build a variable capacitor
 

i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?

for each facing pair, (ie with two fixed and one moving in
between, you'll get two faces)

C=eA/d e=permittivity of free space, A=area, d= separation.

But you must use real scientific/engineering units, not those Yank Hillbilly
ones.

"Jim" wrote in message
...
i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?

In article ,
Jim wrote:

i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?

For a parallel-plate capacitor, capacitance equals

0.0885 e[r] (N - 1) A / t

where A is the area of one plate (in square inches), N is the number
of plates, t is the thickness of the dielectric (separation between
plates, in an air-variable cap) measured in inches, and e[r] is the
dielectric constant of the dielectric (1.0 in the case of air).

A cap with two stator plates and one rotor plate would have twice the
capacitance of a cap with one stator and one rotor, all else being
equal and neglecting parasitic effects.


--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Sat, 16 Jul 2005 13:08:18 -0500, (Jim)
wrote:

i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?

The spacing between the plates is an important factor as well.

You will need to construct it mechanically solid enough that the
spacing does not change much.

The dielectric material properties will have an effect as well,
although smaller.

Happy trails,
Gary (net.yogi.bear)
--
At the 51st percentile of ursine intelligence

Gary D. Schwartz, Needham, MA, USA
Please reply to: garyDOTschwartzATpoboxDOTcom

"Jim" wrote in message
...
i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?
=========================
In the May 2005 issue of 'Electron' ( mag of VERON ,the IARU affiliated
society in the Netherlands) there is an article , by PE1LKT ,on a homebrew
'sliding 'capacitor made of pieces of double sided printed circuit board.
5 plates are fixed with 4 plates sliding in between.
The fixed and sliding plates fit inside grooves ,made in 2 pieces of timber
or polystyrene (alternatively inside glued strips of such material). The
sliding plates are tapered at one end and straight at the other end where
they are also bolted together with double sided PCB material spacers or
bolts and washers. At that end is also a soldered nut which suits a threaded
bar which is supported by a simple 'hose type bearing' with the end coupled
via a piece of rubber or plastic hose to a fixed in-line electric screw
driver.
The electric motor is operated remotely via a three position switch for '
right hand rotation' - 'off ' -'left hand rotation'
The fixed plates are obviously also galvanically connected at the far end
away from the moving plates .

This variable capacitor is used for a transmitting magloop and hence is
subjected to high RF voltages ,hence the plates are appropriately spaced .
Spacing could be minimised for a RX loop

The above variable cap has a capacitance of approx 400 pF ,requiring fixed
plates each with an area of 210 x 170 mm ( 357 cmsq equals approx 55 inch
sq). The sliding plates are slightly shorter.

For a RX loop requiring less plate spacing the plate area can be reduced.

Frank GM0CSZ / KN6WH

On Sat, 16 Jul 2005, Jim wrote:

Date: Sat, 16 Jul 2005 13:08:18 -0500
From: Jim
Newsgroups: rec.radio.amateur.homebrew
Subject: build a variable capacitor

i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?



Do you really need to build this? I know they (365 mfd variables) are hard
to find (I'd love to find a cheap source, expensive sources on the
internet want $10-15 each [google on "365 mfd" to find them]), but if you
go to a thrift store (Salvation Army, etc.), you may find old transistor
clock radios for $1-2 each that have miniature 365 mfd variables that will
do just fine if you take a few minutes to de-solder them from the circuit
board.


Art, W4PON

i need a variable for a receiving loop antenna....
In the May 2005 issue of 'Electron' ( mag of VERON ,the IARU affiliated
society in the Netherlands) there is an article , by PE1LKT ,on a homebrew
'sliding 'capacitor made of pieces of double sided printed circuit board.
5 plates are fixed with 4 plates sliding in between.....

QST has published several homemade-capacitors articles in the last few years:
* A Home-Brew Loop Tuning Capacitor (November, 1994)
* A Homemade High-Power Tuning Capacitor (June, 1983)
* One I recall (but can't find; my QST's are paper, someone with the
QST CDs could do a search) which used concentric tubes with thin
plastic around the inner/inside the outer

Ok, so "few" is relative at my age!-)
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

Of course, you CAN build the capacitor, but unless you want to do it for
fun and/or education, why re-invent the wheel? Dan's Small Parts and
Kits sells several nice capacitors.

http://www.danssmallpartsandkits.net/

The capacitors aren't necessarily "dirt cheap", but a manufactured
capacitor will be physically stable, insuring smooth change in
capacitance as the shaft is rotated.

Scott
N0EDV

Jim wrote:
i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?

"Manufacture" = "Make By Hand"

"Scott" wrote in message
...
Of course, you CAN build the capacitor, but unless you want to do it for
fun and/or education, why re-invent the wheel? Dan's Small Parts and Kits
sells several nice capacitors.

http://www.danssmallpartsandkits.net/

The capacitors aren't necessarily "dirt cheap", but a manufactured
capacitor will be physically stable, insuring smooth change in capacitance
as the shaft is rotated.

Scott
N0EDV

Jim wrote:
i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?

I have recently built a very large pi-network transmatch with two very big
homemade variable capacitors. They can be seen at:
http://www.metaphoria.us/hamradio/transmatch.html

Jozef
WB2MIC

"Polymath" wrote in message
...
"Manufacture" = "Make By Hand"

"Scott" wrote in message
...
Of course, you CAN build the capacitor, but unless you want to do it for
fun and/or education, why re-invent the wheel? Dan's Small Parts and
Kits sells several nice capacitors.

http://www.danssmallpartsandkits.net/

The capacitors aren't necessarily "dirt cheap", but a manufactured
capacitor will be physically stable, insuring smooth change in
capacitance as the shaft is rotated.

Scott
N0EDV

Jim wrote:
i need a variable for a receiving loop antenna. does anyone know how to
determine the area of plates for a given value of cap? for example i
need 365 pf. two half circle plates would be handy, separated by a piece
of plastic or card stock. what surface area for each plate would
approximate that value? does two stator plates with one rotor between
them increase the value if compared to one plate each?