Vacuum Variable

[WB4AM]

Hello,

Not having any experiences with Vacuum Variables I thought maybe I could get an answer here.

I will be building another home brew balance antenna tuner.  The first was built using an Air Variable.

I do get some flash overs using high power from time to time.  When I get the flash overs, I spray the capacitor with
an electronic cleaner and it stops the flash over.

So I was thinking of building the next tuner with a Vacuum Variable to prevent any flash overs.
Although I do wonder if the flash overs are from the outside elements such as close proximity with
the antenna being up in the trees.  (dipole antenna)

My biggest concern is with the Vacuum Variable itself.  The glass Vacuum Variables as I take it uses copper
material constructed to expand and extract to vary the capacitance. 

My question on this, will the copper soon break
due to the constant physical movement of the expansion and the extraction due to tuning and re-tuning due to changing bands
and frequencies?

I have never heard of such a thing but I tend to over think a lot.

Thank you in an advance.

Ken

[KD6VXI]

Never had that problem nor heard of it happening.

Glass being sucked in or breaking, yes.

The bellows breaking, no.

--Shane
KD6VXI

[W1ITT]

I confirm Shane's observations.  I have seen vacuum variables at high power short wave broadcast stations in the Middle East blown to smithereens in direct lightning hits, seen them arrive broken due to shipping mishaps, and saw the results of a couple glass ones that fell off the work bench.  But I've never seen one with the copper innards failed, despite that some are used in autotuners and get exercised a lot.    I'm not sure if the copper used in there is pure copper or some extraterrestrial alloy but dropping one is probably what you need to worry about.  Otherwise, let not your heart be troubled.
73 de Norm W1ITT

[WB4AM]

Never had that problem nor heard of it happening.

Glass being sucked in or breaking, yes.

The bellows breaking, no.

--Shane
KD6VXI

I confirm Shane's observations.  I have seen vacuum variables at high power short wave broadcast stations in the Middle East blown to smithereens in direct lightning hits, seen them arrive broken due to shipping mishaps, and saw the results of a couple glass ones that fell off the work bench.  But I've never seen one with the copper innards failed, despite that some are used in autotuners and get exercised a lot.    I'm not sure if the copper used in there is pure copper or some extraterrestrial alloy but dropping one is probably what you need to worry about.  Otherwise, let not your heart be troubled.
73 de Norm W1ITT

Okay that's great information, enough for me to move forward.  As mentioned I have never heard of it either, but my experience which is not a lot, I mostly
seen these Vacuum Variables in amplifiers and only sometimes in an antenna tuner.  And I often wonder how often they had actually gotten exercised.

Come to think of it now, when I tune from 40 to 160 the air variable only gets nudged a little here and a there, not like the roller inductor does when moving from band to band.

Okay I am convinced!  Thank you Shane and Norm for a quick reply. 
I had a good feeling I would get my answer here, noting but good old experience from one and all.  And I am not suggested that anyone is old here either! 

Again thank you Shane and Norm.

73
Ken

[SA2CLC]

I was also a bit concerned about this when I built my motorized balanced tuner, but I have let the worries go.

http://amfone.net/Amforum/index.php?topic=45862.0

[WB4AM]

Hello,

I had a look at your tuner, looks different from what I am use to seeing but that's not saying much.

Looking at your tuner it looks to have 6 fix connections on the coil.  Is this actually for three bands only?

I ask since it is used with OWL, I am thinking three of those fixed connections are for one leg of the OWL and the other three are
for the other leg, but I actually don't know.

Learning is in order here at my QTH !

Your tuner looks like you put some time into it.  Great job.