Hello All:
I noticed that I wasn't getting any email from the various groups I
belong to, and checked Yahoo groups and each of then said my mail
was bouncing, so I went through their reactivation process and when
I went back to the Yahoo groups, there was no warning message. Yet,
I have not received any messages in the last hour or so. Could
anyone send a "testing" message to the group so I can see if my
reactivation worked?
Thanks,
Barry

开云体育

Bear,

Guess my math-speak was a bit more obscure than the duck-speak in my
direct-conversion receiver. In less mathy technese:

1. While it looks like a pi network, it operates more like an L
matching network. That's because it uses the loop's inductance
partially compensated by the distal capacitor as the inductive leg.

2. No terminating resistor is needed. It's actually a disadvantage
to use one.

3. The impedance of free space only plays in the physics of finding
the loop impedance from first principles—something I've never seen
done outside of university exams. An impedance bridge will do the
trick.

4. The main advantage over the Colpitts-like matching network using
the same parts values is that both capacitors have one side returned
to common. That does not help much in the broadcast band where the
proximal capacitor has a very large value. Above 15 or 20 MHz real
variable capacitors can be used. With traditional variable caps,
being at common becomes a decided advantage for easy construction
and somewhat reduced hand capacitance.

Walking the southern 3 km of the Bothell-Everett highway yesterday
(I was thinking about the next experiment with my 1-evening direct
conversion receiver and got on the wrong bus), I noticed a few
salmon berries looking ripe. The Himalayan blackberries are in
flower, so have a couple weeks before the berries are ready to
harvest. With this year's fashions the lady bears (at least around
here) have blue tummies. So unless you're going to claw your way
into big-leaf maple sap, you might as well work on loops for the
next couple weeks. By the way I arrived in Bothell just in time to
see the correct bus disgorge a passanger who'd been waiting next to
me where I'd gotten on the wrong bus. The way some buses zig-zag
through the industrial parks, walking can be faster.

Regards,

Steven

--- In loopantennas@..., "qrpbear" <qrpbear@y...> wrote:

Steve,

Ahh... thanks for that. I'm still puzzling over the math but

perhaps

they point to a possibility of success.

For those who may not be aware of the "Street Sweepings Loop" it

is

based on the idea of feeding the loop directly from 50 ohm coax

using

the loop inductance itself as part of a Pi-Network. The most
important remaining questions are... what would be the output
impedance of the Pi-net? 377 Ohms (the impedance of free space) or
some other value? Would a resistive output termination be

required?

All this preliminary to building such an animal of course. The

real

answer would be to build it and compare it against a conventional
loop of the same size but Summer is coming on and 'Bear' is more
concerned with eating berries and catching salmon to fatten up on

for

the coming Winter's sleep.... and mebbe finding Lady Bears, too

<G>.

73,

'Bear' NH7SR (Yo! Shoggoth!! Fetch me a beer!!! No... not St.

Pauli

Girl you dumb beast!!!!)

--- In loopantennas@..., "Steven S. Coles"
<steven_coles@y...> wrote:

Bear,

I tried to answer you on the GCC Loop Group. For some reason

the

post didn't appear. So:

Bear,

My shoggoth upchucked allowing me to provide the following

example:

An impedance analyzer gives an impedance of 3 + j215 ohms at 1.6

MHz

for a certain Pioneer AM loop. Let's make the proximal

capacitor

(Cp) 7900 pF and the distal capacitor (Cd) 490 pF. That gives
capacitive reactances –j12.6 ohms and –j203 ohms respectively at

1.6

MHz.

The loop in series with Cd gives

3 + j215 – j203 = 3 + j12

Using the parallel impedance formula gives

(0 – j12.6)(3 + j12)/[ (0 – j12.6)+(3 + j12)] = 50.9 – j2.4 ohms

That's not too bad for the shoggoth and me making some wild

guesses.

Cp could be a 5100 pF cap with a dip switch to add 200, 400,

800,

1600, and 3300 pF in parallel. Cd could be a 360 pF variable

with

200 pF in parallel.

Actually the shoggoth used



and transformed the negative inductor in the upper right network
into a capacitor.

Regards,

Steven

--- In loopantennas@..., Barry Savage <sofistic@s...> wrote:

Hello All:
For some time now, I have been wondering about the electromagnetic

properties of common materials, and find these things rather hard to
discover. The few times I have asked about them on different
electronics-oriented groups, most people have dismissed them as not
workable. I don't want to get too off-topic, but I hope some of you
can point me in a productive direction. I also wanted to ask some
rather nieve questions that I hope someone can either answer or point
me in the right direction. So, here goes.

QUESTIONS:
1. Would it be possible to make a VLF loop-stick out of rebar?

First thing I can think of is that the remnance of rebar would be a
problem, but nowhere have I been able to find the magnetic properties
of such a readily available and cheap material. What about its
permeability, its coercivity? What does its B/H curve look like?

The problem is that it is conductive, and so you'd have I2R losses in
the induced current within the rebar. That is why cores are made out
of nonconductive ferrite, and transformer cores are made of thin
sheets of iron insulated from each other by layers of laquer.

If you look up old projects like building your own Model T spark coil,
or building one of those ring launchers with a bundle of welding rods,
note that they all require you to paint the rods to insulate them from
each other.

So- how about welding rods? Get a pound of the smallest diameter you
can find. Carefully paint them, then bundle them when dry.

2. If a "square" coil is considered optimal, why is an air-core

loop antenna so far out of square? I know aperture is an issue here,
but what are the trade-offs in terms of Q and lack of symmetry between
diameter and coil-depth?

I thought a round coil was optimal. It's just easier to build a square
coil. For the same circumference, a round coil has a larger area
enclosed than any other shape.

BTW, some ham in the state of Washington (don't have the link

offhand, but I can look it up if anyone is interested) has a
spreadsheet called "professor Coyle" that is very good for modeling
coils (both solenoid and spiderweb). I have modified it to include
wire resistance tables and a few other bells and whistles.

Where? Let us see it!

HARE BRAINED IDEAS:
1. What if you built a "steerable null" antenna using two identical

loop antennas at the end of a rotatable beam, and, in turn (heh) each
of the loops were also rotatable? Something Bear said awhile back
triggered this idea off, and I remembered experimenting with a crystal
set from Gollum's Crystal Radio site that used two spider-web coils,
one for the antenna and one for the crystal detector. The coils were
coupled (face to face) at a distance of about 6-inches, but I could
get very responsive results even at a distance of 12-inches apart, so
I played around with rotating one with respect to another and found
the nulling very sharp. I haven't done any measurement of this
arrangement, so this is only an intuitive observation.

BTW, I also built a "box" loop using a wooden "in-basket" I got at a

Michaels craft store, and used the ears of the in-box to mount it on a
base (also from Michaels) so it became an altazimuth loop. It works
very well with my little Radio Shack DX-370 at a distance of about
6-inches. The bottom of the in-box is used as the face of the loop,
the edge of the box is where the coil is wound, and a little poly
variable cap is mounted at the center of the "bottom" of the in-box.

2. Is a helical considered a loop? What if you built a large

helical antenna and treated it as if it were a loop antenna? I mean,
a helical is just a streached-out loop, isn't it? The only difference
I can see is that the distributed capacitance drops and so does the
inter-winding inductance. Humm, what am I missing here?

I seem to recall that someone (QRPBear?) tested this idea with loops
of the same diameter and turns. And found that for an air core, it
made little difference.

The problem is that the further each turn is from the next, the less
they are coupled inductively to each other. So a loop spread out into
a helix may lose efficiency. Hopefully someone with more theory and
experience under their belt can tell us.

Some spread of the turns will reduce interwinding capacitance, but you
quickly reach a point of diminishing returns.

Of course if it is on a ferrite core, that is different. How exactly I
can't say. :'/

3. A few times on this list someone has mentioned mobius-strip

loops; what is the theory behind them and how well do they work? What
are their properties in a vertical and horizontal plane of
orientation? (oops, that was a question, not a HBI)

Mobius strip loops? Hmm... I'm curious too. I can see an easy way to
"wind" one. Use the method of using a ribbon cable- only give it one
turn before you offset and solder. No, that won't work, I just gave it
a mental try and you end up with a one turn loop with a bunch of two
turn shorted turns and a one turn shorted turn. Does the same if you
don't offset. So what is a mobius loop antenna?

Steve Greenfield

We're up to 55 members! And lots of activity.

Remember, if you have not posted yet, all new members are moderated.
So don't be alarmed if your first message takes a few hours or a day
to appear. I get on fairly often to check, subject to Real Life(tm).

Once you've posted, I'll set you unmoderated. Just my way of quietly
blocking spammers.

Steve Greenfield,
moderator of the new loopantennas

Steve et Barry,

I vaguely recall seeing somewhere a description of an ELF loop(stick)
antenna using rebar as the core. Mebbe on <>. I
don't remember for sure.

The helix antenna is a unidirectional antenna commonly used in arrays
of four or more for EME work. It has the same kind of high gain and
narrow beamwidth as long yagis but has circular polarization. Part of
the problem with EME is the return signal from the moon spins in the
opposite direction from the uplink. If you don't select the correct
spin polarity you don't get the downlink signal.

The helix is fed at one end against a screen reflector and the other
end is unconnected. Pitch (spacing between turns) defines the gain
and beamwidth. Since the whole length is active there is no need to
have a decreasing radius unlike a yagi where only the driven element
is active and the directors decrease in length the further they are
from the driven element.

Re the mobius loop. Actually it is not a mobius. If you take a strip
of paper formed into a loop and let it lay on the table you will see
that it roughly defines two loops in a figure 8 when given an
additional half twist. One of the regular Antennex contributors is
working on a loop like this with their planes at 90 degrees to each
other. Seems to me that it could also be done with the planes in
line. Coupling via open-wire or 300 ohm twinlead attached to each
mobius half at the crossover point. It may be, though, that phase
cancellation could occur with the planes in line. Where to insert the
resonating capacitor is a question that remains to be answered as
does whether each half of the mobius should have its own capacitor.
Now that I think of it, if such a creature was fed with a balanced
transmission line the tuning point could be the ATU and no capacitors
required at the antenna.

The other idea I plan to try when I can get the materials and time is
a dual stacked loop sharing a common element... something like a 2:1
rectangular shape with a central shared member creating two 1:1
rectangles. Each square with its own capacitor opposite the common
member and transformer coupled with the usual 1/5th circumference
coupling loop adjacent to the common member or a 50 ohm gamma match
on the central member. The primary idea here being to double the
effective loop area while simultaneously creating a double-tuned
bandpass circuit for increased bandwidth. This would be for a fixed-
tuned transmitting loop for the 30-meter band which is 50 KHz wide.
I'm thinking in terms of a structure that would be about 3' wide and
6' tall. Still fairly compact even for indoor use.

Gotta take a shower and be off to work... hopefully without the
headache I have right now. See y'all in 8 hours or so.

73,

'Bear' NH7SR

--- In loopantennas@..., "Steve" <alienrelics@y...> wrote:

--- In loopantennas@..., Barry Savage <sofistic@s...>

wrote:

Hello All:
For some time now, I have been wondering about the electromagnetic

properties of common materials, and find these things rather hard to
discover. The few times I have asked about them on different
electronics-oriented groups, most people have dismissed them as not
workable. I don't want to get too off-topic, but I hope some of you
can point me in a productive direction. I also wanted to ask some
rather nieve questions that I hope someone can either answer or

point

me in the right direction. So, here goes.

QUESTIONS:
1. Would it be possible to make a VLF loop-stick out of rebar?

First thing I can think of is that the remnance of rebar would be a
problem, but nowhere have I been able to find the magnetic

properties

of such a readily available and cheap material. What about its
permeability, its coercivity? What does its B/H curve look like?

The problem is that it is conductive, and so you'd have I2R losses

in

the induced current within the rebar. That is why cores are made out
of nonconductive ferrite, and transformer cores are made of thin
sheets of iron insulated from each other by layers of laquer.

If you look up old projects like building your own Model T spark

coil,

or building one of those ring launchers with a bundle of welding

rods,

note that they all require you to paint the rods to insulate them

from

each other.

So- how about welding rods? Get a pound of the smallest diameter you
can find. Carefully paint them, then bundle them when dry.

2. If a "square" coil is considered optimal, why is an air-core

loop antenna so far out of square? I know aperture is an issue

here,

but what are the trade-offs in terms of Q and lack of symmetry

between

diameter and coil-depth?

I thought a round coil was optimal. It's just easier to build a

square

coil. For the same circumference, a round coil has a larger area
enclosed than any other shape.

BTW, some ham in the state of Washington (don't have the link

offhand, but I can look it up if anyone is interested) has a
spreadsheet called "professor Coyle" that is very good for modeling
coils (both solenoid and spiderweb). I have modified it to include
wire resistance tables and a few other bells and whistles.

Where? Let us see it!

HARE BRAINED IDEAS:
1. What if you built a "steerable null" antenna using two

identical

loop antennas at the end of a rotatable beam, and, in turn (heh)

each

of the loops were also rotatable? Something Bear said awhile back
triggered this idea off, and I remembered experimenting with a

crystal

set from Gollum's Crystal Radio site that used two spider-web coils,
one for the antenna and one for the crystal detector. The coils

were

coupled (face to face) at a distance of about 6-inches, but I could
get very responsive results even at a distance of 12-inches apart,

so

I played around with rotating one with respect to another and found
the nulling very sharp. I haven't done any measurement of this
arrangement, so this is only an intuitive observation.

BTW, I also built a "box" loop using a wooden "in-basket" I got

at a

Michaels craft store, and used the ears of the in-box to mount it

on a

base (also from Michaels) so it became an altazimuth loop. It works
very well with my little Radio Shack DX-370 at a distance of about
6-inches. The bottom of the in-box is used as the face of the

loop,

the edge of the box is where the coil is wound, and a little poly
variable cap is mounted at the center of the "bottom" of the in-box.

2. Is a helical considered a loop? What if you built a large

helical antenna and treated it as if it were a loop antenna? I

mean,

a helical is just a streached-out loop, isn't it? The only

difference

I can see is that the distributed capacitance drops and so does the
inter-winding inductance. Humm, what am I missing here?

I seem to recall that someone (QRPBear?) tested this idea with loops
of the same diameter and turns. And found that for an air core, it
made little difference.

The problem is that the further each turn is from the next, the less
they are coupled inductively to each other. So a loop spread out

into

a helix may lose efficiency. Hopefully someone with more theory and
experience under their belt can tell us.

Some spread of the turns will reduce interwinding capacitance, but

you

quickly reach a point of diminishing returns.

Of course if it is on a ferrite core, that is different. How

exactly I

can't say. :'/

3. A few times on this list someone has mentioned mobius-strip

loops; what is the theory behind them and how well do they work?

What

are their properties in a vertical and horizontal plane of
orientation? (oops, that was a question, not a HBI)

Mobius strip loops? Hmm... I'm curious too. I can see an easy way to
"wind" one. Use the method of using a ribbon cable- only give it one
turn before you offset and solder. No, that won't work, I just gave

it

a mental try and you end up with a one turn loop with a bunch of two
turn shorted turns and a one turn shorted turn. Does the same if you
don't offset. So what is a mobius loop antenna?

Steve Greenfield

Most excellent tamales!!

A modest proposal...

How about renaming the group to loop_and_compact_antennas? If the
other members are agreeable to broadening the spectrum on this topic
now would seem to be the time to do it rather than later when it
might be more difficult to implement.

Perhaps a poll?

1) Yeah, gimme more!!
2) Nah, I'm a loop purist!!
3) I got here by mistake :-/
4) Spam, spam, spam, spam.

73,

'Bear' NH7SR

--- In loopantennas@..., "Steve" <alienrelics@y...> wrote:

We're up to 55 members! And lots of activity.

Remember, if you have not posted yet, all new members are moderated.
So don't be alarmed if your first message takes a few hours or a day
to appear. I get on fairly often to check, subject to Real Life

(tm).

Once you've posted, I'll set you unmoderated. Just my way of quietly
blocking spammers.

Steve Greenfield,
moderator of the new loopantennas

---

Do you Yahoo!?
Friends. Fun.

Barry,

Hmmm... does anyone have a good source for Mu-metal at all?

It's been in _very_ short supply ever since Mu sank beneath the
waves!

<GD&R>

But doesn't Mu-metal tend to resist flux changes??

'Bear' NH7SR

--- In loopantennas@..., Barry Savage <sofistic@s...>
wrote:

Bear, et al:

Here is something I found on the Public Seismic Network archives.

"New article in the May '96 issue of Scientific American (pg 98)

describes

construction of VLF antenna. Their plans use a 24" piece of rebar

for

the core of the coil. Anyone have a good source for Mu-metal cores

in the

2' range???"

Guess I will look up the SA archives & see what is there.
Barry

Steven,

I wonder which would be more effective at pushing current (pulling
current?) into the loop.. the Pi-net or a Colpitts style circuit akin
to an unbalanced version of the old Army Loop. Guess I'll have to
build it to find out. In either case the wiper contact resistance
will be problematical although the Pi-net should exhibit less hand
capacitance detuning as you pointed out.

Ummm... blue bellies. 'Bear' like blue bellies. 'Bear' like golden
honey brown bellies better but If Lady Bear have plenty salmon 'Bear'
don't care. Trout good, too, but Ahi and Tako Poki even better.

Hmmm... Bothel an industrial park, eh? Next thing I s'pose you're
gonna tell me is that there are no more fields of yellow daffodils in
Fife :-/

73,

'Bear' NH7SR

--- In loopantennas@..., "Steven S. Coles"
<steven_coles@y...> wrote:

Bear,

Guess my math-speak was a bit more obscure than the duck-speak in

my

direct-conversion receiver. In less mathy technese:

1. While it looks like a pi network, it operates more like an L
matching network. That's because it uses the loop's inductance
partially compensated by the distal capacitor as the inductive leg.

2. No terminating resistor is needed. It's actually a

disadvantage

to use one.

3. The impedance of free space only plays in the physics of

finding

the loop impedance from first principles—something I've never seen
done outside of university exams. An impedance bridge will do the
trick.

4. The main advantage over the Colpitts-like matching network

using

the same parts values is that both capacitors have one side

returned

to common. That does not help much in the broadcast band where the
proximal capacitor has a very large value. Above 15 or 20 MHz real
variable capacitors can be used. With traditional variable caps,
being at common becomes a decided advantage for easy construction
and somewhat reduced hand capacitance.

Walking the southern 3 km of the Bothell-Everett highway yesterday
(I was thinking about the next experiment with my 1-evening direct
conversion receiver and got on the wrong bus), I noticed a few
salmon berries looking ripe. The Himalayan blackberries are in
flower, so have a couple weeks before the berries are ready to
harvest. With this year's fashions the lady bears (at least around
here) have blue tummies. So unless you're going to claw your way
into big-leaf maple sap, you might as well work on loops for the
next couple weeks. By the way I arrived in Bothell just in time to
see the correct bus disgorge a passanger who'd been waiting next to
me where I'd gotten on the wrong bus. The way some buses zig-zag
through the industrial parks, walking can be faster.

Regards,

Steven

--- In loopantennas@..., "qrpbear" <qrpbear@y...> wrote:

Steve,

Ahh... thanks for that. I'm still puzzling over the math but

perhaps

they point to a possibility of success.

For those who may not be aware of the "Street Sweepings Loop" it

is

based on the idea of feeding the loop directly from 50 ohm coax

using

the loop inductance itself as part of a Pi-Network. The most
important remaining questions are... what would be the output
impedance of the Pi-net? 377 Ohms (the impedance of free space)

or

some other value? Would a resistive output termination be

required?

All this preliminary to building such an animal of course. The

real

answer would be to build it and compare it against a conventional
loop of the same size but Summer is coming on and 'Bear' is more
concerned with eating berries and catching salmon to fatten up on

for

the coming Winter's sleep.... and mebbe finding Lady Bears, too

<G>.

73,

'Bear' NH7SR (Yo! Shoggoth!! Fetch me a beer!!! No... not St.

Pauli

Girl you dumb beast!!!!)

--- In loopantennas@..., "Steven S. Coles"
<steven_coles@y...> wrote:

Bear,

I tried to answer you on the GCC Loop Group. For some reason

the

post didn't appear. So:

Bear,

My shoggoth upchucked allowing me to provide the following

example:

An impedance analyzer gives an impedance of 3 + j215 ohms at

1.6

MHz

for a certain Pioneer AM loop. Let's make the proximal

capacitor

(Cp) 7900 pF and the distal capacitor (Cd) 490 pF. That gives
capacitive reactances –j12.6 ohms and –j203 ohms respectively

at

1.6

MHz.

The loop in series with Cd gives

3 + j215 – j203 = 3 + j12

Using the parallel impedance formula gives

(0 – j12.6)(3 + j12)/[ (0 – j12.6)+(3 + j12)] = 50.9 – j2.4

ohms

That's not too bad for the shoggoth and me making some wild

guesses.

Cp could be a 5100 pF cap with a dip switch to add 200, 400,

800,

1600, and 3300 pF in parallel. Cd could be a 360 pF variable

with

200 pF in parallel.

Actually the shoggoth used



and transformed the negative inductor in the upper right

network

into a capacitor.

Regards,

Steven

Bear,

Old Bothell is about like always. The changes: The Sammamish River
Trail runs along the (insert guess) river. Univ. of Wash. &
Shoreline Comm. Coll have a shared branch campus overlooking I-405.
Woodinville's 10-watt highway advisory station comes in fine
business. (It announces the local festivals, so it worth a listen
once a month.)

East of I-405 & north of Woodinville is a large technical park.
Another, Canyon Park were I work, is on the Bothell-Everett Highway
maybe 4 miles north of old Bothell. Every so often someone tries to
nickname the strip along I-405 "The Silicon Forest." There's
probably more medical than electronic technology through the
area. "The Pill Forest" just doesn't have the right ring.

In case you haven't noticed, this board's owner/moderator lives in
Tacoma. There are plenty of Puget Sound residents on the technical
boards. Getting them together for any kind of technical test is
like herding mountain lions.

Regards,

Steven

--- In loopantennas@..., "Michael Hebert" <qrpbear@y...>
wrote:

Steven,

I wonder which would be more effective at pushing current (pulling
current?) into the loop.. the Pi-net or a Colpitts style circuit

akin

to an unbalanced version of the old Army Loop. Guess I'll have to
build it to find out. In either case the wiper contact resistance
will be problematical although the Pi-net should exhibit less hand
capacitance detuning as you pointed out.

Ummm... blue bellies. 'Bear' like blue bellies. 'Bear' like golden
honey brown bellies better but If Lady Bear have plenty

salmon 'Bear'

don't care. Trout good, too, but Ahi and Tako Poki even better.

Hmmm... Bothel an industrial park, eh? Next thing I s'pose you're
gonna tell me is that there are no more fields of yellow daffodils

in

Fife :-/

73,

'Bear' NH7SR

All,

Can't post the Rod Newkirk, W9BRD, article on distributed capacitance
multiturn loops but I did find a web site with pics, diagrams and
info for a 7 mHz version at...

<>

Also ran across another site featuring an article about a 12"
diameter 5-30 MHz loop with a rather unique coupling method that I
hadn't seen before. You'll find that one at...

<>

It's entitled "Fun and Games with the G3LZH Miniature Loop Antenna."

Enjoy and 73,

'Bear' NH7SR

开云体育

开云体育

Michael,

Good to see you here and eagerly looking forward to more info on your
loop. Especially how you manage the remote tuning arrangement.

I didn't really mean my proposal to serve as a poll of the members.
That should come from Steve Greenfield as he is the list owner here
and it's his decision if a poll should be done. Nevertheless, thankee
fer da vote <G>

73,

'Bear' NH7SR

--- In loopantennas@..., "Michael Stevenson"
<portstevos@t...> wrote:

1) The more the merrier I think!

Michael Stevenson,
Australia.
----- Original Message -----
From: qrpbear
To: loopantennas@...
Sent: Sunday, June 13, 2004 6:09 AM
Subject: [loopantennas] Re: Whoo hoo!


Most excellent tamales!!

A modest proposal...

How about renaming the group to loop_and_compact_antennas? If the
other members are agreeable to broadening the spectrum on this

topic

now would seem to be the time to do it rather than later when it
might be more difficult to implement.

Perhaps a poll?

1) Yeah, gimme more!!
2) Nah, I'm a loop purist!!
3) I got here by mistake :-/
4) Spam, spam, spam, spam.

73,

'Bear' NH7SR

--- In loopantennas@..., "Steve" <alienrelics@y...>

wrote:

> We're up to 55 members! And lots of activity.
>
> Remember, if you have not posted yet, all new members are

moderated.

> So don't be alarmed if your first message takes a few hours or

a day

> to appear. I get on fairly often to check, subject to Real Life
(tm).
>
> Once you've posted, I'll set you unmoderated. Just my way of

quietly

> blocking spammers.
>
> Steve Greenfield,
> moderator of the new loopantennas


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Service.

Hi All! New to the group, but not to radio. When I was a kid (very
long ago), we used to make our own variable capacitors. Basically,
we'd take a square of wood, varnish it well, and mount a square or
disc of aluminum in the center, and attach a lead to it. This became
the stator. A piece of celluloid (later plastic) was glued over this
stator plate, and a similar piece of aluminum with a lead attached
and a piece of phenolic rod glued in it's center, was placed on top
to become the rotor. Both leads were attached to the board by
fanstock clips. By manipulating (sliding) the rotor plate across the
stator plate, you could go from full mesh to wide open. A friend of
mine and fellow Boy Scout even took india ink and marked the receive
locations of his favorite stations on the celluloid, making
essentially, a rudimentary dial. Now, why am I bringing this all up
(other than to wallow in nostalgia)? It seems to me that variable
capacitors are becoming hard to find these days, and when you can
find them they're pricy, and usually have to be combined with other
items to make a "minimum purchase". Not good for a geezer like me,
living on a fixed income. Does anybody have any experience
homebrewing VCs? If so, maybe you could share some data or formulae
for scaling the plates. Well, talk with you all again soon, Karl