minimal requirement, is to use a good balun to choke off the unbalance currents on the feedline at the antenna's feedpoint.  If you're using ladder line this is a bit more difficult. The concept that ladder line is inherently balanced does NOT prevent unbalanced currents from being included on the line. In this case I have used toroidal cores, large enough that BOTH conductors may pass through, to minimize these currents.
   For coaxial feeds baluns are readily obtainable in both the choke (ferrite) and transformer styles.  The chokes types being more popular and less frequency dependant, and are particularly more effective at the higher bands.  
   For unbalanced antennae, such as verticals, end fed wires, and such; a good radial system is essential.  If ground mounted, they are usually buried and length is less of a factor than if they were unburied provided you have fairly good conducting earth.    In dry climates, tuned radials become more important.   For elevated installations, tuned radials are a must.  No matter what your unbalanced antenna style, or how good your radial system; unbalanced currents are likely simply from the asymmetrical configuration of the radiating elements.  So in any case, a choke coil, or ferrite beads on the feedline are highly recommended.
   The second source of RF is from the antenna itself, which is after all the desired effect.  But if you can't get far enough from the antenna you will obviously be in a high RF field.  Or if your feedline, even with a balun, runs parallel to your antenna, RF can re-couple to the line. {Coupling between any two conductors varies according to the cosine of the angle between them. So even at 45 degrees you can expect about a 50% coupling effect.} While you can't eliminate the field, unless you build a screened enclosure around your shack, you can at least limit the currents that couple to the other conductors. Again, using ferrite beads and rods are effective and can be used anywhere you like. For long conductors, such as coax lines, using several at different places won't hurt a thing.  Caution: avoid spacing them at even multiples of 1/4 wavelengths. For example, spacing two ferrites on a coax 17 feet apart will create a dipole section on ten meters. You can expect some serious RF developing on this section of coax.  
   What about counterpoises?  Well, if you have an RF "bite" problem in your station obviously your operating position is functioning as a voltage node, much like any other antenna end node. If you add a counterpoise, which is simply a 1/4 wave "radiator", you will move the voltage node to the end of that wire. The end attached to you station will become the current node, which will exhibit minimal RF voltage, eliminating the "bite" at your operating position.  What will remain however are the RF circuit currents and all the associated RF radiation in your shack. If applying the counterpoise to minimize interference to TV, Radio, Telephones etc, this solution will probably make things worse.   Counterpoises are best applied to the feedpoint of the antenna, which constitutes a radial. Here again, it is likely that chokes will likely be more effective at eliminating the flow of RF, rather than simply moving the RF around with a  radiating element inside your station.
   So ultimately, the idea is to keep radiation of RF at the antennae exclusively. Balanced loads are most effective at accomplishing this, followed by chokes at the feedpoint.  Chokes elsewhere are the next method to apply at various locations.  In my station there isn't a wire, including power cords, that don't have some sort of choke on them.
   So what if any grounds should be used? Well, here we return to the application for safety from line voltages.  A DC, or even 60 cycle ground, is relatively unaffected by applying chokes to prevent the flow of RF. So your station ground should have a choke applied to eliminate RF currents from flowing.  For an external ground, particularly one feeding buried radials; it is desirable to have as little impedance to RF as possible. This is accomplished best by using heavy cable or preferably dimensionally wide straps. I have found the most cost effective straps can be made from copper roof flashing material.  I cut 2" wide strips of this roughly 20 gauge material for grounds straps. Unlike flat braided cable, corrosion is easily cleaned to permit soldering and it will take a lot of heat when torch soldering to ground rods and tower legs.   If you can't solder, as in the case of aluminum, always use an ample amount of grease between clean connections to prevent corrosion and subsequent RF rectification. This can be a serious cause of harmonic generation causing all sorts of grief, but that's the subject of a different sort (RFI) not being addressed here.  Use grease that does not contain non conductive particles such as lithium (white) grease of Molybdenum disulfide.  Use silicon grease, Vaseline or any of a number of conductive lubricants that are available. Good silicone grease is available from GC Electronics, Allen-Bradley makes a copper laden grease called Kopr-Sheild that I have found to be superb, although few electrical supplies seem to carry it.
   Also, when grounding your equipment, each piece of gear should have it's own ground strap to a common ground point. That ground location should be a large flat plate or strip to minimize any RF voltage drop across it.  I use a 8" x 32" strip of roof flashing screwed to the back of my desk.  This in turn is connected through a large choke (20 turns, 2 layers, 9 over 11, of #0 welding cable around a 4" PVC pipe) to a ground rod driven through the floor of my basement.

   Since we're discussing grounds here I would be remiss not to discuss lightning protection and the role of grounds in that regard.  First, it is important to know that a good ground offers no protection from damage to equipment from lightning surges.  The ground that should be outside your station provides a path for the lightning to prevent fire and explosive damage to buildings and other structures.  
   To protect equipment you must prevent a voltage differential from occurring across your station equipment. Think of  all the connections to your station, each has a potential for establishing a different potential in the event of a lightning strike. So the best solution is to disconnect everything!  This is true even if you are not struck directly. In fact, a good ground in your station could be the worst thing to have, and surge protectors are not great protection either. We're not talking about minor power surges now, were talking about the hand of the Almighty!
   Consider this scenario: You've disconnected your coax and tossed it out the window. You have a good ground in your station and a massive ground on your antenna farm outside.  You're safe right?  Well, let's say you're antennae are struck, or better yet, the guy across the street has a 200 foot pine tree that gets hit.  Where's that energy going? It's being absorbed in the ground of course. In this case the ground will be elevated to some high potential at the strike site. Let's say it's 2,000,000 volts. As the potential dissipates from the strike zone you're station, 200 feet away, has a ground potential of say 1,500,000 volts.  The line neutral feeding your equipment is grounded at the service entrance to your house another 100 feet away. It's at 1,250,000 volts. The "hot" leg from the power company is referenced to the substation, or pole transformer, a mile or so away. It's at 1,000 volts.  Now looking at the differentials across your equipment you have 250,000 volts between chassis ground and power neutral (which isn't going through a switch), 249,000 between neutral and the hot leg, and 499,000 between ground and the hot leg.  But since the hot leg is switched open you don't worry too much about that. And you have a surge protector. Well, the surge protector shorts the hot to the neutral at about 330 volts.  So now your neutral, at your station, just got pulled down much closer to the 1,000 volts of the hot leg. So from the connected neutral to ground you have nearly the 499,000 volts again.  Regardless of whether you have a surge protector or not, these voltages will likely jump the power switch anyway, so the potential (no pun intended) for damage here is considerable.  So do not trust so called lightning protectors, surge protectors or grounds in severe situations. They all serve to limit relatively small potentials and currents which couple to your antenna via radiated energy from strikes beyond a 1/2 mile or so.  That doesn't mean such protectors aren't without merit. In fact, in-line protectors, like the Blitz-Bug, are very good at limiting high voltages that can build up on antennas during stormy conditions even when lightning isn't present. So I don't discourage their use. But for severe lightning conditions disconnect antennas and power connections at a minimum and you should be safe.
   In my station I have added a three pole 100 amp breaker box for power that opens the neutral and both hot legs. (Technically, a switched neutral isn't legal) All legs are surge protected before the box with very large MOVs and a Pi type power line filter to ground. Only the station grounds remain connected to my equipment. I've blown the MOVs twice in fifteen years from nearby strikes with no equipment damage.  I use this method only because it would take a month of Sundays to unplug all the equipment.  
   I hope this provides you with some food for thought on what you may need to do in your station given your operating location and conditions.   Good luck and good operating.

Mark,
KA2QFX
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