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Through-glass antenna power limits
"Denny" wrote in message ps.com... In my search for knowledge I recently went through a big thrash with antenna tuners... Because of cost I built my own variable caps... To keep the size down I experimented with various insulators between the plates, from air up to glass... Being that the tuners on 40 and 20 were end matching a half wave antenna element the circulating tank current was considerable... On 20 meters where the capacitor plates are 1.5" square for resonance the glass would simply shatter at about 800W for 5 seconds... Plexiglass would boil internally in roughly 20 seconds leaving this interesting rippled piece with 'steam' bubbles in the center of the plexi... Ceramic tiles seemed to work OK, but I didn't find any thin ones locally and the thick pieces resulted in plates being awkwardly large... On 80 meters the glass plates are 4" X 6" and the glass runs barely above ambient at 2KW... denny / k8do An very odd result considering that glass dielectric capacitors are sold specifically for high rf current use by people such as AVX and the like. I guess it depends on what type of glass you used. You shouldn't have used your old lead crystal glasses (;-)) 73 Jeff |
Through-glass antenna power limits
Thomas & Karen Mitchell wrote:
... So: I'll stay at 25 when in the car. -- KG7U -------------------------------------------------------------------------- Tom Mitchell, Port Angeles, Washington, USA http://www.olympus.net/personal/kg7u -------------------------------------------------------------------------- Every engineer, worth his pay, which I know, allow at least a 20% excess on ratings. 50 X 20% = 10 watts 50 + 10 = 60 watts I'd run the 65 and examine the glass mount now and then. You'll see if damage begins and switch lower power. Warm regards, JS |
Through-glass antenna power limits
"Richard Clark" wrote in message ... On Mon, 4 Jun 2007 02:43:18 +0100, "Mike Kaliski" wrote: Take a deep breath before transmitting with all the controls set to 11... Hi Mike, Is the warrantee void within 100 yards of a 50KW AM antenna? Or howabout those half megawatt LF stations that litter the continent? 73's Richard Clark, KB7QHC Richard It's all about RF actually within the vehicle. The bodywork acts like a Faraday cage against external influences. But I suspect you know that already :-) A couple of years back a lightning bolt hit a field I was driving past during a storm. The dash completely blanked for 2 seconds and then came back to life - no other ill effects noted - just as well because the road was nearly completely under water at that point. The radio key remote locking can fail if you park too close to a mobile phone mast, or someone is transmitting on 70 cms nearby, but that's about it. As I only operate QRP I hadn't given the matter much thought until I checked in the vehicle manual and saw a footnote. Mike G0ULI |
Through-glass antenna power limits
"Mike Kaliski" wrote in message ... "Richard Clark" wrote in message ... On Mon, 4 Jun 2007 02:43:18 +0100, "Mike Kaliski" wrote: Take a deep breath before transmitting with all the controls set to 11... Hi Mike, Is the warrantee void within 100 yards of a 50KW AM antenna? Or howabout those half megawatt LF stations that litter the continent? 73's Richard Clark, KB7QHC Richard It's all about RF actually within the vehicle. The bodywork acts like a Faraday cage against external influences. But I suspect you know that already :-) A couple of years back a lightning bolt hit a field I was driving past during a storm. The dash completely blanked for 2 seconds and then came back to life - no other ill effects noted - just as well because the road was nearly completely under water at that point. The radio key remote locking can fail if you park too close to a mobile phone mast, or someone is transmitting on 70 cms nearby, but that's about it. As I only operate QRP I hadn't given the matter much thought until I checked in the vehicle manual and saw a footnote. Mike G0ULI I had a similar experience with lightning. The lightning hit a tall comm tower on the side of the road, jumped out the side of the tower, hit the roof of the truck I was in, and then struck a pine tree on the other side of the road. The engine diesd and I coasted to the side of the road. After a few minutes of getting my composurer back together and surveying the damage I was able to start the truck like nothing had happened. I was expecting all the electronics to be fried. Jimmie |
Through-glass antenna power limits
Richard Clark wrote:
On Mon, 4 Jun 2007 02:43:18 +0100, "Mike Kaliski" wrote: Take a deep breath before transmitting with all the controls set to 11... Hi Mike, Is the warrantee void within 100 yards of a 50KW AM antenna? Or howabout those half megawatt LF stations that litter the continent? I'd venture that the concern is not so much radiated field (although, in the nearfield it drops off pretty quick, so a 100W transmitter on a car might well create larger field strengths than that 50kW AM station. It's probably more the potential for conducted RF via the radio's power leads, etc. Imagine that if the chassis of the radio is RF hot, relative to the car body (oh no, that never happens, no waygrin), so now you have significant RF current flowing in the vehicle wiring harness. From a straight field standpoint, it's easy to exceed the ANSI RF field Maximum Permissible Exposure(MPE) with a 2m transmitter and 50W on a car. Combination of the limit being lowest, close proximity to the radiator, etc. (I seem to recall the limit is reached at about 1 meter away from a groundplane/whip antenna at 50W, albeit at 100% duty cycle) But then, you have done your RF safety analysis, right? (Even though there's a categorical exemption from the requirement for routine evaluations for amateur mobile and portable operations, that doesn't relieve you from the requirement for safety). In comparison, you're not going to be driving somewhere the field exceeds the limits, because they'll have a big fence keeping you beyond the distance for uncontrolled exposure limits. As far as the original question about through glass... Glass isn't the lowest loss dielectric to make a capacitor of (heck, even at the 100kHz used in tesla coils, it's lossy enough to crack from the thermal stresses). The glass is the dominant source of loss in using FR-4/G-10 PC boards at microwave frequencies, for instance. However, even a spectacularly bad loss tangent of 0.01 is only going to result in a 1% loss. (wood has a loss tangent of 0.03 or thereabouts at 100MHz, depending on moisture level) A bigger issue would be wires/conductive films for defogging. Jim |
Through-glass antenna power limits
Jeff wrote:
"Denny" wrote in message ps.com... In my search for knowledge I recently went through a big thrash with antenna tuners... Because of cost I built my own variable caps... To keep the size down I experimented with various insulators between the plates, from air up to glass... Being that the tuners on 40 and 20 were end matching a half wave antenna element the circulating tank current was considerable... On 20 meters where the capacitor plates are 1.5" square for resonance the glass would simply shatter at about 800W for 5 seconds... Plexiglass would boil internally in roughly 20 seconds leaving this interesting rippled piece with 'steam' bubbles in the center of the plexi... Ceramic tiles seemed to work OK, but I didn't find any thin ones locally and the thick pieces resulted in plates being awkwardly large... On 80 meters the glass plates are 4" X 6" and the glass runs barely above ambient at 2KW... denny / k8do An very odd result considering that glass dielectric capacitors are sold specifically for high rf current use by people such as AVX and the like. I guess it depends on what type of glass you used. You shouldn't have used your old lead crystal glasses (;-)) Lots of kinds of "glass" out there. fused silica is pretty low loss borosilicate glass (e.g. Pyrex, Kimax, etc.) is medium windowglass (soda lime) is all over the place. |
Through-glass antenna power limits
John Smith I wrote: Thomas & Karen Mitchell wrote: ... So: I'll stay at 25 when in the car. -- KG7U -------------------------------------------------------------------------- Tom Mitchell, Port Angeles, Washington, USA http://www.olympus.net/personal/kg7u -------------------------------------------------------------------------- Every engineer, worth his pay, which I know, allow at least a 20% excess on ratings. 50 X 20% = 10 watts 50 + 10 = 60 watts I'd run the 65 and examine the glass mount now and then. You'll see if damage begins and switch lower power. Warm regards, JS Obviously the engineers who established the maximum power specification would not advocate using greater than 50 watts. ac6xg |
Through-glass antenna power limits
Jim Kelley wrote:
... Obviously the engineers who established the maximum power specification would not advocate using greater than 50 watts. ac6xg 1) No engineer would advocate over-clocking computers the way I do. 2) No engineer would advocate pushing rigs and liners (power amps) to the limits I do. 3) No engineer would advocate doing the mods to equipment I do. 4) No engineer would advocate the antenna experiments I carry out. 6) No engineer would advocate the engine mods to my vette which I have done. 7) No engineer would advocate the levels of exposure to RF I expose myself too. 8) ... That's what makes it fun, now doesn't it? JS |
Through-glass antenna power limits
John Smith I wrote: 1) No engineer would advocate over-clocking computers the way I do. 2) No engineer would advocate pushing rigs and liners (power amps) to the limits I do. 3) No engineer would advocate doing the mods to equipment I do. 4) No engineer would advocate the antenna experiments I carry out. 6) No engineer would advocate the engine mods to my vette which I have done. 7) No engineer would advocate the levels of exposure to RF I expose myself too. 8) ... That's what makes it fun, now doesn't it? It's unquestionably something which separates fools and their money. ac6xg |
Through-glass antenna power limits
Jim Kelley wrote:
... It's unquestionably something which separates fools and their money. ac6xg Yes, well, I am still working on a way to take to the grave with me ... so far, NO PROGRESS! I wish you better luck ... however, I guess others who will benefit from the money will be more than grateful ... granting that I will be unsuccessful in finding a way. JS |
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