vacume caps - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: March 20, 2024, 11:58:33 am
content: are vacume caps more efficient than fixed silver mica?.

author: tggzzz
author_rank: Super Contributor
timestamp: March 20, 2024, 12:17:46 pm
content: Quote from: p.larner on March 20, 2024, 11:58:33 amNo, but airegapd caps are better than plystyureene caps.

author: TimFox
author_rank: Super Contributor
timestamp: March 20, 2024, 03:51:45 pm
content: It's not hard to find real data about vacuum capacitors, which are available at very high prices either fixed or variable capacitance.Seek and ye shall find:https://library.e.abb.com/public/bea07f06039c4a3685bec40a01ac1122/7TKK000115_Jennings_vacuum%20devices_catalog_US.pdf?x-sign=Yi7I0LOgcp0EKk14PWLn0g+EnDe3c/37jNeSjN1jKFU1Dx8kse+gnzGmfTjRrPA6The main reason to use them is their huge RF current ratings, along with high voltage ratings.Jennings does not specifyQor loss, stating that the loss is generally negligible: as a dielectric, vacuum itself has no loss mechanisms until you get to the hugeEfields required for quantum behavior such as vacuum polarization.If your application requires huge RF current or voltage ratings, fixed-value mica capacitors (not silver-mica) are available, useful up to around 3 MHz.At 1 MHz,Qis good, about 2,000.https://www.cde.com/resources/catalogs/HVOLT-CYL.pdf

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 20, 2024, 09:18:29 pm
content: Quote from: p.larner on March 20, 2024, 11:58:33 amIn what context? You are comparing 2 capacitor types that have very different use cases.

author: p.larner
author_rank: Frequent Contributor
timestamp: March 21, 2024, 03:12:16 am
content: The context is for use in an l match atu.

author: radiolistener
author_rank: Super Contributor
timestamp: March 21, 2024, 07:14:12 am
content: for a transmitting magnetic loop antenna vacuum capactior is of course much better, because it has much higher voltage rating.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 21, 2024, 09:24:46 am
content: Quote from: p.larner on March 21, 2024, 03:12:16 amIf being used as a fixed value cap in an L match, I do not see the point to using either of them. I would just use high voltage ceramic caps that cost a couple of cents. For RF applications ceramics are perfectly fine, you just need to power de-rate them by 50%. In my home brew tuner and Tx filters I use 2Kv rated ceramics that cost less than couple of cents each. Given that even with the worst predicted mismatch here, they will see no more than 1Kv their capacitance values will not change with power.By all means use whatever you want, its just a shunt capacitor in essentially a filter, but there is no real benefit in using either silver mica or door knobs.

author: A.Z.
author_rank: Super Contributor
timestamp: March 21, 2024, 11:37:33 am
content: Quote from: vk4ffab on March 21, 2024, 09:24:46 amRob... in case you didn't follow the whole "story" he built an antenna he calls "a doublet" (but isn't, the setup is screwed), fed it with balanced line which in turn is badly installed and finally, after destroying some variable capacitors, built some kinda/sorta ATU allowing him to get "low SWR" (nothing to do with good antenna efficiency) but then, since his ATU can't match some frequencies and/or "arcs" he's now fiddling with it, trying to find a way to "fix it", while the issue is the whole antenna system; add to this that he mostly ignores suggestions and whenever asked for schematics or images he constantly finds an excuse or another to ignore the request, and I think you'll have a more accurate idea of the whole thing and will be able to make your educated guess(note: all of the above can be easily found just by looking at his posts history)

author: iMo
author_rank: Super Contributor
timestamp: March 21, 2024, 11:56:49 am
content: Yep, the OP is the most active creator of new threads on this forum ever..He/she builds a DIY antenna and every bolt and nut means starting a separate thread..

author: radiolistener
author_rank: Super Contributor
timestamp: March 21, 2024, 11:59:02 am
content: Quote from: vk4ffab on March 21, 2024, 09:24:46 amThat is not true, they cannot cost few cents.For example this small one 15 kV 220 pF 25 kVAR cost 8 USD for one piece:But if you want higher voltage/capacitance they are more massive and cost more.For example this one 15 kV 1500pF 90 kVAR cost 22 USD for one piece:But vacuum variable capacitors are more expensive, their price start at about 40 USD.So, if you're don't needs variable capacitor, you can replace it with more cheap constant high voltage capacitor and get economy for 20-30 USD.But I suspect topic starter needs variable capacitor because he building high Q antenna and it has very sharp resonance and too narrow bandwidth, so it requires tuning if you want to receive different frequency

author: A.Z.
author_rank: Super Contributor
timestamp: March 21, 2024, 01:25:53 pm
content: @radiolistener* Door knob capshttp://ur4ll.net/#caps2* Vacuum variable capshttp://ur4ll.net/#caps3_1

author: radiolistener
author_rank: Super Contributor
timestamp: March 21, 2024, 01:48:54 pm
content: Prices almost the same as on our market. On your page 220 pF is 10 USD and 1500 pF is 14 USD.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 21, 2024, 09:00:47 pm
content: Quote from: A.Z. on March 21, 2024, 11:37:33 amI know, the reason why I try and add factual information even though the OP is probably going to ignore it and continue on with his stumbling in the dark method of development is that others might read this thread after the fact and find having quality information in it useful.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 21, 2024, 09:08:02 pm
content: Quote from: radiolistener on March 21, 2024, 11:59:02 amReading and comprehension are essential so as to not quote out of context. The two sentences you ignored qualified my statement. If the OP wanted to use either as a fixed value cap in an L match, I would not use a vacuum, doorknob or silver mica cap, I would use a ceramic that costs cents. Here 5kV 1 nanna 16 cents. Also, there is no advantage to using a $40 doorknob over a ceramic, if your L match tuner is seeing 25kV then you are ham radioing wrong.https://www.lcsc.com/product-detail/Multilayer-Ceramic-Capacitors-MLCC-SMD-SMT_FH-Guangdong-Fenghua-Advanced-Tech-1808B102K502NT_C5120072.htmlAnd of course you can always series the caps if you need a higher voltage rating. But again, an L match should not be seeing extreme voltages, its not a loop antenna. And 10kv rated ceramics can be had for about $3.50.

author: p.larner
author_rank: Frequent Contributor
timestamp: March 22, 2024, 05:49:53 am
content: Fwiw my lashup franken doublet is working fine,the atu l match started off with a 1000pf wide spaced air variable,This was ok matching 80+40m but had a min c that was too high,i removed plates untill it was circa 300pf,This works well >40m too 10m,for 40+80m i have 1400 of silver mica caps switched in paralell with the variable,i just thought the vacume cap would alow better efficeincy+ tuning over the fixed silver mica?.The antenna is used mostly for interG nvis contacts,getting on 80m living in a small flat in a concervation area was difficult!.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 22, 2024, 08:18:35 am
content: Quote from: p.larner on March 22, 2024, 05:49:53 amAs I said in my other post, none that I can think of. Use a vacuum when you need really high voltage rating, mica when you want high stability and a ceramic for low cost. All the other parameters of a capacitor are kind of in the same ballpark, ESR in a filter application might be critical but all three of the above should have ESR better than 0.1 Ohm. All three will do the job in an L Match.The losses in an L Match no so much at the component level but are mostly when it is used at its extremes, so if your tuner can tune say an 800 ohm load and you are trying to tune a 750ohm load, the losses will be highest. Could be as much as 50% power loss in such a situation.

author: A.Z.
author_rank: Super Contributor
timestamp: March 22, 2024, 08:59:32 am
content: Just a note, when we refer to a matching network (using the L topology in this case) it's always a good idea referring to both components of the impedance, that is R and X, in the "L" topology case, losses will be pretty high in some cases, for example when the load to be matched presents very low R and very high X, that is there's high reactance and very low resistance, in such a case one should change the "L" topology between series L/parallel C to series C/ parallel L which is a hell of switching

author: TimFox
author_rank: Super Contributor
timestamp: March 22, 2024, 04:10:51 pm
content: With respect to efficiency using vacuum or mica capacitors:In general, resonant coupling networks (e.g. simple L-C, tapped inductor, pi-networks, L-networks) are characterized by "unloadedQ" and "loadedQ" (after connecting load).They are designed for a desired value of loadedQfrom specified load impedance, and the unloadedQincludes parasitic resistive losses in the inductor(s) and capacitor(s).The power loss from input to output depends on the ratio between unloadedQand loadedQ, where higher unloadedQgives lower loss to the loaded case, since the parasitic losses are lower.Now, unless you use really crummy capacitors, the unloadedQfor a practical network will be dominated by theQof the inductor(s), since decent capacitors haveQnear 1000, while inductors are typically in the 100s.(The Jennings vacuum capacitors have unmeasurably highQ, maybe 25,000; the CDE micas I cited above haveQaround 2000 in their usable frequency range; and class-I ceramics in doorknob or other high-voltage packages are also > 1000.)Therefore, you should concentrate on improving inductorQif efficiency is critical.I have seen reports that with large RF transmitting coils (air core),Qof 1000 is possible.

author: p.larner
author_rank: Frequent Contributor
timestamp: March 26, 2024, 08:15:13 am
content: the inductor is a silver plated roller about 50mm dia with 30 turns end to end with a counter.I just assumed it would be better to use a vac cap as can get one for a good price.

author: p.larner
author_rank: Frequent Contributor
timestamp: March 26, 2024, 08:23:02 am
content: heres the inductor,before cleaning it up.spent about 3hrs working on it,seiezed nuts etc.its now spoton.

author: TimFox
author_rank: Super Contributor
timestamp: March 26, 2024, 03:06:40 pm
content: I have seen new roller inductors with quotedQvalues of 800 (not well-specified,Qvaries with frequency).Can you try estimating your performancequantitativelybased on measuredQwith your cleaned-up parts?You don't need fancy hardware to measureQ, just a bandwidth measurement with a high-Qcapacitor.Obviously, contact resistance can affect the inductor'sQ, and you reach diminishing returns with a capacitorQhigher than the inductor.

author: radiolistener
author_rank: Super Contributor
timestamp: March 26, 2024, 03:19:35 pm
content: Quote from: vk4ffab on March 21, 2024, 09:08:02 pmsuch capacitor will be broken and burned out very quick if you're using it in matching circuit for a very high Q antenna.Quote from: vk4ffab on March 21, 2024, 09:08:02 pmWhen you're trying to transmit with magnetic loop antenna with high Q (about 2000), this is pretty normal, because it has several tens of kilovolt RF due to high Q. And this is where vacuum capacitors are best.There is also constant vacuum capacitors (not variable), they also very good for transmitting with magnetic loop. But variable vacuum capacitor is much more convenient, because with high Q you will have very sharp resonance and very narrow bandwidth, so you will have to tune the matching device with each frequency changeHere is typical example of transmitting magnetic loop antenna for 80-40-30-20 meter bands with vacuum variable capacitor, it uses stepping motor to tune antenna remotely from radio shack:The antenna is feeded with a small loop (gamma match) at the bottom. And vacuum capacitor is used to tune antenna for wanted frequency.Vacuum capacitors is the best choice for such kind of antennas due to it's very high Q a high voltage. But they are very expensive.But I don't recommend to transmit with such kind of antenna, because it has extreme high E and H field strength near antenna which easily exceed all safety limits.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 26, 2024, 10:06:22 pm
content: Quote from: radiolistener on March 26, 2024, 03:19:35 pmDUDE please stop with your endless strawmen arguments showing that you are an internet expert. Go learn to read and understand things. We are not talking about high Q antenna's we are not talking about magnetic loop antennas. The op has a very low Q doublet and an L match. That is the context of everything that has been posted by everyone in this thread other than you. SOMEONE IS WRONG ON THE INTERNET, Seriously. I have a home brew auto tuner L match, it will take a Kw all day long with 12:1 mismatches it does not have doorknobs or vacuum caps in it, it has 2 cent ceramic caps in it. WHY? Because I know how to read and comprehend basic information and perform basic math.

author: coppercone2
author_rank: Super Contributor
timestamp: March 27, 2024, 12:35:35 am
content: With the extra cost savings from the capacitors you can buy Valium. Timfox is the only person that actually answered the thread offering insight behind the behavior of the components. Seeing the comparison between Q's is very interesting.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 27, 2024, 01:55:00 am
content: Quote from: radiolistener on March 21, 2024, 11:59:02 amNo need to suspect anything. Just read his post:Quote from: p.larner on March 21, 2024, 03:12:16 am

author: markus_jlrb
author_rank: Regular Contributor
timestamp: March 27, 2024, 06:44:42 am
content: @vk4ffab,nice work.I have get my vac. caps ten year ago and payed more the 100 Euro for each of the at that time.This kind of caps getting more expensive over time and becoming rear.vy73Markus

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 27, 2024, 08:38:09 am
content: Quote from: markus_jlrb on March 27, 2024, 06:44:42 amHow do you go operating with the loop antenna inside your house Marcus. I have never been in such an antenna restricted place where I have had to get creative to get on air. So how do you get on?

author: radiolistener
author_rank: Super Contributor
timestamp: March 27, 2024, 01:16:36 pm
content: Quote from: vk4ffab on March 26, 2024, 10:06:22 pmDude, I'm not expert, but I have diploma that I learned electronics in college and university and have ham amateur license. I know antenna design theory and electrodynamics. And I have experience in building and using different antennas, include experiments with magnetic loop antennas which I used for transmission. My main interest is DSP design for DDC/DUC radio, but I know hardware part as well, include antenna modelling/simulation and impedance matching things pretty well.So, I'm not sure, what I should learn and understand exactly in your opinion?Even if you use usual antenna tuner, using C0G 2 kV capacitors is not the best choice, because they are going pretty hot during 100W transmission and it leads to self-untune.But if you're dealing with high Q antenna, it has pretty high voltage and current, so cheap SMD ceramic capacitors with 2 kV rating is not suitable here, because they have relatively high loss (for high Q context) and not enough voltage rating.Quote from: vk4ffab on March 26, 2024, 10:06:22 pmDude, I don't see anything in this thread that topic starter wrote about low Q doublet. That's your guess.But topic starter is worried about efficiency, because he mentioned it and it means that he is focused on high Q.This kind of interest is actual for magnetic loop and other high Q antennas.That's why I wrote about magnetic loop.Quote from: Andy Chee on March 27, 2024, 01:55:00 amit appears that this quote a little bit confused me, because topic starter wrote "L" as "l" and I read it as "I". So I decided that he is talking about his own match atu in general.But even for L-match it depends. As I remember my experiments with long wire, I got air variable capacitor breakdown across 3 mm air gap between capacitor plates with feeding just 10 W power and it happen on a classic L-match circuit... So it depends on impedance you're trying to match, Q factor of antenna and Q factor of antenna tuner components. In some cases 2 kV capactior rating will be not enough.I should say that I'm using cheap SMD ceramic 2 kV capacitors in my 100W antenna tuner, but I'm very not satisfied with them. Because they are heating and it leads to untune. I want to replace it with silver mica but currently there is a war, and my purchase abilities are limited.

author: markus_jlrb
author_rank: Regular Contributor
timestamp: March 27, 2024, 07:27:44 pm
content: @vk4ffab,I'm using a QRP TRX and the M-Loop creates only magnetic field radiationin the near field which is not strong absorbed by the brick walls of my house.Due to the M-Loop effectivity (5% to 50% depending of the sw band) and thelow power < 10W the connections are restricted to Europe.I have three different diameters of the loop (0.8m, 1.2m and 1.8m) whichcovers more or less 80 - 10 m operation.When you use a coupling loop that could be shifted along the vertical diameteras well which could be twisted outside the main plain of the main loop youwill get a tuning range of 1:5vy73Markus

author: TimFox
author_rank: Super Contributor
timestamp: March 27, 2024, 08:20:38 pm
content: One last hint about capacitor choice for transmitting circuits:In a resonant coupling network, the circulating ("tank") current is considerably higher than the actual current into the load.That's the relevant parameter to calculate to ensure your capacitor(s) have sufficient current rating and to estimate the loss.

author: markus_jlrb
author_rank: Regular Contributor
timestamp: March 28, 2024, 06:54:50 am
content: @TimFox, vk4ffabbeside the capacity loss, the mechanical/electrical connection between cap. and ind.is most important, as the high cap current is only possible if the connection is doneproperly. Every milli-Ohm counts and decrease the loop efficiency dramatically if exist.That's the reason why my loop has gold plated connectors and the construction allowhigh mechanical pressure between the L and C connection area.As my loop is portable I was not able to construct the connection as a permanent oneby welding like vk4ffab.vy73Markus

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 28, 2024, 07:12:29 am
content: Quote from: markus_jlrb on March 27, 2024, 07:27:44 pmThere are lots of countries in Europe LOL. With the cycle peaking, 10m should see you DX to everywhere even with QRP. My antenna is not that great on 10m, but I still manage some DX. My main bands are 40, 20 and 15m.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 28, 2024, 07:18:30 am
content: Quote from: radiolistener on March 27, 2024, 01:16:36 pmIt’s not a guess. Just read the other posts made by the topic starter:Quote from: p.larner on March 22, 2024, 05:49:53 am

author: coppercone2
author_rank: Super Contributor
timestamp: March 28, 2024, 06:55:04 pm
content: but we can still answer the question in general because of the first post being general and it actually makes the thread better if the more specific question has been answered because its more information

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 28, 2024, 11:18:15 pm
content: Quote from: radiolistener on March 27, 2024, 01:16:36 pmReading and comprehension of what is written. Yeah your technical knowledge is superior to mine, heck lots of people in this thread can make that claim. But I am the only one who asked the OP for clarification of exactly what he was asking rather than assume, because i am not an expert and I am not a mind reader and I do not have all the answers. Reading and comprehension mean sometimes you have to ask questions to better understand what is actually being asked.Quote from: p.larner on March 21, 2024, 03:12:16 amQuote from: p.larner on March 22, 2024, 05:49:53 amNow the OP's question was rather open ended, but he clarified things even before you started posting about magnetic loops. My comments were in the context of the above 2 posts. That the OP was building an L match and he had a doublet. Even if you had read the first of those comments, Post 4, before you made Post 5, you would have understood the context of the thread. Again reading and comprehension. Do you use L match tuners in magnetic loop antenna? Not that I am aware of, but they do use a capacitor, not a fixed capacitor as the OP was talking about but a variable one. What is being asked by the OP can be derived from the context of the thread and thus the context of the answers being given.If you had read and understood the thread, you might have commented in a way that helped the op rather than looking for something wrong on the internet.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 29, 2024, 12:18:47 am
content: Quote from: coppercone2 on March 28, 2024, 06:55:04 pmThe problem is that a specific answer may be irrelevant to the original poster’s query, in this case the irrelevant answer was magnetic loops.Many n00b’s have been lambasted for not providing enough detail in their questions. The Beginners thread description even instructs beginners to provide as much detail as they can, in order to assist contributors to give an accurate and relevant response.

author: p.larner
author_rank: Frequent Contributor
timestamp: March 29, 2024, 12:16:25 pm
content: what bw do you get with your loop on 80m markus?,wide enough for ssb or just data?.

author: coppercone2
author_rank: Super Contributor
timestamp: March 29, 2024, 12:26:08 pm
content: Quote from: Andy Chee on March 29, 2024, 12:18:47 ama thread where people just post to a general topic can be more interesting and long lived then a specific solution to a engineering problem. I mean what are we doing fighting for the front page like its the paper? If your interested in this stuff you end up reading the whole thread three times anyway to see if there is anything new you can use. Not to mention the broader question has a higher chance of giving an alternative approach to something that is just as good or better because its not confined within someones existing engineering dogma related to very specific parts etc. clearly the OP posted a general question indicating that he is willing to read general responses .Just because he posted after the OP to answer someones questions with specifics does not mean we should totally ignore the opening post.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 29, 2024, 09:51:18 pm
content: Quote from: coppercone2 on March 29, 2024, 12:26:08 pmA long time ago, on a forum far far away, back when I was a total noob, I asked what I would consider to be a very simple question about diodes. Being a simple question, you would kind of expect that in the first couple of posts there would be a simple answer. But no. Two internet experts turned that simple question into a 10 page epic thread of them arguing about who was right using 1n4001 and 1n4007. My question never got answered, but we got 10 pages of performance on essentially the same diode.Of course turning this thread into a debate about magnetic loop antenna might be more entertaining, it might even be a better antenna solution given the OP's space situation and sure it gives everyone the opportunity to show how wonderful and technical they are, but what it does not do is answer the very simple question about capacitors.I guess it depends on what school we come from, for me its more about understanding what the actual question is and answering it as best I can rather than every thread being a performance where I can show others how smart I am. And for the OP his questions, while sometimes lacking in detail are mostly very utilitarian in that they seek an answer rather than a discussion of opinions.Actually to the part of your quote in bold, I think that is exactly what should be done until the OP has been given an answer he is satisfied with. This thread is not all about us, its about the OP learning something and gaining better understanding of things. Answering questions that have not been posed especially when the OP has clarified what information he desires is rather poor form indeed and helps no one.

author: coppercone2
author_rank: Super Contributor
timestamp: March 29, 2024, 10:31:16 pm
content: the thread is about us too we are not getting paid it should optimally be mutually beneficial. this is not a likely outcome for many threads but it is what makes us actually go to the forum, because good intention wears out fast (its a portion of why people post but not the whole story).If it gets too out of hand the Op can ask for people to refocus. Part of starting a discussion is managing it, some effort is required... applies in real life too. On a 10 page dumpster fire the mods could step in.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 30, 2024, 09:41:27 pm
content: Quote from: coppercone2 on March 29, 2024, 10:31:16 pmI am more from the Pink school of philosophy, this threads not here for your entertainment. You are right, we are not getting paid, so that gives us the right to follow your friend Miranda and the Fifth and shut the hell up, we are not compelled to do anything. We get to choose what threads we comment in, who we reply to and if we even offer our help at all, because threads are not about us at all, they are about the person who posted them.The quid pro quo if one exists at all is that you get to read what others have posted and maybe learn something along the way. Its not that you get to interject with flame bait and thread derails because of your importance to this forum. Your contributions to this thread could all be deleted and the OP will still have his answers, such is the level of irrelevance you are to this thread.I am sure you think you are important, but the thread would stand without you. I think that is evidence enough disprove your comment about this thread being about you.