Message-ID: <532F7D4648B74CCF825F0D0E596655AF@home2361108df7>
From: "Martin Evans" <books@mollies.freeserve.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <AANLkTimNE73xJS9tHvpE2X7Afd33mWsQm7n-ZdfvG_9+@mail.gmail.com>
Date: Sun, 16 Jan 2011 15:32:50 -0000
MIME-Version: 1.0
Subject: LF: Re: Capacitance switch box design - what is the simplest?
Content-Type: text/plain;
	format=flowed;
	charset="iso-8859-1";
	reply-type=original
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


> 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.