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HIGH Q CAPS FOR VLF LOOP ANTENNA?
On Fri, 28 Oct 2005 18:33:23 -0400, TRABEM wrote:
When you said "Well, this is a good opportunity to examine that tumble down the slope to the Q = 2 (caused by the severe loading of your proposed design)"...................did you mean to say or to infer '(caused by the severe loading of your old (now defunct) series resonant loop design"??? Yes of course. You have asked a number of questions outside of this old (now defunct) design, but you haven't, as far as I can tell, formalized a replacement. The ancillary point that I've made is that the original could work. However, you've never stated any operating specification to test that against. I've offered that all components need to be scrutinized in the face of your goal. We saw where that lead. You've only specified your desire for High Q capacitors (properly, low D capacitors). I offered that ESRs vary widely and could easily derail your goal. The presence of an ESR equal to the 0.06 Ohm of the loop is very well within being guaranteed. It still is. Reg dismissed this as inconsequential. So be it, but being that it is easily remedied through selection, then why toss away half your Q to casual indifference? What Reg actually meant, and he has a hard time with that given he can often be found on both sides of an argument, is that such loss may not matter. There I agree, but this does not advance the topic of High Q Caps for VLF Loop Antenna when they can be obtained. 73's Richard Clark, KB7QHC |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
On Fri, 28 Oct 2005 17:03:55 -0700, Richard Clark
wrote: On Fri, 28 Oct 2005 18:33:23 -0400, TRABEM wrote: When you said "Well, this is a good opportunity to examine that tumble down the slope to the Q = 2 (caused by the severe loading of your proposed design)"...................did you mean to say or to infer '(caused by the severe loading of your old (now defunct) series resonant loop design"??? Yes of course. Agreed! Just wanted to be sure I didn't miss something, so I asked. And, your're right, I haven't formulated a replacement. I bought a 250 foot poll of cable, and it has not been cut. So, it's still in one piece and returnable if I decide not to use it. I was just pondering the alternative of allowing the front end to be untuned. The receiver is susceptible to harmonics, each harmonic of the tuned frequency is down 6 db though. Since the loop would be resonant somewhere on HF, it is probably a bad idea to leave the front end untuned as HF can be unpredictable. so, I am thinking I need some front end selectivity. Reg gave me an example of what he might do. And his antenna came out much cheaper to build and probably easier to put up. I'm also thinking about the method of feeding the signal to the house. It will be around 70 feet from the house, so 90 feet of cable of some sort is needed. I can easily go 700 feet in any of 3 directions, but there is probably no practical need to go that far out into the woods. Since the house is a noisy place for LF and VLF, I have to be concerned about how I feed the antenna. I also think I'd like to have it fed with balanced line to minimize the possibility of the feed line acting as an antenna. If a preamp is used, I have to feed power to the antenna as well, so I will have to wind a common mode filter to do that job as well. So, I got a lot to think about. Although I haven't formulated a plan for a replacement antenna, the series loop is 99.99 percent history. So, I am thinking about it. I have a reading session planned for the late night here so I can reinforce the lesson(s) you and Reg have taught me. And, will probably work on the actual antenna design tomorrow. My caps are on order from Mouser, should be here next week There are a few preamp designs around the web, but none of them seems very well thought out...although they might be well planned. It's possible they are solidly designed, but that the authors haven't shared all the gory details in their web presentation(s). Thanks again. I'll keep you posted if you like......I'd appreciate sending up a red flag if I attempt to commit additional acts of stupidity with regard to whatever I come up with for a design. Regards, T |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
Hi Jim,
I ran some estimates earlier of a much smaller multiturn look with smaller wire. The Q held up much better than I expected and I'm also considering smaller diameter multiturn loops. Appreciate the quotes. Regards, T wrote: To emphasize one idea that Richard Clark has presented: Here is a direct quote from page 6-1 (section 6.2) of the chapter on loop antennas in the (first edition) Antenna Engineering Handbook edited by Jasik (1961): "The radiation pattern of a small loop is identical with that of a small dipole oriented normal to the plane of the loop with the E and H fields interchanged." The use of "magnetic" or other buzz words notwithstanding, a small loop is the dual of a small dipole. Practical considerations might cause one to select a small loop over a small dipole. 73 Mac N8TT |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
TRABEM wrote I ran some estimates earlier of a much smaller multiturn look with smaller wire. The Q held up much better than I expected and I'm also considering smaller diameter multiturn loops. ======================================== With multi-turn loops don't be tempted to neglect wire diameter. And it's well worth while spacing the wires by 1 or 2 wire diameters. Spaced wires reduce RF proximity-effect loss and increase Q. Neatness of construction is of little consequence. Wires need be supported only at the corners of the loop. Keep wires taut enough to prevent them flopping about in the wind. Electrically isolate the main loop. Don't connect it to anything. It will then maintain its good directional properties including its sharp null. Don't even think about screening or shielding the loop. It will serve no useful electrical purpose. It will cause proximity effect to reappear. Connect the smaller coupling loop to the unbalanced receiver via a thin twisted pair of whatever length you like without fear of it picking up any signal of consequence. Remember sensitivity depends ONLY on the enclosed loop area and NOT on the number of turns. So don't be tempted to reduce loop size just because it is multi-turned. Big signals never did anybody any harm. Whereas . . . . . ! I assume your QTH is not adjacent to WWV's antenna. ;o) ---- Reg, G4FGQ. |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
Hi REg,
Looks like I have a plan for a new loop. New loop design: I have a plan for a more appropriately dimensioned loop. At 60 KHz, 4 turns of 2 mm diameter wire, spaced 4 wire diameters apart. 2 meters per side 123 uH, 60,000 pF to resonate 4.7K across the loop. 300 ohm feed impedance at single turn loop feed Q (unloaded) = 101 This allows me to feed the loop with 300 ohm balanced line, which I can easily transform to 50 ohms at the receiver. I'm not sure what the impedance of twisted wire is though, which would be even cheaper than 300 ohm twin lead. Also, my Q will be slightly higher as I can stagger the turns some, so that the wires won't run parallel to each other for the entire length. I was never quite thrilled with a big loop threaded through the trees and supported in that manner, it makes it hard to rotate:: Being able to rotate the loop is a good thing:: Is the 1 turn pick up loop critical??? And, I have another question....... I used rj2loop3 and ran the same numbers as above, except that I separated the wires by 10 wire diameters instead of 4. Instead of seeing the Q improve, it was reduced (from 101) to 93. I expected the Q to improve, not get worse. It seems odd to me. Is there a good reason for this?? Ran it with a very low number and the Q also get worse. So there appears to be an 'optimal' wire winding pitch for optimizing Q? Byr the way, nice software package, thank you for it's use! I assume your QTH is not adjacent to WWV's antenna. ;o) Temporarily, it is on the East Coast of the US. But, at home it is high in the mountains in Northern EU (with no commercial power for miles around). Thanks, T |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
Trabem,
I will try to answer your remaining questions in the order at which they occur. There must be a compromise between size of loop, receiving sensitivity, and the ability to rotate it. Only you can decide. I can suggest only that you bias your opinion towards size matters more than the ability to rotate the thing. Just make sure that the broadside null does not correspond to the direction from which your favourite transmissions come from. The receiving lobes in the polar diagram are very broad. The polar diagram is a figure of 8, like a pair of touching circles. You won't need Eznec. The impedance of the line from the coupling loop to the receiver doesn't matter two hoots. At 60 KHz it is just a pair of wires. A twisted pair of wires has an impedance of very roughly 130 ohms. But at 60 KHz the line length is so short in terms of wavelengths it doesn't matter what its impedance is. The coupling loop can be considered to be directly connected to the 50-ohm receiver. And even with an extremely long line any impedance mismatch loss will be negligible. So forget about 300-ohm balanced line and just use a simple not-tightly-twisted pair. NO IMPEDANCE MATCHING REQUIRED at either end. The size of the small coupling loop inside the main loop matches 50 ohms to a 50-ohm receiver. So ideally the line to the receiver could be 50-ohm coax. But, as I say, it doesn't matter. The size and shape of the small coupling loop is not critical. It can be circular or square. Theoretically, to match the loop to a 50-ohm receiver, it should have an area about 1/25th of the main loop area. To simplify construction the coupling loop can be made self-supporting. Electrically, the thickness of the wire in the coupling loop need be no greater than the wire in the line which connects it to the receiver. The only wire diameter which matters is that of the main loop itself. As the spacing between wires on the main loop increases the RF proximity loss in the loop conductor (related to skin effect) decreases and Q increases. But other things happen when the width of the loop increases with spacing. For example, loop inductance decreases. We are not comparing like with like. And in any case maximisation of Q is not the primary objective. There are other things to be considered. For example, if you want to increase Q then don't bother to increase spacing between turns, just increase wire diameter. But with given wire diameter, the optimum spacing between the wire centres of adjacent turns, to maximise Q, is very crudely about twice the wire diameter. But, as I say, it is very non-critical and you might be better off by increasing wire diameter as it simplifies loop construction. Then, once again, you will have the option of increasing spacing between turns. Compromises are never ending. ;o) Whatever you end up with I can see from your enthusiasm you are enjoying your efforts and will continue to do so. ---- Reg, G4FGQ. ===================================== TRABEM wrote in message ... Hi REg, Looks like I have a plan for a new loop. New loop design: I have a plan for a more appropriately dimensioned loop. At 60 KHz, 4 turns of 2 mm diameter wire, spaced 4 wire diameters apart. 2 meters per side 123 uH, 60,000 pF to resonate 4.7K across the loop. 300 ohm feed impedance at single turn loop feed Q (unloaded) = 101 This allows me to feed the loop with 300 ohm balanced line, which I can easily transform to 50 ohms at the receiver. I'm not sure what the impedance of twisted wire is though, which would be even cheaper than 300 ohm twin lead. Also, my Q will be slightly higher as I can stagger the turns some, so that the wires won't run parallel to each other for the entire length. I was never quite thrilled with a big loop threaded through the trees and supported in that manner, it makes it hard to rotate:: Being able to rotate the loop is a good thing:: Is the 1 turn pick up loop critical??? And, I have another question....... I used rj2loop3 and ran the same numbers as above, except that I separated the wires by 10 wire diameters instead of 4. Instead of seeing the Q improve, it was reduced (from 101) to 93. I expected the Q to improve, not get worse. It seems odd to me. Is there a good reason for this?? Ran it with a very low number and the Q also get worse. So there appears to be an 'optimal' wire winding pitch for optimizing Q? Byr the way, nice software package, thank you for it's use! I assume your QTH is not adjacent to WWV's antenna. ;o) Temporarily, it is on the East Coast of the US. But, at home it is high in the mountains in Northern EU (with no commercial power for miles around). Thanks, |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
On Fri, 28 Oct 2005 13:45:17 -0700, Richard Clark
wrote: On Fri, 28 Oct 2005 20:12:48 GMT, Owen Duffy wrote: Didn't Don Stoner describe a synchronous detector way back. I think I remember reading an article in the mid sixties in "The Sideband Handbook" or similar. I was about 15 then, so a detector that had something like 17 bottles in it seemed overkill when I was copying CW and SSB on an AM receiver (ie diode detector) with BFO. The appeal being an all-mode detector (including DSBSC), but synchrounous detectors didn't seem to catch on in comms receivers, well not until DSP detection... well I don't recall coming across them anyway. Hi Owen, 17 bottles indeed. That seems to strike a resonant chord in the ganglia because my construction was on a utility box of about 3" x 9" x 15" (not counting power supply requirements). We were working from a printed article certainly; and to confirm your recollection, there was a list of modes that could be detected that was long. My perception of the resurgence of interest in synchronous detection (it seems to have many names) is that a considerable body of knowledge evaporated in the 70s and 80s to leave only fragments of what this detector was useful at. I think the appeal of it in the early days of suppressed carrier exploitation by amateurs lay in its application to DSBSC demodulation. It probably fell by the wayside when filter method SSB transceivers became lower in cost. Here is an interesting hypothetical. Australian amateurs at the unrestricted licence grade are subject to the following power restrictions: 16 Transmitter output power (1) Subject to section 15, the licensee must not operate an amateur unrestricted station, using a transmitter output power of more than 400 watts pX, if the emission mode of the station includes: (a) C3F; or (b) J3E; or (c) R3E. (2) The licensee must not operate an amateur unrestricted station, with an emission mode not mentioned in subsection (1), using a transmitter output power of more than 120 watts pY. Since the emission mode for DSBSC is not one of those mentioned in 16(1), then the power limit is 120W pY. If the pX/pY (PEP/Average) power ratio for radiotelephony is somewhere around 12dB to 15dB, that suggests that (using a worst case of 15dB) that the 120W pY DSBSC telephony transmitter is around 3800W pX (PEP), whereas you will note that if we use SSBSC (J3E) we are limited to 400W pX. Doesn't make sense, does it. Did I get the maths wrong? Owen -- |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
On Sat, 29 Oct 2005 21:36:07 GMT, Owen Duffy wrote:
Doesn't make sense, does it. Did I get the maths wrong? Hi Owen, Gad! I left all that computation behind when I finished the exam for FCC 1st Class. (Actually when I finished the Radar portion as I took the entire suite of tests, 3rd, 2nd, 1st, and Radar endorsement in one sitting.) However, I do recognize a strategy when I see it, and I doubt, beyond slipping a decimal the wrong direction, you may have found a loophole. 73's Richard Clark, KB7QHC |
HIGH Q CAPS FOR VLF LOOP ANTENNA?
OK Reg,
You are very patient and your time and expertise is appreciated. Last night I looked at many web articles on loops, many of which had preamps. I was a little discouraged. Many were fed with coax, which seems a little odd, especially since some of these loops had secondary resonances in the MF or HF bands. It seems like a preamp with a little non linearity, some big MF signals nearby, and a feed line that isn't balanced is a bit of a disaster waiting to happen. I know many of the HF radios in use have preamps that can be turned off (actually they are 20 db pads). But, regardless of my receivers susceptibility to out of band signals, I would want to feed my receiver with balanced line...especially since it's much cheaper than coax and should help prevent the feed line from picking up signals. So............followed everything in your last message and it all seems so simple (now).... Getting back to last nights study session. Spent a couple of hours in my 1987 ARRL Handbook and the remainder on the web looking at real life loops published there. The web aspect was really disappointingly devoid of technical jargon, it seems like most of the loop authors just threw something up and it seemed to work-the end:: Do people just throw stuff up without understanding what they're doing, or do they understand and just fail to document the theory?? The Handbook tour was almost as bad. Very little was said about loops except that which applied to the full wave resonant loop and how it can serve as a driven element in a 1 lambda 'guad' type radiator. Other than the theoretical wavelength, the correction factor for wire diameter, there was not more than 2 paragraphs written with useful information on short loops such as I am trying to put up. ------------------------- There was a question I had, perhaps I'm reading between the lines here tho.... YOU SAID: And even with an extremely long line any impedance mismatch loss will be negligible. So forget about 300-ohm balanced line and just use a simple not-tightly-twisted pair. NO IMPEDANCE MATCHING REQUIRED at either end. The size of the small coupling loop inside the main loop matches 50 ohms to a 50-ohm receiver. So ideally the line to the receiver could be 50-ohm coax. But, as I say, it doesn't matter. The size and shape of the small coupling loop is not critical. It can be circular or square. Theoretically, to match the loop to a 50-ohm receiver, it should have an area about 1/25th of the main loop area. To simplify construction the coupling loop can be made self-supporting. With your statement above in mind, are you telling me (or inferring) that any loop can be made to match a 50 ohm line by controlling it's size (area inside the main loop compared to area inside the coupling loop)?? In rjeloop3, the different sized loops have different feed line impedances, ranging from about 200 to 4K ohms. I made my final selection based on the model that matched my existing 300 ohm twin lead. I know you said my line impedance doesn't matter much, and I agree, except that it upsets the tuned front end in the receiver a bit. If it's possible to control this impedance of the small loop, what are the parameters I need to know to adjust mine to different values (other than those given by the software)?? I know you gave me the formula for matching to 50 ohms already. But, if this value can be controlled to customize the impedance, what do I need to know in order to make a match for other values? For my planned 2 meter square loop, my area is 2 X 2 X 4 (2 meters by 2 meters times 4 turns), sm my total loop area is 16 square meters. From the information you gave me (above), I can feed the receiver with 50 ohm feed line if my coupling loop is 16/25 (or about .8 square meters)? I know the matching isn't critical, but I'd like to have a good match for the sake of the front end tuned circuit in the receiver. If you can't say without writing a book, please feel free to decline to answer. If you can give the answer briefly, I'd appreciate some comments however. Hey Reg, was down in the basement where I keep the junk box and found a couple hundred feet of 4 conductor #24 solid copper cable. It's used for connecting telephones indoors. It's even cheaper than 300 ohm twin lead and I was wondering if I could/should use that as a feed line to the house?? Today I surveyed the loop location in the back yard, I have my solder and my 5/16" double weave rayon rope spool out and ready to go. My loop resonating capacitors should be here Wednesday. If all goes well, I could have a working installation by this time next week! Thing's are looking up. Also, Richard suggested a book about loops which I did not find in the library, or even in the reference section. I didn't price it yet, but I need to get some additional technical info. Do you have any suggestions for what to buy for books?? I don't need anything other than short loop theory (without heavy math). Thanks to you and Richard and everyone else who commented in this thread, I learned a great deal. Regards, T At 60 KHz, 4 turns of 2 mm diameter wire, spaced 4 wire diameters apart. 2 meters per side 123 uH, 60,000 pF to resonate 4.7K across the loop. 300 ohm feed impedance at single turn loop feed Q (unloaded) = 101 |
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