On 16 January 2011 15:56, Alan Melia <[email protected]> wrote:

Or "5" caps 1,2,4,8,16...... really 5 switches becasue you may need to
parallel caps to meet these values. I suggest the high voltage pulse-rated
caps that we (Andy et al) were recently talking about using in high power PA
tuning. I use WIMA PHE 428 series, which I think I got from RS (Farnell do a
RIFA equivalent) I guess they were about 75p each a few years ago. Very high
voltages are generated across the loop when you apply power, so its a shame
to subject good quality caps in a sub box that that kind of treatment (yup I
am a test equipment hugger :-))  )

You will probably want a smaller "granularity" than 1nF, so you may need to
include a 470PF and a 500pF wide spaced variable in the box, especially if
as you quote it is a high Q loop.

Alan G3NYK


----- Original Message -----
From: "Martin Evans" <[email protected]>
To: <[email protected]>
Sent: Sunday, January 16, 2011 3:32 PM
Subject: LF: Re: Capacitance switch box design - what is the simplest?


> > across the capacitance can be very many times larger than that at 50
ohms.
> >
> > So, I need *to build a capacitance selection box with values from around
> > 0-30nF with sufficient resolution to fine tune a hi-Q loop*. I will need
> > the
> > capacitors to be rated sufficiently high voltage to cope with any power
> > level I am likely to put into the loop antenna (almost certainly never
> > more
> > than 20-30W though).
> >
> > My questions are these:
> >
> > (a) what is the best *choice of capacitors* - type, voltage working,
pulse
> > behaviour, accuracy, price?
> > (b) what is the *simplest arrangement* to give me the adjustment
accuracy
> > and range I need with the fewest parts and fewest switches?
> >
> > Any advice would be appreciated.
> >
> > 73s
> > Roger G3XBM
>
>
> Well, to do 1nF to 30nF in 1nF steps, you need six caps and six switches -
>
> 1, 2, 2, 5, 10, 10nf caps
>
> all in parallel,  with a (toggle?) switch in series with each capacitor.
>
> Martin  GW3UCJ.
>
>
>
>