ag7ov
| Joined: | Wed, Jul 11th 2018, 12:26 | Roles: | N/A | Moderates: | N/A |
Latest Topics
| Topic | Created | Posts | Views | Last Activity |
|---|---|---|---|---|
| Need advice for burying radials | Sep 12th 2018, 19:38 | 5 | 6,386 | on 26/9/18 |
| Is "balanced" (as in baluns, etc.) a myth? | Sep 5th 2018, 04:32 | 7 | 7,041 | on 10/9/18 |
| Can someone recommend a book? | Aug 12th 2018, 15:20 | 3 | 6,769 | on 13/8/18 |
| Questions about line isolator | Aug 4th 2018, 20:58 | 5 | 6,630 | on 7/8/18 |
| Usable RF power density meter for safety? | Aug 4th 2018, 18:00 | 3 | 7,883 | on 8/11/21 |
Latest Posts
| Topic | Author | Posted On |
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| Need advice for burying radials | ag7ov | on 15/9/18 |
| Thanks, Bill. I didn't know that. | ||
| Need advice for burying radials | ag7ov | on 12/9/18 |
| Hi All, I'm planning an antenna system for my backyard. The idea is to install a permanent ground mount with buried raidials and coax, which will allow a vertical to be put up and taken down easily. The mount will be small and out of the way, and it should not bother the HOA folks. (It can even be covered by some piece of nice-looking landscaping when not in use, such as a vintage wooden bucket.) The antenna need only be up when I'm operating (and even then it won't be visible from the street). I'm thinking of a metal pipe buried about 3 feet in the ground, with concrete to hold it firmly in place and keep it vertical. It will stick out maybe 18 inches or so to permit an antenna to be mounted to it. My idea is to clamp and solder radial wires to the underground section of this pipe, then bury them radiating outward from the mount--maybe 16-20 wires of whatever lengths are practical for where the mount will be in the yard. Then when the antenna is mounted and grounded to the pipe, it will be grounded to the radials automatically. I was thinking of using 12-gauge solid copper with insulation for the radial wires. The reason I'm considering insulation is to help prevent the wires from "rotting" in the ground and ending up in pieces. I could strip away a few inches of insulation at the far end of each wire (farthest from the mount) to couple it electrically to the ground. Any opinions on whether insulated wire is a good way to go, or whether solid copper is a good way to go? Any other thoughts about this plan before I start buying materials? I was also thinking of putting a ground rod into the ground near the mount and running a wire or even a heavy strap to that, which would help for lightning safety if nothing else. I've put up the antenna temporarily in the backyard as a proof of concept, and it seems to work fine. Now I need to come up with a more permanent and usable installation. Any advice from those who may have actually done this before would be most welcome. Thanks, and 73! Dan AG7OV |
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| Is "balanced" (as in baluns, etc.) a myth? | ag7ov | on 10/9/18 |
| Hi John, Thanks for responding. The thing is, the balanced/unbalanced distinction requires a ground reference, which I think you'll see if you look at your own description of it. The distinction is meaningless without a ground reference. In fact, on a signal cable, there is no difference between a balanced and unbalanced signal without a ground reference. Otherwise, we can't say, "This conductor has thus and such a voltage, while that other conductor has thus and such a voltage." A voltage can only be measured across two points. Or, "This conductor comes from thus and such a source impedance, while that other conductor comes from thus and such a source impedance." Impedance to where? The impedance of either conductor with respect to the other must be the same. So when we speak of a single conductor having a voltage, that's shorthand for saying there is a voltage between that conductor and some understood reference point (which is often called "ground," but just as often has little to do with the physical ground). Likewise, when we talk of balanced or unbalanced source or sink impedances, in each case we mean with reference to an external "ground" point. The signal between a power amplifier output and a speaker is not spoken of as "balanced" or "unbalanced" (as I'm sure you know, since you have a background in audio). The distinction has no meaning, because the speaker does not have any kind of a "ground" reference. The amplifier delivers power to the speaker via a voltage across and current through two conductors. By Kirchoff's law, the current must be "equal and opposite" in both conductors. The voltage is measured from one conductor to the other. There is no third point of reference. Hence there can be no balanced/unbalanced distinction. Now, at the amplifer, one of the output terminals could be at the same potential as the power-supply ground, or not. It doesn't matter from the perspective of the signal or from the perspective of the speaker. Either way, the speaker sees one voltage and one current. In practical reality, sometimes one terminal at a power amp is at "ground" potential and somtimes it isn't. It makes no difference, and it's a distinction we simply do not draw. There are no "balanced" versus "unbalanced" speaker inputs. (I mean to the speaker itself, not to an integrated amplifier.) There are no "balanced" versus "unbalanced" speaker cables. If someone were to ask whether a particular power amplifier put out a "balanced" output, generally that person would be laughed at in the industry (again, as I'm sure you know). It's a nonexistent distinction. (Well, it can become important if you want to bridge two amplifiers together, but I don't think that has bearing on this discussion.) RF and audio are very different in a lot of ways. For example, at the lengths we normally use, audio cables do not practically function as transmission lines. We don't care about their characteristic impedance. We don't worry about impedance matching line-level signals because reflections on the cable are not an issue. There is no skin effect to speak of, so the cable shield doesn't act as two separate conductors. Audio signals as a general rule don't radiate electromagnetic waves when running through the wires of normal audio systems (or not enough to concern ourselves with). Yes, both are electrical signals, but there are many considerations that only become important at higher frequencies, where conductors are a significant fraction of a wavelength. Now, I submit that in the case of an antenna system, what we have is like a speaker. What I mean is that there is no external reference point against which to meaningfully measure the individual voltages on the two conductors of the transmission line. We can measure them against the power-supply's DC "ground" potential in the radio, but that is a reference that has no meaning at all from the standpoint of the antenna. From the standpoint of the antenna (as with a speaker), there is simply a voltage across the feed point lines, and there are equal and opposite currents flowing in the feed point lines, and (unlike with an audio signal) there are voltage gradients along the lines, meaning it's possible to measure a significant voltage from one point of a conductor to another point on the same conductor. In any transmission line, the currents and the voltage gradients are equal and opposite so that the E and H fields cancel each other, which is what keeps the line from radiating. At the antenna end of the line, we just have two equal and opposite waves on the two conductors. It is unimportant whether one of the lines is at the power supply "ground" potential in the radio. The result would be exactly the same whether was or it wasn't. (In actual fact, it typically won't be, because of those voltage gradients I mentioned, even when the line is fed from a radio that references its output to ground.) If we feed the coax through a "balun" or an "unun," the result is the same as if we'd fed it from the radio--equal and opposite currents and voltage gradients as referenced to each other. There is no meaningful external point of reference. Even if we wanted to use the radio's "ground" as a point of reference, it wouldn't be correct except at certain precise points along the transmission line. That's not to say the "balun" or "unun" isn't doing something important. It is--isolation. Isolation is an important function, but it has nothing to do with balancing or unbalancing, because the distinction simply doesn't exist in a transmission line. It may be tempting to say that earth ground is the external voltage reference, but the problem is that there is no one earth ground potential. Different parts of the earth are at different potentials, and we know that RF current flows through the earth in the vicinity of many antennas (and elsewhere too). The earth is not a magical "zero" point for voltages. Also, what would we do in space? The same transceiver you use in your shack can be placed in a satellite orbiting the earth, and then where is "ground"? In the world of audio, we don't care about earth ground (except for lightning safety). "Ground" means signal ground, an established common "zero" point of DC power supplies in the system. There's no such thing in an antenna because it doesn't have a power supply. What we're sending to the antenna isn't so much a "signal" (conveying information via a changing voltage). What we're sending to the antenna is power, most of which we hope will be radiated. "Ground" is meaningless here, just like with power amplifiers feeding speakers. By the way, I also made a living running concert sound, owned one recording studio, managed another, worked as a staff engineer in yet another, and built many PA systems, large and small. I say all that not to be competitive. I only want to try to establish my credibility to speak about audio systems. One last clarification: Impedance is a complex-valued function of reactive and resistive elements in a system and frequency. Since impedance varies with frequency, it's fairly common to speak of DC impedance, meaning impedance at the zero point on the frequency curve. In the case of a usable balun, DC impedance should be as close to zero as possible. If not, it's a resistance that dissipates power as heat, which is of course undesirable. Once again, I don't mean to be in any way disrespectful or merely argumentative. I write this because I think there is value in getting our concepts straight. I recognize that this is a difficult thing to do, as these are not simple concepts. As I said before, I am new to ham radio and open to being set straight in my thinking. In the meantime, I don't know any other way of trying to get to the bottom of something like this than to try to clarify my views by hashing them out. I'm hoping this will come across as a cordial discussion, because I mean it that way, sincerely. 73 Dan AG7OV |
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| Is "balanced" (as in baluns, etc.) a myth? | ag7ov | on 5/9/18 |
| Thanks Zak. I sincerely appreciate the conversation, and I am learning from your responses. My thoughts of the moment: Impedance matching is impedance matching. Isolation is isolation. A circuit can be designed to provide either or both. Neither really has anything to do with "balancing" or "unbalancing." Do we call something an "unun" if it provides only impedance matching but not isolation? But I've seen advertised 1:1 "ununs" that are simply isolators. The solid-state transciever output provides a DC path to ground on one side, and it seems to be common practice to bond coax shields together and tie them to earth ground for lightning safety. A 1:1 "choke balun" only presents a high shield impedance at RF. It should basically be a wire at DC, so one side of the antenna should remain grounded at DC. Given a low-DC-impedance output from the radio, the same should hopefully be true for the side fed by the coax center. Let me step back to my original point, though. Coming from the audio world, I understand the balanced/unbalanced distinction for signaling. The point of balanced signaling is rejecting line noise and common-mode power-supply noise using differential input topologies, etc. In that world, there is a clear distinction between balanced and unbalanced signaling at outputs, inputs, and along signal cables. In the world of ham radio antenna systems, there seems to be a desire to draw the same kind of distinction. But it seems a false distinction, made by analogy, but not holding up to scrutiny. I still have yet to hear a description of the balanced/unbalanced transition that doesn't just amount to some combination of isolation and impedance matching. Now, both of those are important, but neither has anything to do with balancing. Understanding what's really going on would seem to help in terms of understanding what is needed in a particular antenna setup. By the way, it's really interesting that you achieved a wide SWR bandwidth on 80 meters with an impedance step-up. I imagine the principle is that large changes in the higher impedance are translated to small changes in the lower impedance? That's an interesting idea that I hadn't heard before. I'll have to look into that on 80-meters. Thanks for the discussion! Dan AG7OV |
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| Is "balanced" (as in baluns, etc.) a myth? | ag7ov | on 5/9/18 |
| Hi Zak, Thank you for replying. I don't doubt (and didn't even before your response) that the distinction is made in amateur practice, and that the end result is something useful. But something useful can come from a flawed concept. I have an extensive background in audio engineering, and there are a ton of silly ideas about "ground loops" in audio systems, which are totally contrary to what's physically happening with noise voltages and currents. But the end result is that people employ certain techniques that work in practical terms. The fact that someone arrives at a correct solution to a problem doesn't prove that the concepts that led them to that solution were correct. It may be useful to have concepts that lead to solutions, but it will be more useful in the long run to have *correct* concepts that lead to solutions. Or at very least we can say, "The situation is mathematically complicated, but in practical terms, here's what you need." May I ask you a question? If your unun on your 80-meter vertical doesn't provide isolation, what purpose does it serve? Better still, can you describe to me the electrical difference between a balun and an unun? I've actually tried modeling the OCFD and transmission line with shield. EZNEC shows shield currents much worse than with a center-fed dipole (depending on how far off center). But a "perfect balun" (which would electrically be an open circuit at RF) makes the shield basically into just another wire in the antenna field. Yes, this will pick up RF like any wire. But it can be isolated at the other end on its way into the shack, too. In my modeling experiments, a reactive impedance of about 6k ohms on the shield is sufficient to reduce currents on it to very low, provided it's oriented orthogonally to the antenna. In the veritcal with an extensive radial system, without an isolator ("balun"), the coax shield is just one more radial. Assuming (as is probably reasonable) that the currents on that side are well distributed among the radials, shield current should go down roughly in proportion to the number of radials. If you picked the entire antenna system up and suspended it on its side in free space, this should still be the case, and for the same reason that people employ counterpoise systems in the shack to disperse RF currents. (But ground losses are probably also a factor in the radial case--if the radials are on or in the ground.) An isolator ("balun") still seems to be a good idea here. I don't mean any of this to come across as disrespectful. Please don't take it that way. It's just that if there is some point to a "balun" other than isolation, or if there is some electrical difference between a "balun" and an "unun" besides the name, somebody ought to be able to describe it plainly. Best regards, Dan AG7OV |
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