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ANTENNA QUESTION
If you can't contribute anything positive to this question, then stay out of
it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! |
ANTENNA QUESTION
John Doe wrote:
... Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Using the latter, at least you would insure the antenna would NOT become preggers! :-) Positive regards, JS |
ANTENNA QUESTION
"John Doe" wrote in message ... If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! John I would have thought that the felt would make the mag mount more likely to slip against the roof. You might not get any scratches, but you probably would be minus one antenna at the end of a fast drive. I have had good results using strips of insulating tape or adhesive backed plastic type materials, similar to the stuff they print stickers on covering the magnet. For best results, you want a soft resiliant type of plastic, something that wont slip against the roof metal. Or have a look for some of that plastic sheet that sticks to glass and smooth surfaces and is used to secure licence or insurance details to car windscreens. That should protect your paintwork and be removable at a moments notice. A favourite trick for mincab or unlicensed cab drivers is to cover the antenna base with a stout sandwich bag and secure it in place with a plastic tie wrap. Take the antenna off the roof and no traces left behind. Any of the above methods should work okay and they will all be a lot thinner than a piece of felt. Mike G0ULI |
ANTENNA QUESTION
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ANTENNA QUESTION
On Nov 3, 5:54 pm, "John Doe" wrote:
If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. You are wasting your time trying to protect the paint when using a magnetic mount. Grit will find its way under your magnet, and within a year of normal driving, you will have scratched paint. Take a deep breath, get out the hole saw or metal punch, and punch a hole in your new car. Your radio will be happier and so will you as the paint around your antenna will not be subjected to the inevitable damage caused by magnet mounts. BTW, before anyone says anything about lowering the value of the vehicle, I've sold and traded-in multiple vehicles with holes punched in them, and not once has anyone ever said a word about the holes. When I trade it in, I just put in a rubber plug and no one notices. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! However, if you have committed yourself to using a magnetic mount, I would suggest using ultra thin vinyl sheeting, this should give you enough grip to keep it on the vehicle. Nevertheless, anything that you use is going to require that you retune the antenna, a longer whip may be also be required because of the decreased capacitance. 73, Dloyd |
ANTENNA QUESTION
On Nov 3, 9:27 pm, Dloyd Lavies wrote:
On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. You are wasting your time trying to protect the paint when using a magnetic mount. Grit will find its way under your magnet, and within a year of normal driving, you will have scratched paint. Take a deep breath, get out the hole saw or metal punch, and punch a hole in your new car. Your radio will be happier and so will you as the paint around your antenna will not be subjected to the inevitable damage caused by magnet mounts. BTW, before anyone says anything about lowering the value of the vehicle, I've sold and traded-in multiple vehicles with holes punched in them, and not once has anyone ever said a word about the holes. When I trade it in, I just put in a rubber plug and no one notices. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! However, if you have committed yourself to using a magnetic mount, I would suggest using ultra thin vinyl sheeting, this should give you enough grip to keep it on the vehicle. Nevertheless, anything that you use is going to require that you retune the antenna, a longer whip may be also be required because of the decreased capacitance. 73, Dloyd Try some 3mil Teflon, Teflon has a very high capacitive coefficent. Jimmie |
ANTENNA QUESTION
On Nov 3, 5:54 pm, "John Doe" wrote:
If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Lose the magmount and drill a hole in your roof. When you sale your truck put a rbber plug in it made for that pupose. Has 0 effect on your resale value. |
ANTENNA QUESTION
JIMMIE wrote:
Try some 3mil Teflon, Teflon has a very high capacitive coefficent. I'm afraid you're misinformed. The dielectric constant of PTFE Teflon is 2.1, or just about twice that of air. Quite a few plastics are higher (e.g. Mylar at 3.2 and PVC around 3.5), and many materials, such as those used for capacitors, have dielectric constants that are a lot higher (e.g., barium titanate at 1500 - 2000). Of course, a lot of the latter aren't physically suited for this application. A long time ago, I had trouble with microstrip line dispersion in a high speed delay line compensation network design. So I chose Teflon for the substrate material because of its *low* dielectric constant. The previous design was on an alumina substrate having a dielectric constant of about 10. The capacitance of two parallel plates is directly proportional to the dielectric constant and the plate surface area, and inversely proportional to the plate spacing. So putting 0.1 inch of Teflon between the plates gives you the same capacitance as putting the plates 0.05 inch apart with air between. This isn't to say that Teflon might not be a good choice. It's a very low loss material, and chemically very inert. It's soft so won't scratch, but it's slippery which might be a disadvantage. It's also subject to cold flow, but there probably won't be enough pressure for that to be a problem. Roy Lewallen, W7EL |
ANTENNA QUESTION
Try some 3mil Teflon, Teflon has a very high capacitive coefficent. But seems like teflon - being so slippery - would contribute to the antenna slipping off to roof.?? |
ANTENNA QUESTION
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Hash: SHA1 Dloyd Lavies wrote: On Nov 3, 5:54 pm, "John Doe" wrote: lowering the value of the vehicle, I've sold and traded-in multiple vehicles with holes punched in them, and not once has anyone ever said a word about the holes. When I trade it in, I just put in a rubber plug and no one notices. don't mean to go off topic, but I bought a new hyundai in 2000, and it came from the manufacturer with a hole in the roof! Roof antennas seem to be popular for cell phones and GSP. It had one of those plastic plugs, and it looked perfectly normal. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFHLULgQuDJiZ/QrH0RAm1WAJ9KWNU5YGOxGvq894zrLAjPMlDecgCguzfE NkkyCuyT/exew4JGV7grCmM= =tDsG -----END PGP SIGNATURE----- |
ANTENNA QUESTION
"Dloyd Lavies" wrote in message ps.com... On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. You are wasting your time trying to protect the paint when using a magnetic mount. Grit will find its way under your magnet, and within a year of normal driving, you will have scratched paint. Take a deep breath, get out the hole saw or metal punch, and punch a hole in your new car. Your radio will be happier and so will you as the paint around your antenna will not be subjected to the inevitable damage caused by magnet mounts. BTW, before anyone says anything about lowering the value of the vehicle, I've sold and traded-in multiple vehicles with holes punched in them, and not once has anyone ever said a word about the holes. When I trade it in, I just put in a rubber plug and no one notices. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! However, if you have committed yourself to using a magnetic mount, I would suggest using ultra thin vinyl sheeting, this should give you enough grip to keep it on the vehicle. Nevertheless, anything that you use is going to require that you retune the antenna, a longer whip may be also be required because of the decreased capacitance. 73, Dloyd I've been down that road, too. The mag mount is OK for short term use, say for a day or so but it is not a good idea to leave one on the vehicle for any length of time. As Dloyd pointed out, grit WILL find a way to get under the mount and it WILL damage the paint. I learned the hard way that moisture will also linger under the mount and the paint will be further damaged. Then, too, is the possibility of the antenna striking branches, door tops and other low-hanging obstacles and when the magnet slides across the roof, guess what? You got it! More damage to the paint. At VHF/UHF frequencies you will not see enough of a change in the standing wave to be concerned with if you use a buffer between the magnet and the body. It will be miniscule. Dloyd is correct again when he advises you to do it the right way. Drill a hole. Mount the antenna and have fun. Oh, as an addendum? I once had a magnetic mount antenna stolen from my car. It was a 2 meter quarter wave whip but that didn't phase the CBer who cut the coax and stole it. And before you jump on me and ask how I know it was a CBer, read on. I had engraved part of my soc number on the underside of the antenna. I just happened to visit a local CB shop one afternoon and saw my antenna on a shelf and the owner of the shop told me it was for sale for five dollars. Without going into further detail, I arranged for a local detective to visit the shop and buy the antenna, whereupon the owner of the shop was promptly arrested for selling stolen property. The icing on the cake, so to speak, was when the detective discovered that the owner also had several stolen CB radios for sale as well. The CBer who stole the antenna was also caught and charged. 73! name and call withheld |
ANTENNA QUESTION
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Hash: SHA1 Not Dloyd wrote: I've been down that road, too. The mag mount is OK for short term use, say for a day or so but it is not a good idea to leave one on the vehicle for any length of time. I don't see what the big deal is. I have a mag mount on my car and haven't noticed any scratches. It doesn't move at all. It is a 2 meter antenna, and the magnet is very strong. Do other mag mounts typically have week magnets? -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFHLUV1QuDJiZ/QrH0RAuDiAJ9st7Dbb0vbUWsZTEPgouvtIuHM3wCfRE0j PuFFEpw7djeBuAAHr0IANJs= =VgJ5 -----END PGP SIGNATURE----- |
ANTENNA QUESTION
Stefan Wolfe wrote:
I think that when considering plastic and mica type materials for high capacitance values, component engineers do not necessarily look at dielectric constant as the most important property. Mylar, mica and polyethylene are common capacitor materials and have rather low dielctic constants just a little higher than teflon. What makes them most attractive is their ability to be manufactured in very thin sheets (increased C) and their high dielectric strengths (increased V ratings). Teflon is higher cost but is very good for high voltage ratings due to superior dielectric strength. Aluminum and tantalum oxides tend to have very high dielectric constants due to the capability of these metal oxide molecules to store electrons. What is more desirable however is their property of being very thin. What makes them a problem is their relatively low dielectric strengths thus their low voltage ratings. Actually, there are a lot more considerations yet. Some plastics are self-healing, so a momentary arc won't permanently destroy the capacitor. And some have exceptionally low leakage current. Various materials also have widely different temperature coefficients. In general, the very high k (dielectric constant) ceramics have higher temperature coefficients than lower k materials. Some ceramics, like the very high k ceramics used for Z5U and similar capacitors, are also hygroscopic, microphonic, piezoelectric, and their k varies with frequency and voltage. Many capacitors, depending largely on the dielectric, also have nonlinear properties such as "soak" (dielectric absorption) and "hook". Loss, expressed as loss tangent, ESR, or power factor, is also often an important consideration, and it can be very different for different dielectrics. Choosing the right capacitor for a particular job can be pretty demanding. This one, fortunately, is easier than some. Roy Lewallen, W7EL |
ANTENNA QUESTION
"Cecil Moore" wrote in message et... John Doe wrote: Positive comments only! I use aluminum foil. -- 73, Cecil http://www.w5dxp.com At least one dualband magmount, no longer offered by MFJ, came with an aluminum foil bottom. I have used several of them and seen no adverse effect on the paint -- and I whiz along at highway speeds a lot. |
ANTENNA QUESTION
In message om, Dloyd
Lavies writes On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. You are wasting your time trying to protect the paint when using a magnetic mount. Grit will find its way under your magnet, and within a year of normal driving, you will have scratched paint. I used a 7" magmount for a 5/8 WL on 2m. It had a rubber boot. I used it most days to and from work, which involved putting it on and taking it off twice each day. And then there was other use. Each time, before I put it on, I wiped the mounting spot (in the centre of the roof) with a soft rag (with maybe a bit of spit if nobody was looking). Four years later, when I got rid of the car, the mounting spot was extremely highly polished, which (being a company car) is more than you could say for the rest it. -- Ian |
ANTENNA QUESTION
John
Nobody seems to have mentioned these... - Tune the antenna length to allow for the change in capacitance? - Increase the diameter of the magnet to increase the capacitance... Cheers Bob John Doe wrote: I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. |
ANTENNA QUESTION
"John Doe" wrote in message ... If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Maybe black, electrical tape on the bottom of the mag mount (applied to that mag mount, of course.) Would be thinner, but I suspect it might still cause some problems with increased capacitance. Count paint that mag mount base with liquid electrical tape, which I get at my local ACE Hardware store. But then, you could still get some grit underneath the mount. Saran Wrap? Very thin. Might be your best bet. Tough and flexible. Maybe that would help... Good luck. Dave |
ANTENNA QUESTION
On Nov 4, 4:00 am, Ian Jackson
wrote: In message om, Dloyd Lavies writes On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. You are wasting your time trying to protect the paint when using a magnetic mount. Grit will find its way under your magnet, and within a year of normal driving, you will have scratched paint. I used a 7" magmount for a 5/8 WL on 2m. It had a rubber boot. I used it most days to and from work, which involved putting it on and taking it off twice each day. And then there was other use. Each time, before I put it on, I wiped the mounting spot (in the centre of the roof) with a soft rag (with maybe a bit of spit if nobody was looking). Four years later, when I got rid of the car, the mounting spot was extremely highly polished, which (being a company car) is more than you could say for the rest it. -- Ian Interesting Ian, I suppose the fact that you removed the antenna twice a day and wiped the spot made a substantial difference. In my experience, I did not remove the antenna daily, but as a lot of amateurs, I used the magnet mount in lieu of punching a hole. After a year, the paint had terrible scratches where the magnet had been placed. Nevertheless, this still does not solve John's problem with the detuning he is experincing when using a rubber boot or other protective device. What John does not say, and I would like to know, is why he is unable to retune the antenna to compensate for the change in capacitance. Will the antenna not resonate? or is there some physical limitation, such as the radiating element being too short, ect. 73 Dloyd |
ANTENNA QUESTION
Ian Jackson wrote:
I used a 7" magmount for a 5/8 WL on 2m. It had a rubber boot. My Larsen mag mount came with a thin sheet of aluminum across the bottom. After much use, the aluminum started to tear. I replaced the factory aluminum with ordinary aluminum foil taped to the base. It's not as sturdy as the original but it costs virtually nothing to replace. -- 73, Cecil http://www.w5dxp.com |
ANTENNA QUESTION
On Nov 3, 5:54 pm, "John Doe" wrote:
If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Consider drilling an NMO mount and get the Larsen whip for 27-30 MHz. |
ANTENNA QUESTION
Can't Drill any holes - but thanks for the suggestion.
wrote in message oups.com... On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Consider drilling an NMO mount and get the Larsen whip for 27-30 MHz. |
ANTENNA QUESTION
Thanks Stefan - I will keep that in mind.
"Stefan Wolfe" wrote in message ... "John Doe" wrote in message ... If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! The thickness of the balloon will result in a much larger capacitance to the 'ground' plane counterpoise so it should be a big improvement over the felt. |
ANTENNA QUESTION
"Cecil Moore" wrote in message ... Ian Jackson wrote: I used a 7" magmount for a 5/8 WL on 2m. It had a rubber boot. My Larsen mag mount came with a thin sheet of aluminum across the bottom. After much use, the aluminum started to tear. I replaced the factory aluminum with ordinary aluminum foil taped to the base. It's not as sturdy as the original but it costs virtually nothing to replace. -- 73, Cecil http://www.w5dxp.com Thanks Cecil - I think that this might be the best possibility of all. 73's |
ANTENNA QUESTION
"Dloyd Lavies" wrote in message ups.com... On Nov 4, 4:00 am, Ian Jackson wrote: In message om, Dloyd Lavies writes On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. You are wasting your time trying to protect the paint when using a magnetic mount. Grit will find its way under your magnet, and within a year of normal driving, you will have scratched paint. I used a 7" magmount for a 5/8 WL on 2m. It had a rubber boot. I used it most days to and from work, which involved putting it on and taking it off twice each day. And then there was other use. Each time, before I put it on, I wiped the mounting spot (in the centre of the roof) with a soft rag (with maybe a bit of spit if nobody was looking). Four years later, when I got rid of the car, the mounting spot was extremely highly polished, which (being a company car) is more than you could say for the rest it. -- Ian Interesting Ian, I suppose the fact that you removed the antenna twice a day and wiped the spot made a substantial difference. In my experience, I did not remove the antenna daily, but as a lot of amateurs, I used the magnet mount in lieu of punching a hole. After a year, the paint had terrible scratches where the magnet had been placed. Nevertheless, this still does not solve John's problem with the detuning he is experincing when using a rubber boot or other protective device. What John does not say, and I would like to know, is why he is unable to retune the antenna to compensate for the change in capacitance. Will the antenna not resonate? or is there some physical limitation, such as the radiating element being too short, ect. 73 Dloyd Dloyd - The radiating element will be too short if it is cut, and there is a problem with it resonating. |
ANTENNA QUESTION
On Nov 3, 1:54 pm, "John Doe" wrote:
If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Just use a very thin piece of plastic. Some companies that make mag- mounts sell circular pieces with adhesive to stick to the antenna base. But you can just cut out a disk from a zip-lock bag and put underneath the magnet. It won't slip around. The thicker the material, the more it affects the performance because it reduces the capacitance to the metal that the antenna is placed on. Thinner is better here. By the way, someone told me to try felt years ago, and I did. The antenna had more tendency to slip around, and it DID mar the paint on my car, more than if I hadn't used anything. Last comment: a balloon would probably get water in it, and keep the antenna base wet for a long time after it rains (at least here in Portland where I live!) Not a great idea. And balloons aren't known for their tougness. Gary |
ANTENNA QUESTION
On Nov 4, 10:48 am, "John Doe" wrote:
"Dloyd Lavies" wrote in message ups.com... On Nov 4, 4:00 am, Ian Jackson wrote: In message om, Dloyd Lavies writes On Nov 3, 5:54 pm, "John Doe" wrote: If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. You are wasting your time trying to protect the paint when using a magnetic mount. Grit will find its way under your magnet, and within a year of normal driving, you will have scratched paint. I used a 7" magmount for a 5/8 WL on 2m. It had a rubber boot. I used it most days to and from work, which involved putting it on and taking it off twice each day. And then there was other use. Each time, before I put it on, I wiped the mounting spot (in the centre of the roof) with a soft rag (with maybe a bit of spit if nobody was looking). Four years later, when I got rid of the car, the mounting spot was extremely highly polished, which (being a company car) is more than you could say for the rest it. -- Ian Interesting Ian, I suppose the fact that you removed the antenna twice a day and wiped the spot made a substantial difference. In my experience, I did not remove the antenna daily, but as a lot of amateurs, I used the magnet mount in lieu of punching a hole. After a year, the paint had terrible scratches where the magnet had been placed. Nevertheless, this still does not solve John's problem with the detuning he is experincing when using a rubber boot or other protective device. What John does not say, and I would like to know, is why he is unable to retune the antenna to compensate for the change in capacitance. Will the antenna not resonate? or is there some physical limitation, such as the radiating element being too short, ect. 73 Dloyd Dloyd - The radiating element will be too short if it is cut, and there is a problem with it resonating.- Hide quoted text - - Show quoted text - I wasn't implying that you should cut the element. What I was trying to understand/suggest, is whether the whip is too short already. Have you checked the antenna with an analyzer? Is it resonate anywhere on 10? Dloyd |
ANTENNA QUESTION
" wrote in message ... "Stefan Wolfe" wrote in message ... sign your own mr crow not mine |
ANTENNA QUESTION
Roy Lewallen wrote: JIMMIE wrote: Try some 3mil Teflon, Teflon has a very high capacitive coefficent. I'm afraid you're misinformed. The dielectric constant of PTFE Teflon is 2.1, or just about twice that of air. Quite a few plastics are higher (e.g. Mylar at 3.2 and PVC around 3.5), and many materials, such as those used for capacitors, have dielectric constants that are a lot higher (e.g., barium titanate at 1500 - 2000). Of course, a lot of the latter aren't physically suited for this application. A long time ago, I had trouble with microstrip line dispersion in a high speed delay line compensation network design. So I chose Teflon for the substrate material because of its *low* dielectric constant. The previous design was on an alumina substrate having a dielectric constant of about 10. The capacitance of two parallel plates is directly proportional to the dielectric constant and the plate surface area, and inversely proportional to the plate spacing. So putting 0.1 inch of Teflon between the plates gives you the same capacitance as putting the plates 0.05 inch apart with air between. This isn't to say that Teflon might not be a good choice. It's a very low loss material, and chemically very inert. It's soft so won't scratch, but it's slippery which might be a disadvantage. It's also subject to cold flow, but there probably won't be enough pressure for that to be a problem. Roy Lewallen, W7EL It was a joke. I actually tried it on my wifes car, she wouldnt let me drill a hole. The antenna stayed on about 3 minutes, just long enough for her to get out of the neighborhood and get up to about 45MPH, then it slid off and scratched the side of her car.Slipperiness is a BIG disadvantage. Jimmie |
ANTENNA QUESTION
In message . com,
JIMMIE writes Roy Lewallen wrote: JIMMIE wrote: Try some 3mil Teflon, Teflon has a very high capacitive coefficent. I'm afraid you're misinformed. The dielectric constant of PTFE Teflon is 2.1, or just about twice that of air. Quite a few plastics are higher (e.g. Mylar at 3.2 and PVC around 3.5), and many materials, such as those used for capacitors, have dielectric constants that are a lot higher (e.g., barium titanate at 1500 - 2000). Of course, a lot of the latter aren't physically suited for this application. A long time ago, I had trouble with microstrip line dispersion in a high speed delay line compensation network design. So I chose Teflon for the substrate material because of its *low* dielectric constant. The previous design was on an alumina substrate having a dielectric constant of about 10. The capacitance of two parallel plates is directly proportional to the dielectric constant and the plate surface area, and inversely proportional to the plate spacing. So putting 0.1 inch of Teflon between the plates gives you the same capacitance as putting the plates 0.05 inch apart with air between. This isn't to say that Teflon might not be a good choice. It's a very low loss material, and chemically very inert. It's soft so won't scratch, but it's slippery which might be a disadvantage. It's also subject to cold flow, but there probably won't be enough pressure for that to be a problem. Roy Lewallen, W7EL It was a joke. I actually tried it on my wifes car, she wouldnt let me drill a hole. The antenna stayed on about 3 minutes, just long enough for her to get out of the neighborhood and get up to about 45MPH, then it slid off and scratched the side of her car.Slipperiness is a BIG disadvantage. Use RUBBER. -- Ian |
ANTENNA QUESTION
"Stefan Wolfe" wrote in message ... "Dave" wrote in message ... "John Doe" wrote in message ... If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Maybe black, electrical tape on the bottom of the mag mount (applied to that mag mount, of course.) Would be thinner, but I suspect it might still cause some problems with increased capacitance. Dave Dave (and John Doe), the idea is to incease capacitance. Large capacitance is a solution to the problem not a cause of problems, agree? I don't know, but if that's the case, than a thinner medium between the mag mount and the roof would have a higher capacitance than a thicker one, like felt. Or it least I think it would... If so, than Saran Wrap might be the answer. Dave |
ANTENNA QUESTION
On 5 Nov, 06:11, "Dave" wrote:
"Stefan Wolfe" wrote in message ... "Dave" wrote in message ... "John Doe" wrote in message om... If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! Maybe black, electrical tape on the bottom of the mag mount (applied to that mag mount, of course.) Would be thinner, but I suspect it might still cause some problems with increased capacitance. Dave Dave (and John Doe), the idea is to incease capacitance. Large capacitance is a solution to the problem not a cause of problems, agree? I don't know, but if that's the case, than a thinner medium between the mag mount and the roof would have a higher capacitance than a thicker one, like felt. Or it least I think it would... If so, than Saran Wrap might be the answer. Dave- Hide quoted text - - Show quoted text - KISS method Use a stronger magnet Art |
ANTENNA QUESTION
Jimmie D wrote:
Interesting, when I replaced the mica insulator on a screen bypass cap with Teflon the capacitance increased. I guess this is because the Teflon was considerably thinner than the mica. That would be the reason. The dielectric constant of mica is around 5-7 depending on the type. So for the same thickness, mica would give you at least 2-1/2 times the capacitance of Teflon. Dielectric constants for a wide range of materials are readily available on the web. Google is your friend. Roy Lewallen, W7EL |
ANTENNA QUESTION
ah well it was nice while it lasted
"John Doe" wrote in message ... If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! |
ANTENNA QUESTION
too bad the topic dies
"John Doe" wrote in message ... If you can't contribute anything positive to this question, then stay out of it, with your trash! I have a 10m base loaded mag mount antenna, that I had to put a piece of felt over the magnet as not to scratch a new vehicle. When I did this I apparently changed the capacitance between the mag mount and the roof of the vehicle. Someone suggested that I might want to replace the felt with either a large balloon or a large prophylactic as it is thinner and the capacitance would then return to almost where it should be. Positive comments only! |
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