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antenna theory made easy
"Jeff Liebermann" wrote in message ... On Thu, 23 Jan 2014 15:52:40 -0700, "Irv Finkleman" wrote: From all the reading I have done, the Magnetic Loop Antenna seems to be a good performer, and ideally suited to locations where large or long antennas are out of the question. Irv VE6BP I agree. A magnetic loop will both fit and work best at your location. No grounding system or ground radials required. I don't have a magnetic loop of my own but I've helped build 3 of them (all different) with local hams. Some things to think about before blundering forward: 1. Where are you and the loop going to live? The Q of the loop is very high which means you're going to be retuning the loop every time you change frequency more than a few KHz. If the loop is outside on the balcony, and you're inside near the radio, you're going to be running back and forth all day long. The only saving grace is if you're doing PSK31, which lives on a single frequency per band, and therefore doesn't require constant retuning. Think about motor driven capacitor tuning. I already have a small 12v motor and have tested it with a number of different capacitors. With a simple 10k wirewound pot I can slow it down to almost nothing, or bring it back up to some value, and that beautiful pot just gets lukewarm. Using a DPDT toggle switch I can control the direction as well. Remote tuning on a shoestring! 2. Magnetic loops generate rather high voltage across the tuning capacitor. You'll probably need either a wide spaced cheese cutter type capacitor, a Jennings vacuum variable capacitor ($150 to $350 on eBay), or a butterfly type variable capacitor. I prefer the butterfly, but they're difficult to find. So, build your own: http://www.alexloop.com/artigo21.html arrangements: http://www.alexloop.com I will be operating QRP and have a good selection of all types of capacitors, some fairly high voltage although many of the QRP designs I have seen will allow a simple broadcast radio capacitor (not those new plastic jobbies) to operate -- sometimes as high as 20W1 3. I've seen problems with the loop tuning drifting with temperature. It's not the transmit power but heating from the sun. Even if you're planning on sitting on one frequency all day, you should check for tuning drift. I plan to cut a big cloth circular case to drop over the loop. It will be some nice flowery design, and a plan to put it over the back of a lawnchair so that anyone who walks by and looks up to my 3rd floor balcony will see what appears to be a nice comfortable highback chair! That's mainly for subterfuge! At the same time, having been experimenting with assorted antennas over the years (before I began to deteriorate) I had a big yard full of wire, ground rods, and a tower with all kinds of things hanging off of it. I always kept an eye on the SWR bridge for any variations, so any of that sun heating drift will be watched carefully. Thanks for the tip though! For all my reading that is one thing I hadn't encountered before. 4. Magnetic loops are somewhat directional. You may need some method of spinning the loop for maximum signal. When mounted vertically, the loop is horizontally polarized. When hung over the balcony rail horizontally, it's vertically polarized. See photos of mounting I plan to mount it vertically using a piece of 2X4 sitting in a big pot or tub filled with gravel. Anticipating lots of experimentation once I get on the air, I can drill it, nail it, or whatever for various forms of support, and even clamp a whip on it. I also anticipate that I will eventually turn it into something akin to swiss cheese, and when that comes about, I can replace my 'cheapo balcony tower' with ease. I will also constuct a small platform to hold the tub/pot and by using some of the small plastic swivel casters I'll make the whole works rotatable. 5. Magnetic loop calculators: http://www.66pacific.com/calculators/small_tx_loop_calc.aspx http://www.aa5tb.com/aa5tb_loop_v1.22a.xls 6. Losses. If you plug the numbers into an antenna simulator, you'll find that the losses are HUGE. If you go to the bottom of the page at: http://www.aa5tb.com/loop.html there's a graph of losses versus different loop material sizes. Notice the 1" and 1" (0.5 ohms) plots. Only 0.5 ohms of added resistance and the losses increase by about 16dB at 3.5 MHz. That's a nice way of saying you can't just throw it together. Everything has to be soldered carefully and no sliding contacts on the caps. Your MFJ antenna analyzer will be handy for testing the construction. I'm counting on the MFJ Analyzer for lots of help. I'm very away of the losses, and where they occur and will be careful. As far as losses are concerned, when running QRP why worry? Under good band conditions like we had in the good old days (with propagation ever again improve?) you can work the world with only a few watts. some guys do it with milliwatts! I only want 75M for very local work within about 200Km, but I know I'll be quite efficient on 20 which is where I have always had the most fun. A few of the magloops I've looked at on the net were not all soldered -- many just used nuts, bolts and starwashers. Whenever I ran across them, I'd get back to the builder/operator and ask how they worked, and had they taken the resistance losses into consideration. In all cases they were happy with the way things were and making lots of contacts. One of the Manufactured loops is made of aluminim strips bolted together, and it seems to get good ratings on EHam. On hand, I also have a yoyo antenna, a Miracle Whip antenna, and the MP1 which will be on the air as soon as I can measure the radials and see how it works indoors. Did I mention two Slinky's that I want to try -- first as a short vertical, and then as a helical magnetic loop! I can hardly wait to get going -- and I'm getting closer every day! Thanks for the tips Jeff -- your contributions to the group are always great. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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antenna theory made easy
On Thu, 23 Jan 2014 21:06:12 -0700, "Irv Finkleman"
wrote: I already have a small 12v motor and have tested it with a number of different capacitors. With a simple 10k wirewound pot I can slow it down to almost nothing, or bring it back up to some value, and that beautiful pot just gets lukewarm. Using a DPDT toggle switch I can control the direction as well. Remote tuning on a shoestring! Loose the potentiometer and replace is with a gearbox reducer. You want to retain all the torque and power from the drive motor. Also, don't forget to hi voltage insulate the motor, so that you don't arc over to the motor in transmit. I plan to cut a big cloth circular case to drop over the loop. It will be some nice flowery design, and a plan to put it over the back of a lawnchair so that anyone who walks by and looks up to my 3rd floor balcony will see what appears to be a nice comfortable highback chair! That's mainly for subterfuge! If the lawn chair has an aluminum frame, it will detune the magnetic loop antenna. All plastic lawn chair is required. Also, don't fire up the transmitter when someone is sitting in the lawn chair. At the same time, having been experimenting with assorted antennas over the years (before I began to deteriorate) I had a big yard full of wire, ground rods, and a tower with all kinds of things hanging off of it. I always kept an eye on the SWR bridge for any variations, so any of that sun heating drift will be watched carefully. Thanks for the tip though! For all my reading that is one thing I hadn't encountered before. It's only a problem with high Q antennas. Figure on a Q of 100 to 200 for the lower bands. On 80 meters, that's a 2:1 VSWR bandwidth of about 25 to 35 KHz. It doesn't take much to drift the antenna off frequency. Most magnetic loop users don't notice because they're constantly tweaking the tuning capacitor. I plan to mount it vertically using a piece of 2X4 sitting in a big pot or tub filled with gravel. So much for the disguise antenna. Don't forget to pound a ground rod into the gravel. It won't do anything useful, but will make an interesting conversation starter as to whether a ground is necessary. Anticipating lots of experimentation once I get on the air, I can drill it, nail it, or whatever for various forms of support, and even clamp a whip on it. I also anticipate that I will eventually turn it into something akin to swiss cheese, and when that comes about, I can replace my 'cheapo balcony tower' with ease. Let me know when you're ready to build my inflatable antenna tower. I will also constuct a small platform to hold the tub/pot and by using some of the small plastic swivel casters I'll make the whole works rotatable. Lazy Susan works well. I use that for my direction finder hacks. Use lots of grease if left outside as the bearings tend to rust. I'm counting on the MFJ Analyzer for lots of help. I'm very away of the losses, and where they occur and will be careful. As far as losses are concerned, when running QRP why worry? If you only have a little power to work with, then losses become even more important than if you had hundreds of watts to waste. Under good band conditions like we had in the good old days (with propagation ever again improve?) you can work the world with only a few watts. some guys do it with milliwatts! I only want 75M for very local work within about 200Km, but I know I'll be quite efficient on 20 which is where I have always had the most fun. They still do it with milliwatts. http://hflink.com/jt65/ I'm not sure what's wrong with propagation these days. Probably global warming, or a government conspiracy. A few of the magloops I've looked at on the net were not all soldered -- many just used nuts, bolts and starwashers. They don't work well. One of those I rescued was initially thrown together but not soldered on the assumption that future modifications would be easier without soldering. It didn't work until after it was soldered. Whenever I ran across them, I'd get back to the builder/operator and ask how they worked, and had they taken the resistance losses into consideration. In all cases they were happy with the way things were and making lots of contacts. You can also make a fair number of contacts with a dummy load. I know, I've done it by accident. I also made a fair number of contacts on one Field Day using a coax cable that was not connected to any antenna. In some cases, a dummy load or no antenna would be an improvement over some of the home brew antennas I've seen. One of the Manufactured loops is made of aluminim strips bolted together, and it seems to get good ratings on EHam. You can test that yourself with your MFJ antenna analyzer. Build a loop that way. Measure the VSWR curve and impedance at resonance. Then, rattle the loop a bit to move the bolts a little. Measure again. My guess is you'll see substantial changes every time you bang on the antenna. It might also be interesting to use an ESR (equivalent series resistance) meter to measure the DC resistance. That's also going to vary. On hand, I also have a yoyo antenna, a Miracle Whip antenna, and the MP1 which will be on the air as soon as I can measure the radials and see how it works indoors. Anything with amazing, magic, miracle, ultimate, or other superlatives is usually over-rated. Did I mention two Slinky's that I want to try -- first as a short vertical, and then as a helical magnetic loop! A what? Never mind. I don't have time to model a pretzel made from a Slinky. I can hardly wait to get going -- and I'm getting closer every day! Oh-oh. Slow down. It might be more fun doing cut-n-try, but it's much easier if you plan your antenna carefully, and build it once, not 20 times. Thanks for the tips Jeff -- your contributions to the group are always great. Y'er welcome. Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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antenna theory made easy
On Fri, 24 Jan 2014 10:10:01 -0800, Jeff Liebermann
wrote: They still do it with milliwatts. http://hflink.com/jt65/ Sorry, wrong link: http://kk4dsd.com/2012/06/07/jt65-power-calculator/ Note that the power levels are tyically well below 5 watts. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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antenna theory made easy
On 1/24/2014 1:10 PM, Jeff Liebermann wrote:
On Thu, 23 Jan 2014 21:06:12 -0700, "Irv Finkleman" wrote: If the lawn chair has an aluminum frame, it will detune the magnetic loop antenna. All plastic lawn chair is required. Also, don't fire up the transmitter when someone is sitting in the lawn chair. [OT] Reminds me of a friend who put up a ground mounted HF vertical in his back yard (back in the 60's). He was sitting in the shack checking it out when his wife let the dog out. The dog saw the new fire hydrant in the back yard, but my friend was ready. Just as the dog lifted his leg, my friend hit the key - with a full KW going to the antenna. From then on, the dog gave the antenna a wide berth, always watching it suspiciously. -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
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antenna theory made easy
On Fri, 24 Jan 2014 15:06:27 -0500, Jerry Stuckle
wrote: Reminds me of a friend who put up a ground mounted HF vertical in his back yard (back in the 60's). He was sitting in the shack checking it out when his wife let the dog out. The dog saw the new fire hydrant in the back yard, but my friend was ready. Just as the dog lifted his leg, my friend hit the key - with a full KW going to the antenna. From then on, the dog gave the antenna a wide berth, always watching it suspiciously. Mythbusters did a test of urinating on an electrified 3rd rail. http://www.youtube.com/watch?v=pDY-0ijiOEQ It didn't work. Surface tension causes the urine stream to break up into non-connected globules, which will not conduct electricity. Later, they managed to get it to work with a 3 inch stream. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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antenna theory made easy
On 1/24/2014 6:16 PM, Jeff Liebermann wrote:
On Fri, 24 Jan 2014 15:06:27 -0500, Jerry Stuckle wrote: Reminds me of a friend who put up a ground mounted HF vertical in his back yard (back in the 60's). He was sitting in the shack checking it out when his wife let the dog out. The dog saw the new fire hydrant in the back yard, but my friend was ready. Just as the dog lifted his leg, my friend hit the key - with a full KW going to the antenna. From then on, the dog gave the antenna a wide berth, always watching it suspiciously. Mythbusters did a test of urinating on an electrified 3rd rail. http://www.youtube.com/watch?v=pDY-0ijiOEQ It didn't work. Surface tension causes the urine stream to break up into non-connected globules, which will not conduct electricity. Later, they managed to get it to work with a 3 inch stream. Who cares about the 3rd rail? That's 60hz, not RF. A HUGE difference. And they probably weren't 3-4" from the third rail like the dog was. Do you understand ANYTHING? -- ================== Remove the "x" from my email address Jerry, AI0K ================== |
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antenna theory made easy
On Fri, 24 Jan 2014 20:35:38 -0500, Jerry Stuckle
wrote: On 1/24/2014 6:16 PM, Jeff Liebermann wrote: On Fri, 24 Jan 2014 15:06:27 -0500, Jerry Stuckle wrote: Reminds me of a friend who put up a ground mounted HF vertical in his back yard (back in the 60's). He was sitting in the shack checking it out when his wife let the dog out. The dog saw the new fire hydrant in the back yard, but my friend was ready. Just as the dog lifted his leg, my friend hit the key - with a full KW going to the antenna. From then on, the dog gave the antenna a wide berth, always watching it suspiciously. Mythbusters did a test of urinating on an electrified 3rd rail. http://www.youtube.com/watch?v=pDY-0ijiOEQ It didn't work. Surface tension causes the urine stream to break up into non-connected globules, which will not conduct electricity. Later, they managed to get it to work with a 3 inch stream. Who cares about the 3rd rail? That's 60hz, not RF. A HUGE difference. And they probably weren't 3-4" from the third rail like the dog was. The 3rd rail is usually about 1500 VAC. Last time I checked, there has to be current flowing though the conduction path. The losses involved in jumping the air gap are just too high to electrocute the dog. The RF field from the antenna might cook the dog, but not electrocute. If you hadn't witten "Just as the dog lifted his leg" I wouldn't have said anything. Incidentally, I've gotten some rather nasty RF burns in the past. In all cases, I was in physical contact with the antenna. Unless the transmitter can produce an arc or possibly a corona discharge, there's not going to be much in the way of RF current or electrocution. Do you understand ANYTHING? Are you able to discuss anything technical without resorting to insults? -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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antenna theory made easy
On 1/25/2014 2:11 AM, Jeff Liebermann wrote:
On Fri, 24 Jan 2014 20:35:38 -0500, Jerry Stuckle wrote: On 1/24/2014 6:16 PM, Jeff Liebermann wrote: On Fri, 24 Jan 2014 15:06:27 -0500, Jerry Stuckle wrote: Reminds me of a friend who put up a ground mounted HF vertical in his back yard (back in the 60's). He was sitting in the shack checking it out when his wife let the dog out. The dog saw the new fire hydrant in the back yard, but my friend was ready. Just as the dog lifted his leg, my friend hit the key - with a full KW going to the antenna. From then on, the dog gave the antenna a wide berth, always watching it suspiciously. Mythbusters did a test of urinating on an electrified 3rd rail. http://www.youtube.com/watch?v=pDY-0ijiOEQ It didn't work. Surface tension causes the urine stream to break up into non-connected globules, which will not conduct electricity. Later, they managed to get it to work with a 3 inch stream. Who cares about the 3rd rail? That's 60hz, not RF. A HUGE difference. And they probably weren't 3-4" from the third rail like the dog was. The 3rd rail is usually about 1500 VAC. Last time I checked, there has to be current flowing though the conduction path. The losses involved in jumping the air gap are just too high to electrocute the dog. The RF field from the antenna might cook the dog, but not electrocute. If you hadn't witten "Just as the dog lifted his leg" I wouldn't have said anything. Try again, troll. Here in Washington, DC, it's 750V. And the rest of your comment is pure BS, also. Incidentally, I've gotten some rather nasty RF burns in the past. In all cases, I was in physical contact with the antenna. Unless the transmitter can produce an arc or possibly a corona discharge, there's not going to be much in the way of RF current or electrocution. So what? Do you understand ANYTHING? Are you able to discuss anything technical without resorting to insults? I get real tired of trolls who continue to show their ignorance while trying to contradict science. -- ================== Remove the "x" from my email address Jerry Stuckle ================== |
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antenna theory made easy
On Friday, January 24, 2014 5:16:15 PM UTC-6, Jeff Liebermann wrote:
Mythbusters did a test of urinating on an electrified 3rd rail. It didn't work. I got a severe static electricity shock in Odessa, TX one time while urinating into a hotel john. I saw the arc between my stream and the water in the john. The hotel had wool carpets and the humidity was very low. -- 73, Cecil, w5dxp.com |
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antenna theory made easy
On Fri, 24 Jan 2014 19:44:38 -0800 (PST), W5DXP
wrote: On Friday, January 24, 2014 5:16:15 PM UTC-6, Jeff Liebermann wrote: Mythbusters did a test of urinating on an electrified 3rd rail. It didn't work. I got a severe static electricity shock in Odessa, TX one time while urinating into a hotel john. I saw the arc between my stream and the water in the john. The hotel had wool carpets and the humidity was very low. Ouch. I feel your pain. Wikipedia claims 3 million volts/meter (or about 75,000 volts/inch) which seems a bit high: http://en.wikipedia.org/wiki/Dielectric_strength#Breakdown_field_strength I've always used 10,000 volts/inch but I don't recall the conditions for which that's accurate. In any case, you had quite a (static) high voltage buildup, which could jump the broken parts of the urine flow. 1000 watts into a ground mounted HF vertical? Is that a good idea? It might be too close to the operator to be within accepted RF exposure (MPE) limits. Checking: http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65b.pdf See table 4a on Pg 23. For 1000 watts the minimum distances for a typical 0dBi gain antenna is: Band Controlled Uncontrolled (meters) 160m 0.5 0.7 80m 0.6 1.3 40m 1.1 2.5 20m 2.2 4.8 15m 3.2 7.2 10m 4.5 10. So, if he's operating on 20 meters, a ground mounted antenna with no gain and 1000 watts can't be any closer than about 14 ft from the operator. Maybe with a big yard and if you don't care about cooking the dog or the neighbors. In order to put the voltage node of the vertical near where the dogs urine stream can reach, it would need to be fairly close to the ground. That leaves very little room for the ground radials, any possible control box, or a balun. Seem an odd vertical antenna with the loading coil at ground level. Maybe for 160 meters or a mono band antenna. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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