"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?


Here is the "How to Build an Air Variable Capacitor" article you are looking
for:
http://www.eham.net/articles/5217

w9gb

Without following your link.....well done that man!

"Jozef" wrote in message
...
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

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

The discussions in this thread make me wonder... it seems like the
tough part of making variable caps in the "classical" shape is making
the plates. In my distant past, I was involved in making modest
quantities of punched aluminum parts, and found a shop that was willing
to make a punch/die set for the parts, even though my initial order was
for only a couple hundred of them. (Eventually I ordered enough to
wear out the punch/die set and they made another one...probably did
10,000 or so total.) Once you pay for the punch/die, the parts become
pretty cheap, and the uniformity is vastly superior to what you could
reasonably do by hand. So, the question becomes, if the plates (rotors
and stators) were available in maybe two or three different basic
sizes, how many frustrated hams would be interested in buying them?
Might it be enough to make the punch economical? Would you be willing
to pay, say, two dollars per plate for 0.0625" thick plates with 3"
rotor diameter? A set of 25 such plates, using 0.1" gap (good for
maybe 7kV peak?--provided the edges are properly rounded), would give
you about 190pF. Do the people who would be trying to build such
things already have the ability to do the rest of the parts, or would
they need to be included too? Does someone already sell kits of
capacitor parts?

That's obviously way beyond what the OP needs for a receiving loop, of
course! For that, I'd probably use a varactor diode...or find a radio
receiver to scrap one out of if I wanted to stay mechanical.

Cheers,
Tom

K7ITM wrote:
The discussions in this thread make me wonder... it seems like the
tough part of making variable caps in the "classical" shape is making
the plates. In my distant past, I was involved in making modest
quantities of punched aluminum parts, and found a shop that was willing
to make a punch/die set for the parts, even though my initial order was
for only a couple hundred of them. (Eventually I ordered enough to
wear out the punch/die set and they made another one...probably did
10,000 or so total.) Once you pay for the punch/die, the parts become
pretty cheap, and the uniformity is vastly superior to what you could
reasonably do by hand. So, the question becomes, if the plates (rotors
and stators) were available in maybe two or three different basic
sizes, how many frustrated hams would be interested in buying them?
Might it be enough to make the punch economical? Would you be willing
to pay, say, two dollars per plate for 0.0625" thick plates with 3"
rotor diameter? A set of 25 such plates, using 0.1" gap (good for
maybe 7kV peak?--provided the edges are properly rounded), would give
you about 190pF. Do the people who would be trying to build such
things already have the ability to do the rest of the parts, or would
they need to be included too? Does someone already sell kits of
capacitor parts?

For short runs today laser cutting would be better -- the programming
and setup costs are much lower than tooling. The quality will approach
that of a good die set and exceed that of a poor one. There will be a
point where things'll be cheaper to punch out with a die set, but it'd
probably be in the thousands if not tens of thousands of plates.

That's obviously way beyond what the OP needs for a receiving loop, of
course! For that, I'd probably use a varactor diode...or find a radio
receiver to scrap one out of if I wanted to stay mechanical.

Cheers,
Tom

I wouldn't use a varactor to tune a recieving loop because of intermod.
Antique Radio Supply and others have various variable caps for sale.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Good point about other tooling methods (laser cutting not the only one
available), but the question remains, how many people would actually be
interested in buying plates, at what cost levels? (At the time I was
making the other parts, soft tooling was cheaper for very short runs,
but the shop that did the punch/die was set up to make such tools at
very low cost and easily beat the soft tooling for modest runs. You
just need to decide how many of your parts you want, and go shopping.)

As for using varactors on receivers loops, if you use back-to-back
diodes and don't have to deal with seriously big signals, distortion
should be at worst a very minor problem. For a fairly high Q tuned
antenna, the signal voltage across the diodes for signals you're not
tuned to is pretty small. Of course, the reason it's so hard to find
small (receiving size) mechanical caps now is that their job has been
taken over...by varactors.

Cheers,
Tom

On 2005-07-20, K7ITM wrote:
So, the question becomes, if the plates (rotors
and stators) were available in maybe two or three different basic
sizes, how many frustrated hams would be interested in buying them?

This would be a good emachineshop.com order. You can choose any of the
processes that people are discussing on this thread, depending on the
expected volume. When I played with their estimating software, my
experience was that pretty much everything cost roughly:

$100/ea in qty 1 = $100
$11/ea qty 10 = $110
$1.20/ea qty 100 = $120
$0.13/ea qty 1000 = $130

etc. I figured the biggest risk in using them would be the temptation to
get 10x more units for 10% more and then have a zillion whatevers (eg
rotor plates around the garage. I've been trying to think of a project
that needs 100 identical widgets, and a homemade air variable might be it.

--
Ben Jackson

http://www.ben.com/

In article .com,
K7ITM wrote:

Good point about other tooling methods (laser cutting not the only one
available), but the question remains, how many people would actually be
interested in buying plates, at what cost levels? (At the time I was
making the other parts, soft tooling was cheaper for very short runs,
but the shop that did the punch/die was set up to make such tools at
very low cost and easily beat the soft tooling for modest runs. You
just need to decide how many of your parts you want, and go shopping.)

At the price you suggested (a couple of dollars per plate), the cost
of putting together a good-sized air variable is going to be fairly
fierce. It may be somewhat less than the cost of a new, commercially
manufactured AVC, but it's probably not competitive with the cost of
used-but-serviceable air variables at hamfest/fleamarket prices. You
might find that it was appealing only to those who needed a fairly
specific size/capacitance/standoff-voltage combination, not available
used.

You might find it attractive to specialize a bit. One not-uncommon
application for large air variable caps these days, is people who want
to make a magnetic-loop transmitting antenna. In this application,
keeping resistive losses in the cap to a minimum is very desirable
to help keep the Q and efficiency as not-terribly-low as possible.

One way to do this is to use a welded rotor/shaft assembly, and welded
stator plates, to reduce resistive losses. This is beyond the
capabilities of most homebrewers, I think. An alternative is to use a
soldered construction, which can't be done easily with aluminum but
which is relatively easy if your shaft and spacers are brass, and the
plates are either brass, or PC-board material (double-sided would work
best).

Hence, you might increase the demand for such stamped or laser-cut
plates if you were able to offer them in materials other than
aluminum. The materials cost for brass would probably be higher than
for aluminum, but you might be able to offer plates in double-sided
FR4 for prices no more than aluminum.

--
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!

Cool, Ben. Thanks. I'm wondering what part you put in to get 13 cents
each at 1000 parts. That would be very attractive, but I keep getting
over a dollar a plate for 3" diameter semicircles with a tab/hole for
the shaft, for rotor plates. Admittedly, the 0.063" aluminum thickness
is more than needed for many applications, but dropping it to 0.024"
doesn't help a lot. I don't figure that making your own cap for low
power applications is going to be very attractive, so I'm figuring the
larger sizes. Also, what machining technique did you pick?

Cheers,
Tom

Dave Platt wrote:
In article .com,
K7ITM wrote:


Good point about other tooling methods (laser cutting not the only one
available), but the question remains, how many people would actually be
interested in buying plates, at what cost levels? (At the time I was
making the other parts, soft tooling was cheaper for very short runs,
but the shop that did the punch/die was set up to make such tools at
very low cost and easily beat the soft tooling for modest runs. You
just need to decide how many of your parts you want, and go shopping.)


At the price you suggested (a couple of dollars per plate), the cost
of putting together a good-sized air variable is going to be fairly
fierce. It may be somewhat less than the cost of a new, commercially
manufactured AVC, but it's probably not competitive with the cost of
used-but-serviceable air variables at hamfest/fleamarket prices. You
might find that it was appealing only to those who needed a fairly
specific size/capacitance/standoff-voltage combination, not available
used.

You might find it attractive to specialize a bit. One not-uncommon
application for large air variable caps these days, is people who want
to make a magnetic-loop transmitting antenna. In this application,
keeping resistive losses in the cap to a minimum is very desirable
to help keep the Q and efficiency as not-terribly-low as possible.

One way to do this is to use a welded rotor/shaft assembly, and welded
stator plates, to reduce resistive losses. This is beyond the
capabilities of most homebrewers, I think. An alternative is to use a
soldered construction, which can't be done easily with aluminum but
which is relatively easy if your shaft and spacers are brass, and the
plates are either brass, or PC-board material (double-sided would work
best).

Hence, you might increase the demand for such stamped or laser-cut
plates if you were able to offer them in materials other than
aluminum. The materials cost for brass would probably be higher than
for aluminum, but you might be able to offer plates in double-sided
FR4 for prices no more than aluminum.

At two bucks a pop for plates you _ought_ to be burning up most of your
money in setup, tooling and handling, with very little going to the
actual material.

Hence brass shouldn't be too much more expensive.

I might be more interested in a kit that would let me saw plates out of
FR4 and assemble a precision cap -- so if you supplied frame & shaft
pieces that would accept .031 or .062 FR4 that would be a cool thing.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On 2005-07-20, K7ITM wrote:
Cool, Ben. Thanks. I'm wondering what part you put in to get 13 cents
each at 1000 parts.

That was just an example. I just made a quick 3" diameter rotor with
center hole, laser cut from 6061T6 .047 and 1 is $103, 10 are $123,
100 are $264. It's not quite as steep a curve as the other parts I was
playing with. Since that's laser cutting you can group parts, so if I
throw on a stator too it only goes up to $404 for 100. If I go to slightly
thinner mild steel it's $254 for 100 pairs. Not sure how much difference
Al vs steel makes for this application.

I bet the turret punch would be cheaper, but it can't make the convex
curves.

If you make a die for the rotor it looks like the tooling is about $1k
but the per-part cost can go down to 15 cents each at quantity 10k (for
a net price of about 27 cents each). On the other hand, the lead time
is 72 days!

--
Ben Jackson

http://www.ben.com/