Topband: Submerging variable caps in oil assubstituteforvacuumvariables

[mapa50 at windstream.net]

I contacted George A. Sanford about the oil used in the variable cap in the HP4815a and he replied that the oil was used for a shock absorber effect for frequency stability in the instrument and not for any other reason. I had a need for a vac variable and have a large air variable that I had planed to submerge in oil, but have not yet found a suitable oil.  

       73 es DX Pat H. Armstrong  KF5YZ

---- Carl <km1h at jeremy.mv.com> wrote: 
> There are several on the HP forum that are famililar with that product 
> Hardy.
> I also have one but claim no expertise since it still works well.
> hp_agilent_equipment at yahoogroups.com
> 
> Carl
> KM1H
> 
> ----- Original Message ----- 
> From: "Hardy Landskov" <n7rt at cox.net>
> To: "Tom W8JI" <w8ji at w8ji.com>; "Bill Wichers" <billw at waveform.net>
> Cc: <n4zr at contesting.com>; <topband at contesting.com>; "HAROLD SMITH JR" 
> <w0rihps at sbcglobal.net>; "Shoppa, Tim" <tshoppa at wmata.com>
> Sent: Sunday, February 02, 2014 8:58 AM
> Subject: Re: Topband: Submerging variable caps in oil 
> assubstituteforvacuumvariables
> 
> 
> > FYI All,
> > The HP4815A Vector Impedance Meter submersed the main tuning capacitor in 
> > an oil bath of some kind to get the capacitance up. Apparently dissipation 
> > factor was not of concern when the unit was designed. If there are any 
> > retired HP folks out there they may be able to identify what they used. I 
> > have read some years ago that hydraulic jack oil was very close.
> > I need to open mine up and replace one of the capacitors because the 
> > oscillator will not start on the higher frequency ranges. I am gun shy at 
> > this point until I know exactly what I am dealing with.
> > 73 Hardy N7RT
> >
> >
> > ----- Original Message ----- 
> > From: "Tom W8JI" <w8ji at w8ji.com>
> > To: "Bill Wichers" <billw at waveform.net>
> > Cc: <n4zr at contesting.com>; <topband at contesting.com>; "HAROLD SMITH JR" 
> > <w0rihps at sbcglobal.net>; "Shoppa, Tim" <tshoppa at wmata.com>
> > Sent: Sunday, February 02, 2014 6:16 AM
> > Subject: Re: Topband: Submerging variable caps in oil as 
> > substituteforvacuumvariables
> >
> >
> >> The major issue with dielectrics is dissipation factor at 2 MHz, which 
> >> affects losses and Q. Dissipation factor is not published all the time. I 
> >> can't find dissipation factor for mineral oil.
> >>
> >>
> >>
> >> ----- Original Message ----- 
> >> From: "Bill Wichers" <billw at waveform.net>
> >> To: "Tom W8JI" <w8ji at w8ji.com>
> >> Cc: "HAROLD SMITH JR" <w0rihps at sbcglobal.net>; "Shoppa, Tim" 
> >> <tshoppa at wmata.com>; <n4zr at contesting.com>; <topband at contesting.com>
> >> Sent: Saturday, February 01, 2014 4:05 PM
> >> Subject: Re: Topband: Submerging variable caps in oil as 
> >> substituteforvacuum variables
> >>
> >>
> >> I was reading this thread and all the concerns about oil in the 
> >> capacitor. Has anyone ever thought about trying SF6 as a dielectric? It's 
> >> commonly used in high voltage (hundreds of kilovolts) switchgear by 
> >> utilities.
> >>
> >> Just a thought, more curiosity than anything else.
> >>
> >> -Bill
> >>
> >> Sent from my iPhon
> >>
> >> On Jan 30, 2014, at 5:32 AM, "Tom W8JI" <w8ji at w8ji.com> wrote:
> >>
> >>> Still I am intrigued by the thought of a remote tuning capacitor via 
> >>> hydraulic tubing :-). The capacitor plates could be as simple as two 
> >>> concentric cylinder conductors with appropriate spacers. I betcha crud 
> >>> collecting on the top of the oil would set voltage limit.>>>
> >>>
> >>> I would be as concerned, or more concerned, with the dissipation factor 
> >>> of the oil at short wave frequencies.
> >>>
> >>> The thing that worries me is I cannot recall every seeing a single good 
> >>> high-Q oil-dielectric capacitor above power line and audio frequencies. 
> >>> As a matter of fact, many years ago I tried to use a surplus 20-40kV oil 
> >>> capacitor from Fair Radio as a plate blocking capacitor, and it 
> >>> overheated so badly it exploded.
> >>>
> >>> I looked for HF data on mineral oil as a dielectric and couldn't find 
> >>> anything. That would be my main concern. I guess I could stick mineral 
> >>> oil between the plates of a capacitor and see what happens to Q.
> >>>
> >>>
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