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#31
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Flex dryer vent hose loop antenna
On 11/8/2015 10:23 PM, Jeff Liebermann wrote:
On Sun, 8 Nov 2015 18:02:19 -0500, rickman wrote: On 11/8/2015 3:44 PM, Jeff Liebermann wrote: You might want to grab and read this: "The Underestimated Magnetic Loop HF Antenna V1.2" by Leigh Turner VK5KLT http://802.11junk.com/jeffl/antennas/magnetic-loop/_The%20Underestimated%20Magnetic%20Loop%20HF%20Ant enna_V1.%202.pdf You may have read a previous version. The author updated it recently and posted it to the Yahoo Magloop forum in the files section: https://groups.yahoo.com/neo/groups/MagLoop/info I couldn't find it posted anywhere else on the interknot, so I took the liberty of posting it to my web pile without permission. I just skimmed it quickly but offhand, it looks like a very good explanation of how a magnetic loop antenna works, without going excessively into technobabble and equations. Thanks. I signed up for that group, but only by email. To join Yahoo these days you have to give a mobile number and I'm not willing to do that. I signed up long ago and have been able to bypass that requirement. Eventually, I'm sure Yahoo, Google, Microsoft, and others will make it mandatory. What ****es me off is that the various vendors involved claim that it's a security feature, when it's really a bad excuse for cell phone calling and SMS spamming. Here's a possible solution: http://www.burnerapp.com Essentially, it's a throw away phone number service. I just found the URL, so I haven't had time to try it or decode the cryptic description on the web page. I found a discussion of this problem which listed a website for getting phone numbers, but it seems Yahoo won't work with this. I'm not sure what Burner app is about, but it looks like they give you a burner phone number which will relay texts and calls. But that just means you have to share your phone number with *them*. I've been getting the emails talking about this file (mostly people saying they can't download it) but no one has made it available outside of that group... until now. I'm not sure of the legality or if it's ethically correct. I would get a bit irritated if someone posted a copy of my work, instead of a link to the original. However, as soon as I find it publicly posted elsewhere by the author, I'll take mine down. Technically it is copyrighted. I'm not sure if that restricts the sharing of the file or not, I think so if he hadn't shared it openly elsewhere. Since it is there for anyone to download, I'm not sure there can be a problem unless he says you need to take it down. Some interesting comments on the magloop article: http://www.brisdance.com/vk4amz/VK5KLT.html -- Rick |
#32
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Flex dryer vent hose loop antenna
On 11/8/2015 10:46 PM, Jeff Liebermann wrote:
On Sun, 8 Nov 2015 19:12:38 -0500, rickman wrote: On 11/8/2015 5:14 PM, Jeff Liebermann wrote: You were asking about using a better dielectric than air. I found this article: http://www.qrpbuilder.com/downloads/loop%20antenna%20110310.pdf which includes construction details for a piston capacitor arrangement using various dielectrics. On Pg 8 is a chart of various plastics, with dielectric constant, dielectric strength, and dissipation factor. For cheap, the author recommends UHMW (polypropylene), which is one tenth the cost of PTFE (Teflon). Yeah, but he doesn't address the issue of temperature dependance of Er. I don't even see it in his table. True, but I don't think tempco is critical or required. A practical loop antenna, with sufficiently high Q and narrow bandwidth, will require an automatic tuning arrangement. I managed to built one where the operating bandwidth on 80 meters was less than the occupied bandwidth of a SSB signal. With that critical a frequency tuning, manual or fixed tuning isn't going to work. Once you have an automatic tuner, compensating for thermal drift is easy. I'm not familiar with automatic tuners that can tune the antenna while in use. One of the issues someone pointed out was that the dielectric can heat up from the energy absorbed during transmission. Is an antenna tuner real time in this case? I crunched some numbers and found 100's of PPM change in tuned frequency due to ambient temperature change over the course of a year for an antenna with an air or vacuum tuning capacitor. I can find ceramic dielectrics that would be lower than this and even in the opposite direction to offset the natural drift. But I can't find this info for PEX. Incidentally, one of the problems I'm fighting is that the loop tuning is different between TX and RX because the impedance of the receiver and transmitter are slightly different and enought to detune the loop. That's another problem that an automagic tuner should fix. The text says he used PEX (cross linked polyethylene) for the capacitor, but I can't find much info on the electrical characteristics. The one that is hardest to find is the temperature dependence of Er. I don't see PEX in his table at all. Very odd. I use PEX plumbing pipe for coil forms, spacers, standoffs, and spreaders. Works well, but seems a bit expensive. This might help: http://www.smeter.net/daily-facts/11/fact21.php Er = 2.3 and 60-90 kV/mm I'm not terribly concerned with the actual value of Er and even the dielectric strength. What is important to me is the temperature coefficient of Er. Here's something on building a trombone capacitor: http://www.eham.net/ehamforum/smf/index.php?topic=70784.0;wap2 Er = 2.25 Interesting, but nearly every discussion I find on loop antennas has a lot of fluff content. Here is some from this discussion, "By the way PEX is cross linked polyethylene and is superior to using sheet Teflon in this instance." Unless the reason is stated for considering PEX superior to Teflon, I haven't learned anything. I'm certainly not going to take an anonymous person's word for it. This looks a bit more authoritative: http://www.comfortprosystems.com/sites/comfortprosystems.com/files/cps_aquaheat_pex_pressdrop_tec-04.pdf Er = 2.3 The info on PEX that I can find on the web indicates it may have problems with use outdoors, but maybe this antenna isn't intended to be used outdoors. Just about everything plastic has problems with UV embrittlement. The best fix I've found is Krylon clear acrylic spray. http://yarchive.net/electr/plastic_uv_resistances.html Hint: Search Yarchive and Google for posting by Dr Barry L. Ornitz WA4VZQ. Lots of really good info on materials, chemicals, processes, and antennas. I'm talking about water impacts. Humidity and rain soak into materials. Some by absorption, others by infiltration into micro-cracks. I saw some materials that talked about water trees in PEX. This is not a universal problem in all plastics. Incidentally, if you dive into the Yahoo magloop files sections, there are some photos of the insides of the MFJ-1786 mag loop. http://www.mfjenterprises.com/Product.php?productid=MFJ-1786 https://groups.yahoo.com/neo/groups/MagLoop/files/MFJ-1786/MFJ-1786%20coupling%20loop/ Two things worthy of notice. All the aluminum parts are brazed or welded together and the matching(?) coil inside the box appears to be silver plated. If MFJ's reputation for cheap construction is to be believed, they would not silver plate anything if a cheaper alternative would work. I can't get to the MagLoop files. I don't know anything about MFJ's reputation. There are simple facts about silver that make it only very slightly better than copper for RF circuits. I know that you can increase the size of the conductor by less than 5% as an alternative to using silver plating if the electrical characteristics are the goal. Can you explain why silver is required? The numbers don't show it. -- Rick |
#33
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Flex dryer vent hose loop antenna
On 11/9/2015 12:59 AM, rickman wrote:
On 11/8/2015 10:46 PM, Jeff Liebermann wrote: On Sun, 8 Nov 2015 19:12:38 -0500, rickman wrote: On 11/8/2015 5:14 PM, Jeff Liebermann wrote: You were asking about using a better dielectric than air. I found this article: http://www.qrpbuilder.com/downloads/loop%20antenna%20110310.pdf which includes construction details for a piston capacitor arrangement using various dielectrics. On Pg 8 is a chart of various plastics, with dielectric constant, dielectric strength, and dissipation factor. For cheap, the author recommends UHMW (polypropylene), which is one tenth the cost of PTFE (Teflon). Yeah, but he doesn't address the issue of temperature dependance of Er. I don't even see it in his table. True, but I don't think tempco is critical or required. A practical loop antenna, with sufficiently high Q and narrow bandwidth, will require an automatic tuning arrangement. I managed to built one where the operating bandwidth on 80 meters was less than the occupied bandwidth of a SSB signal. With that critical a frequency tuning, manual or fixed tuning isn't going to work. Once you have an automatic tuner, compensating for thermal drift is easy. I'm not familiar with automatic tuners that can tune the antenna while in use. One of the issues someone pointed out was that the dielectric can heat up from the energy absorbed during transmission. Is an antenna tuner real time in this case? I crunched some numbers and found 100's of PPM change in tuned frequency due to ambient temperature change over the course of a year for an antenna with an air or vacuum tuning capacitor. I can find ceramic dielectrics that would be lower than this and even in the opposite direction to offset the natural drift. But I can't find this info for PEX. Incidentally, one of the problems I'm fighting is that the loop tuning is different between TX and RX because the impedance of the receiver and transmitter are slightly different and enought to detune the loop. That's another problem that an automagic tuner should fix. The text says he used PEX (cross linked polyethylene) for the capacitor, but I can't find much info on the electrical characteristics. The one that is hardest to find is the temperature dependence of Er. I don't see PEX in his table at all. Very odd. I use PEX plumbing pipe for coil forms, spacers, standoffs, and spreaders. Works well, but seems a bit expensive. This might help: http://www.smeter.net/daily-facts/11/fact21.php Er = 2.3 and 60-90 kV/mm I'm not terribly concerned with the actual value of Er and even the dielectric strength. What is important to me is the temperature coefficient of Er. Here's something on building a trombone capacitor: http://www.eham.net/ehamforum/smf/index.php?topic=70784.0;wap2 Er = 2.25 Interesting, but nearly every discussion I find on loop antennas has a lot of fluff content. Here is some from this discussion, "By the way PEX is cross linked polyethylene and is superior to using sheet Teflon in this instance." Unless the reason is stated for considering PEX superior to Teflon, I haven't learned anything. I'm certainly not going to take an anonymous person's word for it. This looks a bit more authoritative: http://www.comfortprosystems.com/sites/comfortprosystems.com/files/cps_aquaheat_pex_pressdrop_tec-04.pdf Er = 2.3 The info on PEX that I can find on the web indicates it may have problems with use outdoors, but maybe this antenna isn't intended to be used outdoors. Just about everything plastic has problems with UV embrittlement. The best fix I've found is Krylon clear acrylic spray. http://yarchive.net/electr/plastic_uv_resistances.html Hint: Search Yarchive and Google for posting by Dr Barry L. Ornitz WA4VZQ. Lots of really good info on materials, chemicals, processes, and antennas. I'm talking about water impacts. Humidity and rain soak into materials. Some by absorption, others by infiltration into micro-cracks. I saw some materials that talked about water trees in PEX. This is not a universal problem in all plastics. Incidentally, if you dive into the Yahoo magloop files sections, there are some photos of the insides of the MFJ-1786 mag loop. http://www.mfjenterprises.com/Product.php?productid=MFJ-1786 https://groups.yahoo.com/neo/groups/MagLoop/files/MFJ-1786/MFJ-1786%20coupling%20loop/ Two things worthy of notice. All the aluminum parts are brazed or welded together and the matching(?) coil inside the box appears to be silver plated. If MFJ's reputation for cheap construction is to be believed, they would not silver plate anything if a cheaper alternative would work. I can't get to the MagLoop files. I don't know anything about MFJ's reputation. There are simple facts about silver that make it only very slightly better than copper for RF circuits. I know that you can increase the size of the conductor by less than 5% as an alternative to using silver plating if the electrical characteristics are the goal. Can you explain why silver is required? The numbers don't show it. The silver is simply optimization. If you can make your coil wire 5% bigger, you should have already done that. Then if you want to optimized 1 + 0.05, silver plate it. Mikek |
#34
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Flex dryer vent hose loop antenna
On Mon, 9 Nov 2015 01:59:06 -0500, rickman wrote:
I'm not familiar with automatic tuners that can tune the antenna while in use. The basic benefits of having a remote controller a 1. RF safety and you're not part of the antenna system. 2. A controller is easier to umm.... control. 3. Automatic remote tuning reacts to changes when you're not looking at the VSWR meter. 4. White knuckle tuning is difficult. Let the servos do the work. 5. It's the only effective way to tune a loop mounted on a tall pole, tower, or roof. One of the issues someone pointed out was that the dielectric can heat up from the energy absorbed during transmission. Is an antenna tuner real time in this case? Yes in receive. Probably not in transmit. In receive, you can tune all you want and nothing will explode or catch fire. In transmit, you can easily tune through full power and arc over the tuning capacitor. Auto tuning also goes through the optimum VSWR point several times during the tuning cycle. You transmitter may not like operating into a high VSWR load during tuning. The solution is to tune at low tranmit power levels. Once the lowest VSWR point is found, you can increase your power. However, that usually prevents you from "tweaking" the tuning at full power, which is what you need to compensate for thermal drift. It's also difficult to tune with any modulation other than CW. I think (not sure) that some controllers have this ability, probably with warnings and disclaimers. I wouldn't trust it. Basically, to make it work requires a VWSR sensor and calculator that works when there's modulation, and a tuning capacitor that can tolerate moving while passing high currents. Arcing and welding the bearing and bushings might be a problem. I crunched some numbers and found 100's of PPM change in tuned frequency due to ambient temperature change over the course of a year for an antenna with an air or vacuum tuning capacitor. I can find ceramic dielectrics that would be lower than this and even in the opposite direction to offset the natural drift. But I can't find this info for PEX. I looked and also didn't find anything. The problem is that you don't find tempco data for plumbing parts that were not intended to be used for RF components. I'm not terribly concerned with the actual value of Er and even the dielectric strength. What is important to me is the temperature coefficient of Er. Again, I think you might be trying to solve a problem that has already been solved by automatic tuning (which you need anyway). Unless you plan to transmit endlessly, you can simply press the tune button on the controller a few times per hour, and be done with trying to temperature stabilize the loop. Interesting, but nearly every discussion I find on loop antennas has a lot of fluff content. RF is magic. It's difficult to explain some things. Here is some from this discussion, "By the way PEX is cross linked polyethylene and is superior to using sheet Teflon in this instance." Unless the reason is stated for considering PEX superior to Teflon, I haven't learned anything. I'm certainly not going to take an anonymous person's word for it. The world is divided between practitioners of theory and of practice. Those who favor can explain anything, but can't build anything that actually works. Those that favor practice tend to build strange contraptions that they can't explain. That's also probably the main source of what you call "fluff". Sometimes, I run into a theoretician that knows which end of the soldering iron to grab, but they are rare. For the record, I'm a practitioner of practice, trial-n-error, and magic. I have difficulty explaining some things, and I think you've seen my horrid math. I'm talking about water impacts. Humidity and rain soak into materials. Some by absorption, others by infiltration into micro-cracks. I saw some materials that talked about water trees in PEX. This is not a universal problem in all plastics. I come from the marine radio part of the business. Water and corrosion are key parts of the marine radio problem. Many materials are hygroscopic and will absorb moisture. Apply some RF and the water boils out, as in a microwave oven. Do it too fast, and the material can crack. Of course, the tuning will change. Lots of info on plastics selection for minimal water absorption found online. For example: http://www.curbellplastics.com/technical-resources/pdf/water-absorption-plastics.pdf Polypropylene would be my first choice for minimal water absorption. However, it requires UV protection, which for RF applications means some kind of conformal coating. (Adding carbon black is a bad idea as it causes heating problems). There are simple facts about silver that make it only very slightly better than copper for RF circuits. I know that you can increase the size of the conductor by less than 5% as an alternative to using silver plating if the electrical characteristics are the goal. Can you explain why silver is required? The numbers don't show it. Antennas are mounted outdoors where exposed copper is an invitation to corrosion. If one must protect the copper with something, why not use silver, which also improves its RF characteristics? My magloop nightmare come true: http://www.mixw.co.uk/MagLoop/magloopF.htm -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#35
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Flex dryer vent hose loop antenna
On 11/9/2015 12:08 PM, amdx wrote:
On 11/9/2015 12:59 AM, rickman wrote: On 11/8/2015 10:46 PM, Jeff Liebermann wrote: On Sun, 8 Nov 2015 19:12:38 -0500, rickman wrote: On 11/8/2015 5:14 PM, Jeff Liebermann wrote: You were asking about using a better dielectric than air. I found this article: http://www.qrpbuilder.com/downloads/loop%20antenna%20110310.pdf which includes construction details for a piston capacitor arrangement using various dielectrics. On Pg 8 is a chart of various plastics, with dielectric constant, dielectric strength, and dissipation factor. For cheap, the author recommends UHMW (polypropylene), which is one tenth the cost of PTFE (Teflon). Yeah, but he doesn't address the issue of temperature dependance of Er. I don't even see it in his table. True, but I don't think tempco is critical or required. A practical loop antenna, with sufficiently high Q and narrow bandwidth, will require an automatic tuning arrangement. I managed to built one where the operating bandwidth on 80 meters was less than the occupied bandwidth of a SSB signal. With that critical a frequency tuning, manual or fixed tuning isn't going to work. Once you have an automatic tuner, compensating for thermal drift is easy. I'm not familiar with automatic tuners that can tune the antenna while in use. One of the issues someone pointed out was that the dielectric can heat up from the energy absorbed during transmission. Is an antenna tuner real time in this case? I crunched some numbers and found 100's of PPM change in tuned frequency due to ambient temperature change over the course of a year for an antenna with an air or vacuum tuning capacitor. I can find ceramic dielectrics that would be lower than this and even in the opposite direction to offset the natural drift. But I can't find this info for PEX. Incidentally, one of the problems I'm fighting is that the loop tuning is different between TX and RX because the impedance of the receiver and transmitter are slightly different and enought to detune the loop. That's another problem that an automagic tuner should fix. The text says he used PEX (cross linked polyethylene) for the capacitor, but I can't find much info on the electrical characteristics. The one that is hardest to find is the temperature dependence of Er. I don't see PEX in his table at all. Very odd. I use PEX plumbing pipe for coil forms, spacers, standoffs, and spreaders. Works well, but seems a bit expensive. This might help: http://www.smeter.net/daily-facts/11/fact21.php Er = 2.3 and 60-90 kV/mm I'm not terribly concerned with the actual value of Er and even the dielectric strength. What is important to me is the temperature coefficient of Er. Here's something on building a trombone capacitor: http://www.eham.net/ehamforum/smf/index.php?topic=70784.0;wap2 Er = 2.25 Interesting, but nearly every discussion I find on loop antennas has a lot of fluff content. Here is some from this discussion, "By the way PEX is cross linked polyethylene and is superior to using sheet Teflon in this instance." Unless the reason is stated for considering PEX superior to Teflon, I haven't learned anything. I'm certainly not going to take an anonymous person's word for it. This looks a bit more authoritative: http://www.comfortprosystems.com/sites/comfortprosystems.com/files/cps_aquaheat_pex_pressdrop_tec-04.pdf Er = 2.3 The info on PEX that I can find on the web indicates it may have problems with use outdoors, but maybe this antenna isn't intended to be used outdoors. Just about everything plastic has problems with UV embrittlement. The best fix I've found is Krylon clear acrylic spray. http://yarchive.net/electr/plastic_uv_resistances.html Hint: Search Yarchive and Google for posting by Dr Barry L. Ornitz WA4VZQ. Lots of really good info on materials, chemicals, processes, and antennas. I'm talking about water impacts. Humidity and rain soak into materials. Some by absorption, others by infiltration into micro-cracks. I saw some materials that talked about water trees in PEX. This is not a universal problem in all plastics. Incidentally, if you dive into the Yahoo magloop files sections, there are some photos of the insides of the MFJ-1786 mag loop. http://www.mfjenterprises.com/Product.php?productid=MFJ-1786 https://groups.yahoo.com/neo/groups/MagLoop/files/MFJ-1786/MFJ-1786%20coupling%20loop/ Two things worthy of notice. All the aluminum parts are brazed or welded together and the matching(?) coil inside the box appears to be silver plated. If MFJ's reputation for cheap construction is to be believed, they would not silver plate anything if a cheaper alternative would work. I can't get to the MagLoop files. I don't know anything about MFJ's reputation. There are simple facts about silver that make it only very slightly better than copper for RF circuits. I know that you can increase the size of the conductor by less than 5% as an alternative to using silver plating if the electrical characteristics are the goal. Can you explain why silver is required? The numbers don't show it. The silver is simply optimization. If you can make your coil wire 5% bigger, you should have already done that. Then if you want to optimized 1 + 0.05, silver plate it. Why can't you make the wire 5% bigger again? Where exactly is the cost? -- Rick |
#36
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Flex dryer vent hose loop antenna
On 11/9/2015 7:52 PM, Jeff Liebermann wrote:
On Mon, 9 Nov 2015 01:59:06 -0500, rickman wrote: I'm not familiar with automatic tuners that can tune the antenna while in use. The basic benefits of having a remote controller a 1. RF safety and you're not part of the antenna system. 2. A controller is easier to umm.... control. 3. Automatic remote tuning reacts to changes when you're not looking at the VSWR meter. 4. White knuckle tuning is difficult. Let the servos do the work. 5. It's the only effective way to tune a loop mounted on a tall pole, tower, or roof. A remote controller is not an automatic tuner. Automatic implies the controller tunes the resonance while a remote controller is just that, a control to adjust the tuning remotely. I don't see how this could work while transmitting. The tuning needs to be done separately. One of the issues someone pointed out was that the dielectric can heat up from the energy absorbed during transmission. Is an antenna tuner real time in this case? Yes in receive. Probably not in transmit. In receive, you can tune all you want and nothing will explode or catch fire. In transmit, you can easily tune through full power and arc over the tuning capacitor. Auto tuning also goes through the optimum VSWR point several times during the tuning cycle. You transmitter may not like operating into a high VSWR load during tuning. How does an automatic tuner operate? When you mention VSWR, how is that affected by tuning? The solution is to tune at low tranmit power levels. Once the lowest VSWR point is found, you can increase your power. However, that usually prevents you from "tweaking" the tuning at full power, which is what you need to compensate for thermal drift. It's also difficult to tune with any modulation other than CW. I think (not sure) that some controllers have this ability, probably with warnings and disclaimers. I wouldn't trust it. Basically, to make it work requires a VWSR sensor and calculator that works when there's modulation, and a tuning capacitor that can tolerate moving while passing high currents. Arcing and welding the bearing and bushings might be a problem. I crunched some numbers and found 100's of PPM change in tuned frequency due to ambient temperature change over the course of a year for an antenna with an air or vacuum tuning capacitor. I can find ceramic dielectrics that would be lower than this and even in the opposite direction to offset the natural drift. But I can't find this info for PEX. I looked and also didn't find anything. The problem is that you don't find tempco data for plumbing parts that were not intended to be used for RF components. Substances have properties regardless of usage. PEX is not just used for plumbing. It is also used in electrical cables where the dielectric properties are very important. I'm not terribly concerned with the actual value of Er and even the dielectric strength. What is important to me is the temperature coefficient of Er. Again, I think you might be trying to solve a problem that has already been solved by automatic tuning (which you need anyway). Unless you plan to transmit endlessly, you can simply press the tune button on the controller a few times per hour, and be done with trying to temperature stabilize the loop. That's not how it was presented to me. The suggestion was that tuning will change during transmission due to heating from the power being transmitted. Of course that depends on not just the Er dependance with temperature, but also the dissipation factor. So perhaps with material of a sufficiently low DF the Er dependence on temperature is not so important. Interesting, but nearly every discussion I find on loop antennas has a lot of fluff content. RF is magic. It's difficult to explain some things. You mean people don't understand it. Anything that is understood can be explained. If you don't understand it, you can't explain it. Even so, that's not fluff. Fluff is when things are described in non-rigorous ways like, "solder joints result in sub-optimum performance". Hard to prove or disprove. Clearly they will have some effect even if that effect too small to be measurable. "Sub-optimal" is pretty meaningless in general until you define the details of "optimal". Here is some from this discussion, "By the way PEX is cross linked polyethylene and is superior to using sheet Teflon in this instance." Unless the reason is stated for considering PEX superior to Teflon, I haven't learned anything. I'm certainly not going to take an anonymous person's word for it. The world is divided between practitioners of theory and of practice. Those who favor can explain anything, but can't build anything that actually works. Those that favor practice tend to build strange contraptions that they can't explain. That's also probably the main source of what you call "fluff". Sometimes, I run into a theoretician that knows which end of the soldering iron to grab, but they are rare. For the record, I'm a practitioner of practice, trial-n-error, and magic. I have difficulty explaining some things, and I think you've seen my horrid math. I think the world is *not* divided at all, rather there is a range of abilities on both theory and practice scales with independent values. Fluff is fluff no mater what you are good at. If someone can't recognize fluff, then they are missing a lot of understanding. I'm used to seeing this sort of lack of rigor in many pursuits, cars, sports, etc. I've also seen it in safety. Lots of people use seat of the pants concepts in analyzing safety. It shows up very easily when you simply ask questions about the source of the info. Same with the amateur design of antennas. Lots of talk, but very little data in most cases, like with solder joints. I'm talking about water impacts. Humidity and rain soak into materials. Some by absorption, others by infiltration into micro-cracks. I saw some materials that talked about water trees in PEX. This is not a universal problem in all plastics. I come from the marine radio part of the business. Water and corrosion are key parts of the marine radio problem. Many materials are hygroscopic and will absorb moisture. Apply some RF and the water boils out, as in a microwave oven. Do it too fast, and the material can crack. Of course, the tuning will change. Lots of info on plastics selection for minimal water absorption found online. For example: http://www.curbellplastics.com/technical-resources/pdf/water-absorption-plastics.pdf Polypropylene would be my first choice for minimal water absorption. However, it requires UV protection, which for RF applications means some kind of conformal coating. (Adding carbon black is a bad idea as it causes heating problems). Plastic in a tuning capacitor should be protected from the elements in other ways. Conformal coating is not really needed if the entire capacitor is in a box, even a transparent plastic box as long as it blocks the UV. Most plastics do. There are simple facts about silver that make it only very slightly better than copper for RF circuits. I know that you can increase the size of the conductor by less than 5% as an alternative to using silver plating if the electrical characteristics are the goal. Can you explain why silver is required? The numbers don't show it. Antennas are mounted outdoors where exposed copper is an invitation to corrosion. If one must protect the copper with something, why not use silver, which also improves its RF characteristics? My solution would be to use aluminum instead. Copper is not really superior in a meaningful way and costs a lot more. My magloop nightmare come true: http://www.mixw.co.uk/MagLoop/magloopF.htm I don't get your point here. -- Rick |
#37
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Flex dryer vent hose loop antenna
On 11/10/2015 1:41 AM, rickman wrote:
On 11/9/2015 12:08 PM, amdx wrote: The silver is simply optimization. If you can make your coil wire 5% bigger, you should have already done that. Then if you want to optimized 1 + 0.05, silver plate it. Why can't you make the wire 5% bigger again? Where exactly is the cost? You can. I don't think ultimate optimizing is for you. Just make your coil with as much surface area as room will allow. Then know it could have just a tiny bit less loss if you had the silver. That tiny bit of loss will not be noticed in use, except for that little nagging thought... I once had a 6 or 7 turn loop made with 1/4" copper tubing*, Q was about 800 at 1MHz. I could have made it with 1/2" tubing, probably would have had higher Q. I guess the limits are money and how you want to limit physical size and maximum inductance you can use. Did I miss any? Mikek *It had a vacuum variable mounted on it, I had it in the garage and my young son moved it and broke the glass on the vacuum variable, it was a sad day for me. |
#38
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Flex dryer vent hose loop antenna
On 11/10/2015 2:02 AM, rickman wrote:
On 11/9/2015 7:52 PM, Jeff Liebermann wrote: My solution would be to use aluminum instead. Copper is not really superior in a meaningful way and costs a lot more. Money rears it's head as an issue, aluminum is cheaper. +1 for aluminum. Resistivity is 65% higher for aluminum. I'd give that a -1, however, if maximizing Q is not important in your design then still at +1. Oxidation: Copper oxide vs Aluminum oxide. Aluminum oxide is an insulator. How does that affect skin resistance? I don't know, does it just shrink your tubing by the thickness of the oxide? Copper oxide is said to be a semiconductor, has much, much lower resistance than aluminum oxide. Page 5, http://www.ets-lindgren.com/pdf/emctd_1293_weibler.pdf Or just varnish it. Bottom of page, http://hamwaves.com/coils/en/ Aluminum is a great material to work with. My magloop nightmare come true: http://www.mixw.co.uk/MagLoop/magloopF.htm I don't get your point here. Build one, monitor your dreams. Jeff, I don't like the parallel sections, seems like it could be made more complicated using x, y, and z right angles. Nap on that! :-) Mikek |
#39
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Flex dryer vent hose loop antenna
On 11/10/2015 9:12 AM, amdx wrote:
On 11/10/2015 1:41 AM, rickman wrote: On 11/9/2015 12:08 PM, amdx wrote: The silver is simply optimization. If you can make your coil wire 5% bigger, you should have already done that. Then if you want to optimized 1 + 0.05, silver plate it. Why can't you make the wire 5% bigger again? Where exactly is the cost? You can. I don't think ultimate optimizing is for you. Just make your coil with as much surface area as room will allow. Then know it could have just a tiny bit less loss if you had the silver. That tiny bit of loss will not be noticed in use, except for that little nagging thought... I once had a 6 or 7 turn loop made with 1/4" copper tubing*, Q was about 800 at 1MHz. I could have made it with 1/2" tubing, probably would have had higher Q. I guess the limits are money and how you want to limit physical size and maximum inductance you can use. Did I miss any? People seem to go nuts with ideas that you need to optimize every little thing without any evidence to show the significance of the impact on performance. Your example is perfect. Increasing the copper tube from 1/4 inch to even just 3/8 inch would more than make up for silver plating and not really cost that much more. There are guys who talk about using single piece, 3 inch copper tube bent into a loop to avoid having solder joints when using straight pieces even though those solder joints will be about the same resistance as a quarter inch of the tube or a microscopic increase in the resistance. Then they conveniently forget about the resistance of the clamp connection to the vacuum variable capacitor swamping out the solder joint resistance even more. It makes me want to scream, "Enough of the maddness"! -- Rick |
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Flex dryer vent hose loop antenna
On 11/10/2015 10:10 AM, amdx wrote:
On 11/10/2015 2:02 AM, rickman wrote: On 11/9/2015 7:52 PM, Jeff Liebermann wrote: My solution would be to use aluminum instead. Copper is not really superior in a meaningful way and costs a lot more. Money rears it's head as an issue, aluminum is cheaper. +1 for aluminum. Resistivity is 65% higher for aluminum. I'd give that a -1, however, if maximizing Q is not important in your design then still at +1. I can overcompensate for the increased resistivity by using larger tubing and still save $$$, +1 for aluminum. Resistivity is not resistance. Oxidation: Copper oxide vs Aluminum oxide. Aluminum oxide is an insulator. How does that affect skin resistance? I don't know, does it just shrink your tubing by the thickness of the oxide? How many atoms is that in aluminum? So we are talking nanometers? I started with a tube that is 100% larger, so still lower conductivity. Copper oxide is said to be a semiconductor, has much, much lower resistance than aluminum oxide. Page 5, http://www.ets-lindgren.com/pdf/emctd_1293_weibler.pdf Or just varnish it. Bottom of page, http://hamwaves.com/coils/en/ Aluminum is a great material to work with. The oxides are irrelevant. -- Rick |
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