Magnetic Loop Performance
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I could use some help with a MagLoop. I constructed a 48" loop from 3.8"
copper pipe. I have a 51 pf capacitor across the open end of the loop. I am feeding the loop with a 4 turn coil around the copper pipe opposite of the capacitor. Before checking the loop I tested a carbon resistor on the balun (ant side). Using a 50ohm resistor on the ant side of a 1:1 Balun which was fed with a 2' piece of 50 ohm coax and with the loop disconnected I get the following readings from a MFJ 259B antenna analyzer: Freq Resistance X 9.7mhz 66ohm 0 10 68 0 12 77 0 14 89 0 17 109 0 21 126 0 27 128 0 27.6 78 0 Reattaching the feed coil and the balun I get the following readings Freq Resistance X 9.7mhz 18ohm 77 10 20 81 12 43 119 15 163 203 16 303 282 16.5 406 120 16.56 440 0 17 547 0 21 >650 21.3 467 114 24 95 191 27 38 129 27.6 34 120 29.1 21 104 I ran all these test trying to find out what the resonance is for my 51pf capacitor. Rather than helping me, I am more confused. I was not finding anything like the normal response I have been led to believe I should experience with a magloop. E.g. peek noise at the resonance frequency. John AD2F ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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John - when you say 4 turn coil around the copper pipe, what exactly do you
mean? What I'm wondering is, what is the orientation of the 4 turn loop with respect to the copper tubing loop? You don't mean that you wound the 4 turns around the copper pipe do you? The two loops should be in the same plane, perhaps offset, but you should be able to stick a rod through both loops. Bob - W3DK -----Original Message----- From: Elecraft [mailto:[hidden email]] On Behalf Of johnpierce Sent: Monday, October 24, 2016 8:23 PM To: [hidden email] Subject: [Elecraft] Magnetic Loop Performance I could use some help with a MagLoop. I constructed a 48" loop from 3.8" copper pipe. I have a 51 pf capacitor across the open end of the loop. I am feeding the loop with a 4 turn coil around the copper pipe opposite of the capacitor. Before checking the loop I tested a carbon resistor on the balun (ant side). Using a 50ohm resistor on the ant side of a 1:1 Balun which was fed with a 2' piece of 50 ohm coax and with the loop disconnected I get the following readings from a MFJ 259B antenna analyzer: Freq Resistance X 9.7mhz 66ohm 0 10 68 0 12 77 0 14 89 0 17 109 0 21 126 0 27 128 0 27.6 78 0 Reattaching the feed coil and the balun I get the following readings Freq Resistance X 9.7mhz 18ohm 77 10 20 81 12 43 119 15 163 203 16 303 282 16.5 406 120 16.56 440 0 17 547 0 21 >650 21.3 467 114 24 95 191 27 38 129 27.6 34 120 29.1 21 104 I ran all these test trying to find out what the resonance is for my 51pf capacitor. Rather than helping me, I am more confused. I was not finding anything like the normal response I have been led to believe I should experience with a magloop. E.g. peek noise at the resonance frequency. John AD2F ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by johnpierce
What does 48" mean? Diameter? radius? circumference?
Where did the 51 pf value come from. Most small mag loops have variable caps to tune them. What is the "balun" you mention? "4 turn coil around the copper pipe?" Not sure what you mean. Small mag loops are pretty simple, and very well characterized. Most are fed as resonant transformers. More info would really help. 73, Fred K6DGW - Sparks NV DM09dn - Northern California Contest Club - CU in the Cal QSO Party 7-8 Oct 2017 - www.cqp.org On 10/24/2016 5:22 PM, johnpierce wrote: > I could use some help with a MagLoop. I constructed a 48" loop from 3.8" > copper pipe. I have a 51 pf capacitor across the open end of the loop. I > am feeding the loop with a 4 turn coil around the copper pipe opposite of > the capacitor. Before checking the loop I tested a carbon resistor on the > balun (ant side). Using a 50ohm resistor on the ant side of a 1:1 Balun > which was fed with a 2' piece of 50 ohm coax and with the loop disconnected > I get the following readings from a MFJ 259B antenna analyzer: ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by johnpierce
First off, Is your loop 48 inches diameter or total circumference of 48
inches? Second, is your cap a butterfly cap. Those camps make a huge difference in performance. Although I have successfully used a double ganged cap with excellent results. Using a toroid to connect the loop with the turns on the toroid and the pipe going through the loop may be a better set up. Spacing on the coil turns and contact with the loop will change dramatically and I would highly recommend a faraday loop for feeding/driving the loop. make this coil/loop a circle with diameter 1/5 the diameter of the loop. For a 48 inch diameter loop that would be about 9.5 inches diameter. My loop worked great with a 1/4 inch copper tube 1/5 the diameter of the main loop fed directly across the coax, that is, braid on one side center on the other. I adjusted the matching by moving the loop up and down on the central axis. It worked great and would take 350 watts with out heating up, providing the resonant frequency of the loop was matched with the capacitor. My cap was rated at 2500 voltage break down. You might consider a gamma match with your loop, Although this will make the loop favor one side over the other but only marginally. Personally, I have never had much luck with just turns on wire on the loop to make it work. I know it is in the literature but it never worked repeatability for me because of the variability of the construct. I never used a 1to1 balun on any of my loops. And never got kissed by RF coming back at me. I could see that a wire wound choke balun may introduce inductance in the match and cause problems where as a bead balun may not cause problems. Also you are expecting way to much from this loop. Loops only work well for 1 maybe 2 bands at a time. This is limited by the variability of the capacitor and the inter capacitance of the construct. As you go up in frequency your need way way less capacitance and soon your loop will be unmatchable. Your 48 inch loop makes the conductor length of 12.5 feet. This is way big for 15 meters because it is greater than a 1/4 wave at that frequency and loops only function as a loop if they are a fraction of a quarter wave in frequency. You loop, if it is 48 inches in diameter is ideal for 30 and 20 meters and with the right capacitor will be usable on 40 with reasonable efficiency. A 2.5 foot diameter loop for the high bands is more in the ideal range for 20-10 meter operation. Because the total circumference of the loop is only 8 feet which is shorter than a 1/4 wave at the lowest frequency, 28 mhz. A loop this size will best work on 15-10 meters and will be matchable to work with decreased efficiency on 20. It calculates to about 48 % on 20 with all the other values much higher in efficiency as the frequency goes up. The Problem is with the matching capacitor minimum and maximum values and the intrinsic capacitance of the total system changing the resonance. Finding capacitors has always been the weak point in Loop construction. Vacuum variables with high Q values are the best widest frequency solution and can run the most power to the antenna because of their high voltage break down. The problem is the fragility and the cost factor of the capacitor. I shopped EBAY and found air variable double gang or butterfly caps that would work over a wide value of frequency for pennies compared to the dollars spent on vacuum variables, and for less than 100 watts, worked quite well. This website has helped me immeasurabley when I was constructing loops. For me this was the best calculator and guide to construction. http://www.66pacific.com/calculators/small_tx_loop_calc.aspx Vy 73, Morgan Bailey NJ8M On Mon, Oct 24, 2016 at 7:22 PM, johnpierce <[hidden email]> wrote: > I could use some help with a MagLoop. I constructed a 48" loop from 3.8" > copper pipe. I have a 51 pf capacitor across the open end of the loop. I > am feeding the loop with a 4 turn coil around the copper pipe opposite of > the capacitor. Before checking the loop I tested a carbon resistor on the > balun (ant side). Using a 50ohm resistor on the ant side of a 1:1 Balun > which was fed with a 2' piece of 50 ohm coax and with the loop disconnected > I get the following readings from a MFJ 259B antenna analyzer: > > > > Freq Resistance X > > 9.7mhz 66ohm 0 > > 10 68 0 > > 12 77 0 > > 14 89 0 > > 17 109 0 > > 21 126 0 > > 27 128 0 > > 27.6 78 0 > > > > Reattaching the feed coil and the balun I get the following readings > > Freq Resistance X > > 9.7mhz 18ohm 77 > > 10 20 81 > > 12 43 119 > > 15 163 203 > > 16 303 282 > > 16.5 406 120 > > 16.56 440 0 > > 17 547 0 > > 21 >650 > > 21.3 467 114 > > 24 95 191 > > 27 38 129 > > 27.6 34 120 > > 29.1 21 104 > > > > I ran all these test trying to find out what the resonance is for my 51pf > capacitor. Rather > > than helping me, I am more confused. I was not finding anything like the > normal response > > I have been led to believe I should experience with a magloop. E.g. peek > noise at the resonance frequency. > > > > John > > AD2F > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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