vacume caps

[p.larner]

are vacume caps more efficient than fixed silver mica?.

[tggzzz]

are vacume caps more efficient than fixed silver mica?.

No, but airegapd caps are better than plystyureene caps.

[TimFox]

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+gnzGmfTjRrPA6
The main reason to use them is their huge RF current ratings, along with high voltage ratings.
Jennings does not specify Q or loss, stating that the loss is generally negligible:  as a dielectric, vacuum itself has no loss mechanisms until you get to the huge E fields 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, Q is good, about 2,000.
https://www.cde.com/resources/catalogs/HVOLT-CYL.pdf

[vk4ffab]

are vacume caps more efficient than fixed silver mica?.

In what context? You are comparing 2 capacitor types that have very different use cases.

[p.larner]

The context is for use in an l match atu.

[radiolistener]

for a transmitting magnetic loop antenna vacuum capactior is of course much better, because it has much higher voltage rating.

[vk4ffab]

The context is for use in an l match atu.

If 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.

[A.Z.]

If being used as a fixed value cap in an L match, I do not see the point to using either of them.

Rob... 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)

[iMo]

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..
 

[radiolistener]

I would just use high voltage ceramic caps that cost a couple of cents.

That 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

[A.Z.]

@radiolistener

* Door knob caps
http://ur4ll.net/#caps2

* Vacuum variable caps
http://ur4ll.net/#caps3_1

 

[radiolistener]

Prices almost the same as on our market. On your page 220 pF is 10 USD and 1500 pF is 14 USD.

[vk4ffab]

If being used as a fixed value cap in an L match, I do not see the point to using either of them.

Rob... 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)

I 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.

[vk4ffab]

I would just use high voltage ceramic caps that cost a couple of cents.

That 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

Reading 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.html

And 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.

[p.larner]

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!.

[vk4ffab]

i just thought the vacume cap would alow better efficeincy+ tuning over the fixed silver mica?.

As 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.

[A.Z.]

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

[TimFox]

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 "unloaded Q" and "loaded Q" (after connecting load).
They are designed for a desired value of loaded Q from specified load impedance, and the unloaded Q includes parasitic resistive losses in the inductor(s) and capacitor(s).
The power loss from input to output depends on the ratio between unloaded Q and loaded Q, where higher unloaded Q gives lower loss to the loaded case, since the parasitic losses are lower.
Now, unless you use really crummy capacitors, the unloaded Q for a practical network will be dominated by the Q of the inductor(s), since decent capacitors have Q near 1000, while inductors are typically in the 100s.
(The Jennings vacuum capacitors have unmeasurably high Q, maybe 25,000; the CDE micas I cited above have Q around 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 inductor Q if efficiency is critical.
I have seen reports that with large RF transmitting coils (air core), Q of 1000 is possible.

[p.larner]

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.

[p.larner]

heres the inductor,before cleaning it up.spent about 3hrs  working on it,seiezed nuts etc.its now spoton.

[TimFox]

I have seen new roller inductors with quoted Q values of 800 (not well-specified, Q varies with frequency).
Can you try estimating your performance quantitatively based on measured Q with your cleaned-up parts?
You don't need fancy hardware to measure Q, just a bandwidth measurement with a high-Q capacitor.
Obviously, contact resistance can affect the inductor's Q, and you reach diminishing returns with a capacitor Q higher than the inductor.

[radiolistener]

Here 5kV 1 nanna 16 cents.

such capacitor will be broken and burned out very quick if you're using it in matching circuit for a very high Q antenna.

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.

When 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 change


Here 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.

[vk4ffab]

Here 5kV 1 nanna 16 cents.

such capacitor will be broken and burned out very quick if you're using it in matching circuit for a very high Q antenna.

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.

When 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 change


Here 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.

DUDE 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.

[coppercone2]

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.

[Andy Chee]

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

No need to suspect anything. Just read his post:

The context is for use in an l match atu.