Questions about line isolator
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| Aug 4th 2018, 20:58 | |
ag7ovTotal Topics: 0 Total Posts: 0 |
Hi All, I have a model B2KLISO line isolator from BuxComm. According to the sticker on the isolator, it isolates both the center and shield of the coax, not just the outside of the shield (as with some choke baluns, etc.) I'm using this on 2 meters with a vertical antenna. This isn't a "ground plane" antenna. It's a 5/8-wave vertical designed for an HT. It's mounted to a BNC connector on an elbow at the back of an antenna tuner that sits on the windowsill in a second-floor bedroom. With no ground plane, I understand that the antenna needs something to "push against." The vertical element must couple capacitively to some conductor or conductors that complete the circuit so that RF current (displacement current) can flow in the antenna. Without this current--no radiation. (I'm new to how antennas work, so feel free to set me straight if I'm getting this wrong.) Since my rig is only about 8 feet from the antenna, my concern is that the radio itself, and possibly even the power lines in my house, may form this "other half" of the antenna system. A choke on the coax can help keep RF off the outer shield, but there is still an RF potential (voltage) that exists from the radio chassis to the vertical element, meaning the radio chassis can still act like the other half of the antenna, especially since it's so close by. Based on what this isolator promises, my hope is that it actually isolates the radio fully from the antenna system (in the same way a power transformer isolates its secondary from its primary). In a power transformer, parasitic capacitance can make the isolation imperfect, but it's generally quite good. Perhaps the same is true of this B2KLISO device? This is important to me because I'm trying to do everything I can to mitigate RF exposure "in the shack," given I'm using an indoor antenna in fairly close proximity to the radio. Can anyone comment on how this kind of line isolator works--whether my understanding is correct? Any other thoughts about how my antenna system should be set up? Note that I've attached two tuned lengths of wire to the ground lug of the tuner on the windowsill. As well as trying to keep the tuner chassis at a low RF voltage, these can help form the other half of the antenna system. The system works, in the sense that I can receive and transmit effectively. But I don't want to test its isolation effectiveness by going around touching things, and I don't (yet) have a way to measure RF in the room. So the hope is that someone can give me a feel for whether this setup ought to work as I'm expecting it to. Any thoughts? Thanks, Dan AG7OV |
| Aug 6th 2018, 10:01 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
None of your efforts are going to have much effect on your RF exposure other than building a Faraday cage around your body. You are in the near field radiation of the antenna. Bill |
| Aug 6th 2018, 14:42 | |
ag7ovTotal Topics: 0 Total Posts: 0 |
Hi Bill, Thanks for the response. Understood about antenna nearfield. My thinking was there must be some difference between being 8 feet from a radiating conductor and being 2 feet from one, even in the nearfield. If the radio and the coax can be kept from radiating, that would seem to increase safety. (The calculations say that 6 feet should be safe.) I understand that being in the nearfield throws calculations off. But I imagine there are still things that can be done that change the RF power density at the radio. Just my thoughts. Appreciate the response. Dan |
| Aug 6th 2018, 20:26 | |
WA0CBWTotal Topics: 0 Total Posts: 0 |
Couple of points: The radio and the coax don't normally radiate RF. The coax "conducts" RF from the radio to the antenna. The radio normally doesn't radiate RF and not in amounts to be concerned with. Several factors can cause RF to be "conducted" down the outside of the coax shield where it can cause the radio to malfunction or cause contact RF burns when touching metal parts of the radio. This can be eliminated or reduced by fixing the problem at the antenna or adding a choke (or line lsolator) at the antenna. Bill |
| Aug 7th 2018, 00:00 | |
ag7ovTotal Topics: 0 Total Posts: 0 |
Hi Bill, Thanks again. Under some circumstances, I'm led to understand, the coax and radio do radiate RF. Unless I'm reading incorrectly, the ARRL Antenna Book says that with an end-fed antenna, the outside of the coax, the station equipment, and even the power lines in the station can all become part of the antenna system. I suppose that must mean they can radiate. My understanding might be off, but this is what the books seem to say. This other path back to the original energy source becomes "the other side" of the antenna--unless you take measures to mitigate that. I started with the assumption that my antenna is an end-fed one, since it's a vertical with no radials. What I was asking about was effective ways of mitigating the problems with that--especially in my situation. People fix it at the antenna, I guess, by putting in a dipole, or some such. But it looks to me (again, mostly from what I've read in books like the ARRL Antenna Book) like another option is just to use that end-fed antenna and take steps to mitigate the problems in the shack. I was thinking along those lines. At any rate, since posting this, I've switched to a balanced antenna in the attic, which I think fixes the problem at the antenna. I appreciate your input. I'm still trying to learn about how antenna systems work, so any other feedback would be welcome. Best regards, Dan |
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