Re: Topband: alternative to vacuum variables
I've tried doorknob caps, they get hot, drift and I've had a couple explode! I've used air variables, they work well until a bug crawls between the plates. I finally gave up and went with vacuum variables - work great, no issues! BTW, I've got four vacuum variables listed on QTH.COM right now at reasonable prices. Three at $50 each under "Miscellaneous" And One motorized for $100 listed under "Antenna Tuners" 73 Lloyd - N9LB -Original Message- From: Topband [mailto:topband-boun...@contesting.com]On Behalf Of Pete Smith N4ZR Sent: Thursday, January 24, 2013 1:34 PM To: topband@contesting.com Subject: Topband: alternative to vacuum variables One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? -- 73, Pete N4ZR Check out the Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com. For spots, please go to your favorite ARC V6 or VE7CC DX cluster node. _ Topband Reflector _ Topband Reflector
Re: Topband: alternative to vacuum variables
I don't know much antenna theory, ask me something regarding human biology & I might be a better authority. However, back when I lived in IL in the early 90's, I used to have a tower and not enough space to put a good radial field down. What I did to get on 160 was make a shortened delta loop with the apex at the top of the tower and center fed at the base, held off the tower by around 7-12". I had a relatively small, and inexpensive vacuum variable at the feedpoint at the bottom and ran a wire up the tower held off by spacers from that feedpoint till the capacitance was sufficient that at the least capacitance from the VV, I was 1:1 at 1.800. I had a servo attached to the VV and enclosed everything in a plastic mailbox and mounted that at the tower. I had the matching servo in the shack and was able to adjust that small VV to give me 1:1 across the entire 160 meter band. I believe the VV was a 5KV 300pf variety. This may not be what you need to do with your setup but it allowed me to use a cheap VV and surplus servos to give me a perfect feedpoint match and I had no issues with my old 77SX with this setup. And when running QRP I had fantastic results. Wish my current Inv-L with 60 130' radials worked nearly as well. Gary KA1J _ Topband Reflector
Re: Topband: alternative to vacuum variables
- Original Message - From: "Tom W8JI" To: Sent: Friday, January 25, 2013 1:00 PM Subject: Re: Topband: alternative to vacuum variables Yes, there is a great solution, I'm using it for almost 10 years. The issue with capacitors in 160m is the dielectric an most capacitors get hot and change the capacitance. I used 19 x 62 pFNPO 3KV capacitor to get 1200pF. Never had a failure. Just keep them apart , don't let two capacitor touch This is an age old problem highlighted with amplifiers. The problem with higher value ceramic doorknob capacitors is temperature coefficient, not so much the heat. It is impossible to find NP0 (negative positive zero) in a thick dielectric ceramic. Typically the transmitting doorknob cap has to be less than 100pF to get an NP0. A 200 pF 5 or 7.5 kV was typically measured in the N1000-2000 range, more often than not worse than advertised. 170 pF 5 kV's of better selection could get into the N150-300 range. Most of the reason I used to parallel multiple transmitting doorknobs was to keep the temperature coefficient reasonable, not for heat. Multi-layer transmitting chip ceramics and some disk capacitors are available in NP0 or the equivalent. Mica capacitors are generally very stable, so the old surplus WWII screw terminal block micas work pretty well and are often just a few bucks. We should remember coaxial cables are transmission lines. Because of that, they have pronounced standing waves. With any significant electrical length in degrees the capacitance will NOT be what we calculate for capacitance per foot. At just a few degrees they deviate from calculated because they start to have significant open stub effect. This also results in higher voltage at the open end than we have applied across the feed end. With longer stubs (even though we might imagine them capacitors) the voltage step up can be significant. This was a problem in more than one amateur antenna using coax to make cheap capacitors, the Unihat vertical being one of them that comes to mind. There are many cheap alternatives, but there are few cheap universally good alternatives except perhaps a multitude of lower value temperature stable caps in parallel, or mica or air insulated caps. 73 Tom A problem with old TX micas is leakage as they have deteriorated with age. Ive had to toss about half of mine since they wont even pass at 1000V from one of my vintage cap testers. Id still have resevations about the remainder in HV RF situations. Carl KM1H _ Topband Reflector
Re: Topband: alternative to vacuum variables
Yes, there is a great solution, I'm using it for almost 10 years. The issue with capacitors in 160m is the dielectric an most capacitors get hot and change the capacitance. I used 19 x 62 pFNPO 3KV capacitor to get 1200pF. Never had a failure. Just keep them apart , don't let two capacitor touch This is an age old problem highlighted with amplifiers. The problem with higher value ceramic doorknob capacitors is temperature coefficient, not so much the heat. It is impossible to find NP0 (negative positive zero) in a thick dielectric ceramic. Typically the transmitting doorknob cap has to be less than 100pF to get an NP0. A 200 pF 5 or 7.5 kV was typically measured in the N1000-2000 range, more often than not worse than advertised. 170 pF 5 kV's of better selection could get into the N150-300 range. Most of the reason I used to parallel multiple transmitting doorknobs was to keep the temperature coefficient reasonable, not for heat. Multi-layer transmitting chip ceramics and some disk capacitors are available in NP0 or the equivalent. Mica capacitors are generally very stable, so the old surplus WWII screw terminal block micas work pretty well and are often just a few bucks. We should remember coaxial cables are transmission lines. Because of that, they have pronounced standing waves. With any significant electrical length in degrees the capacitance will NOT be what we calculate for capacitance per foot. At just a few degrees they deviate from calculated because they start to have significant open stub effect. This also results in higher voltage at the open end than we have applied across the feed end. With longer stubs (even though we might imagine them capacitors) the voltage step up can be significant. This was a problem in more than one amateur antenna using coax to make cheap capacitors, the Unihat vertical being one of them that comes to mind. There are many cheap alternatives, but there are few cheap universally good alternatives except perhaps a multitude of lower value temperature stable caps in parallel, or mica or air insulated caps. 73 Tom _ Topband Reflector
Re: Topband: alternative to vacuum variables
Using a thicker wire, or multiple wires in a drop, always helps. Last single thin wire I used was around 1970. :-) - Original Message - From: Pete Smith N4ZR To: Tom W8JI Cc: topband@contesting.com Sent: Friday, January 25, 2013 8:30 AM Subject: Re: Topband: alternative to vacuum variables Thanks, Tom. I thought I recalled this advice from an earlier reflector post, but couldn't find it. Thanks also to everyone else who replied, and the many interesting ideas My single 97-foot tower has two tri-band yagis and a 2-element shortened 40m yagi at the top, as well as an 80-meter lazy-vee array deployed all around it. When I set up the shunt feed years ago, I did a lot of climbing around on it, trying to find a 50-ohm point. Only after not being able to find one did I install the shunt feed at an arbitrary point (about 50 feet, and yes, it's a single wire) and configure an Omega match with a couple of 3000v 300pf variable capacitors from Henry Radio. I know it is a very sub-optimum system (not enough radials either), but I have it set for a 1:1 SWR at 1825 and it satisfies my occasional needs for a 160-meter transmitting antenna. See you all in CQ 160. 73, Pete N4ZR Check out the Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com. For spots, please go to your favorite ARC V6 or VE7CC DX cluster node. On 1/24/2013 9:59 PM, Tom W8JI wrote: One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? Coax isn't the best for two reasons: 1.) Transmission line effects increase voltage at the open end. This aggravates arcing issues 2.) They are lossy Here's something to think about, Pete. The voltage across a capacitor is I^2 * Xc So if you have a shunt feed and use a long, thin, wireyou not only decrease bandwidth and efficiency, you also increase the voltage. A thin shunt feed wire requires higher Xc (lower capacitance), and that makes voltage increase and BW narrow. Use a wire cage and tap the shunt wire down at a 50 ohm point, and voltage goes way down. Do it the lazy way and use an omega match, and you give up bandwidth and need higher voltage parts. 73 Tom _ Topband Reflector
Re: Topband: alternative to vacuum variables
Thanks, Tom. I thought I recalled this advice from an earlier reflector post, but couldn't find it. Thanks also to everyone elsewho replied, and the many interesting ideas My single 97-foot tower has two tri-band yagis and a 2-element shortened 40m yagi at the top, as well as an 80-meter lazy-vee array deployed all around it. When I set up the shunt feed years ago, I did a lot of climbing around on it, trying to find a 50-ohm point. Only after not being able to find one did I install the shunt feed at an arbitrary point (about 50 feet, and yes, it's a single wire) and configure an Omega match with a couple of 3000v 300pf variable capacitorsfrom Henry Radio. I know it is a very sub-optimum system (not enough radials either), but I have it set for a 1:1 SWR at 1825 and it satisfies my occasional needs for a 160-meter transmitting antenna. See you all in CQ 160. 73, Pete N4ZR Check out the Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com. For spots, please go to your favorite ARC V6 or VE7CC DX cluster node. On 1/24/2013 9:59 PM, Tom W8JI wrote: One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? Coax isn't the best for two reasons: 1.) Transmission line effects increase voltage at the open end. This aggravates arcing issues 2.) They are lossy Here's something to think about, Pete. The voltage across a capacitor is I^2 * Xc So if you have a shunt feed and use a long, thin, wireyou not only decrease bandwidth and efficiency, you also increase the voltage. A thin shunt feed wire requires higher Xc (lower capacitance), and that makes voltage increase and BW narrow. Use a wire cage and tap the shunt wire down at a 50 ohm point, and voltage goes way down. Do it the lazy way and use an omega match, and you give up bandwidth and need higher voltage parts. 73 Tom _ Topband Reflector
Re: Topband: alternative to vacuum variables
On 2013-01-24, at 2:33 PM, Pete Smith N4ZR wrote: > One of the reasons I have mot gone to high power on 160 is the cost of > capacitors for my omega match - at least $300. Anyone know of any workable > alternative? I remember someone writing about using coiled-up RG-8, RG-213 > or maybe Teflon coax. Where can I find more information? > Hi Pete, I've used a short length of RG-58 coax for just that purpose in one application here, to supplement the "C" from my air variable at the base of one of my extended inverted "L's"... There's nothing at all wrong with that, I don't think, as long as the coax is simply "padding" existing C---I don't think you'd want to use the coax for a LOT of capacitance, though: if I recall correctly, one foot of RG-58 has some 25-pfd. of capacitance---to get, say, 200-pfd., would you really want an 8-foot coil of coax in your system...? ~73~ de Eddy VE3CUI - VE3XZ _ Topband Reflector
Re: Topband: alternative to vacuum variables
Pete Smith N4ZR wrote: > One of the reasons I have mot gone to high power on 160 is the cost of > capacitors for my omega match - at least $300. Anyone know of any > workable alternative? I remember someone writing about using > coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? Yes, there is a great solution, I'm using it for almost 10 years. The issue with capacitors in 160m is the dielectric an most capacitors get hot and change the capacitance. I used 19 x 62 pFNPO 3KV capacitor to get 1200pF. Never had a failure. Just keep them apart , don't let two capacitor touch each other. Just put them in parallel. You can us a receiver variable capacitor to find the valor necessary to tune the antenna, then measure it and replace with X 62pF in parallel. I used PTFE tape for pipes tread to protect the packet from the weather. Capacitor, ceramic. 62pF 3,000V. CERA-MITE 564CC0GAA302EL620J You can find it in several stores, here the price is U$ 1 each http://www.electronicsurplus.com/Item/15444/CERA-MITE%20-%20Capacitor_%20cer amic_%2062pF%203_000V_%20-%20564CC0GAA302EL620J/ Regards Jose Carlos N4IS _ Topband Reflector
Re: Topband: alternative to vacuum variables
One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? Coax isn't the best for two reasons: 1.) Transmission line effects increase voltage at the open end. This aggravates arcing issues 2.) They are lossy Here's something to think about, Pete. The voltage across a capacitor is I^2 * Xc So if you have a shunt feed and use a long, thin, wireyou not only decrease bandwidth and efficiency, you also increase the voltage. A thin shunt feed wire requires higher Xc (lower capacitance), and that makes voltage increase and BW narrow. Use a wire cage and tap the shunt wire down at a 50 ohm point, and voltage goes way down. Do it the lazy way and use an omega match, and you give up bandwidth and need higher voltage parts. 73 Tom _ Topband Reflector
Re: Topband: alternative to vacuum variables
> Making capacitors out of coax seems like an easy solution, however > they end up being fairly lossy due to copper loss. Think about > it: you would not deliberately make a capacitor long and skinny. One way around this is to use multiple shorter sections in parallel. One can also use series/parallel arrangements of fixed "doorknob" capacitors to obtain the necessary voltage/current rating to pad a lower capacity/wider spaced "breadslicer". 73, ... Joe, W4TV On 1/24/2013 8:39 PM, Rick Karlquist wrote: Pete Smith N4ZR wrote: One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? Making capacitors out of coax seems like an easy solution, however they end up being fairly lossy due to copper loss. Think about it: you would not deliberately make a capacitor long and skinny. The only thing that makes them workable at all is that the heat generated is spread out over a large area. You would have to carefully evaluate your proposed design to see if it were feasible. Rick N6RK _ Topband Reflector _ Topband Reflector
Re: Topband: alternative to vacuum variables
Pete Smith N4ZR wrote: > One of the reasons I have mot gone to high power on 160 is the cost of > capacitors for my omega match - at least $300. Anyone know of any > workable alternative? I remember someone writing about using coiled-up > RG-8, RG-213 or maybe Teflon coax. Where can I find more information? Making capacitors out of coax seems like an easy solution, however they end up being fairly lossy due to copper loss. Think about it: you would not deliberately make a capacitor long and skinny. The only thing that makes them workable at all is that the heat generated is spread out over a large area. You would have to carefully evaluate your proposed design to see if it were feasible. Rick N6RK _ Topband Reflector
Re: Topband: alternative to vacuum variables
Pete, I used a big 300pf breadslicer with somewhere in the 7.5 to 9KV range ( I forget the spacing, it was a vintage Johnson) and a couple of 857 size ceramic caps for my Omega at a prior QTH. Smaller spacing and lighter duty fixed caps didnt survive the 1200W of my own amp but the rebuild was tested with a customers Alpha 77DX and it held together fine. This was in the mid 80's and it was all housed in a plywood box. The other cap was a standard smaller spacing, dont remember the value but its still partially together out in a storage trailer and I can measure it. I seem to remember it was an even older 1000pf Cardwell. Carl KM1H - Original Message - From: "Pete Smith N4ZR" To: Sent: Thursday, January 24, 2013 2:33 PM Subject: Topband: alternative to vacuum variables One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? -- 73, Pete N4ZR Check out the Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com. For spots, please go to your favorite ARC V6 or VE7CC DX cluster node. _ Topband Reflector - No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1430 / Virus Database: 2639/5554 - Release Date: 01/24/13 _ Topband Reflector
Topband: alternative to vacuum variables
One of the reasons I have mot gone to high power on 160 is the cost of capacitors for my omega match - at least $300. Anyone know of any workable alternative? I remember someone writing about using coiled-up RG-8, RG-213 or maybe Teflon coax. Where can I find more information? -- 73, Pete N4ZR Check out the Reverse Beacon Network at http://reversebeacon.net, blog at reversebeacon.blogspot.com. For spots, please go to your favorite ARC V6 or VE7CC DX cluster node. _ Topband Reflector
