-----Original Message----- > > Here is a solution from a very old QST, > > It's not from QST. > > and all I can say is it works > > > > for me. My shack is on the 2nd floor and I run several rigs w/ amps > > . I > > was having RF everywhere until I tried this: > > - Run coax ( RG-8 or similar) from rig to a real good ground . Put a > > .01 cap across the shield and center conductor at both ends. Cap needs > > to be 2KV or better. You will be using the center conductor as your > > connection to rig and the ground rod.You should also use an ATU as > > mentioned in an earlier reply.
If it is not from QST, do you know where it is from? Do you know where (or how) I can get a reprint of it? My shack is on the second floor, too. And I believe that The Wireman used to sell a kit containing the capacitors! Dan Allen KB4ZVM K2 S/N 1757 --------------------------------- Those capacitors look like "shorts" at RF. So the center conductor is tied to the shield at RF, but not at D-C. Direct current isn't the problem. Tying the shield to the center makes the impedance of the cable slightly less because it's slightly bigger to the RF. Even so, at RF it's still just a wire from the second story to ground. If you force a wire to either a low impedance (grounded) or high impedance (insulated) at one end, it will show at the opposite end the opposite impedance if it's 1/4 wavelength long or an odd multiple of 1/4 wavelength long, and it'll show at the opposite end the *same* impedance if it's 1/2 wave long or a multiple of 1/2 wavelength long. Everything in between will be something "in between". Even a marginal RF ground will cure problems caused by no RF ground in some situations. Even a hunk of coax. The things you can do to provide a low(er) impedance ground than a wire is to make the wire larger, or parallel a lot of wires to ground, all separated, such as running a large wide sheet of metal from the rig to the ground. If you're on the second floor, that's seldom practical for those who live in the same building with our "shacks" ;-) My shack is, fortunately, on the ground floor about 3 feet from where the large metal conduit for the underground utilities emerges from the earth. It's also where all the other services - telephone, cable, etc. - are grounded along with the mains ground. And my RF ground goes there. Since it's only a few feet away, I use a 2-foot wide strip of copper attached to the wall from the rig to a point on the wall opposite the mains conduit. Then a pair of ground wires makes the run through the wall to the mains conduit. Each wire is about 12 inches long. Two in parallel reduce the impedance of one. To RF, they look like on FAT conductor. I didn't suggest that because, from the 2nd floor, it'd take very large piece of metal to show low enough impedance. But all is not lost. You can operate free from RF when you aren't close to ground. It's been done by everything from rigs in Zeppelins to Hams stuck in the attic! The "trick" is to keep the impedance of all RF antenna circuits at the rig low. A 50 ohm dummy load does that, but it's a lousy radiator. But it demonstrates the concept: keep the impedance low. Feeding an antenna at a current loop (low-impedance) point will do that. That's why Hams seldom have trouble with center-fed dipoles. The impedance at the center of a dipole is low - typically not far from 50 ohms. Bringing the feed point of the dipole directly to the rig or through a length of coax that has a low SWR keeps the impedance at the rig low. The impedance is low because each half of a dipole is 1/4 wavelength long. The far ends are insulated, forcing their impedance high. So the ends at the rig are low impedance - typically 20 to 30 ohms. Two of them makes a 50 ohm load for the rig. The famous "Zeppelin" antennas did the same for a rig floating high in the sky held aloft by a bag of hydrogen. An open wire feed line, exactly 1/4 wavelength long, was connected to the rig. At the far end one side of the feeder was connected to nothing at all: it was well insulated. The other side of the feeder was connected to a 1/2 wavelength hunk of wire floating out behind the ship in the air stream. The 1/2 wavelength long wire showed a high impedance to the feeder at its end. The feed line, being 1/4 wave long matched that high impedance while showing a low impedance at the rig. The other side of the feed line, being insulated at the far end, also showed a low impedance at the rig in the airship. Low impedance at the rig meant no RF to cause sparks and burns which meant the airship didn't catch fire. If your not using a "Zepp"eline antenna you can do the same thing. Just hook a 1/4 wave long piece of wire to the chassis of your rig. Yes, that means you need one for each frequency you operate on. That's the bad news. The good news is that it's generally adequate to use only one for each Amateur band. As I mentioned above, you don't need a perfect ground. The "problem" with such counterpoises, other than the need to string them out and insulate the far ends, is that they, too can radiate, just like 1/2 of a center fed dipole radiates. If the end, hot with RF, is near house wiring, etc. that can pick up the RF voltage, you might still see troubles. Also, a portion of your power will be radiated by the counterpoise which might be largely absorbed by the building or other nearby objects. In an ideal counterpoise, you'd have two 1/4 wave wires running in opposite directions with the ends out in the clear. The RF in each wire would be in phase, so their RF fields would cancel, resulting in little radiation and loss. That's why "Ground Plane" types of antennas show more than one radial arranged in a symmetrical pattern. When that's done, the center point of the radials, typically at the base of the radiator, is at a very low impedance for RF and the radials radiate very little power. Moxon and others have studied the needs of such "ground" systems and come up with some interesting suggestions that might apply to you. One point is that the counterpoise can be much shorter than 1/4 wavelength if you resonate it with an inductor. Moxon suggests using several identical radials less than 1/4 wave long tied together and all resonated in common with an inductor. The multiple radials minimize the power radiated by the system while the inductor resonates the system at 1/4 wave for the lowest impedance at the rig. That can be a lot of work, which is why most Hams in "attic" situations rely on some sort of balanced feed with a low impedance, like the famous Zepp antenna, or a center fed 1/2 wave, etc. Keep in mind, if you are using an antenna that is designed to be a low impedance, the SWR on the coax must be very low. If it's allowed to rise, then voltage maxima will appear along the line. That's exactly what we're measuring when we measure the SWR. If a voltage maxima appears near the rig, you'll likely find "RF in the shack". In that case the SWR on that feeder isn't low enough. You need a better match out at the antenna. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
