Hello All,

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]

I'm interested in getting my feet wet in building a receiver, but don't
want to buy a 'kit'. I'd like to build a radio that has a knob I can
somewhat tune it with. I've been really looking at regenerative radios
and superheterodynes, although a simple radio is more important than
how well it really performs.

One common theme in almost every 'simple' radio design I can find, is
that they *ALL* use Air variable capacitors. While I'm sure these were
everywhere 20 years ago, going to every major electronics supply house
in Portland, Oregon has yielded me only with small trimmer capacitors,
which AFAIK, will not do for a 'tuner'. I'm aware I can order online
for about 10-20$ and purchase an air variable, but I'd like to see if
any of you have any suggestions on what might work as an alternative?

Varactor diode seemed like a neat idea, although I guess they suffer
from temperature fading. Would this make it impratical for a shortwave
or AM receiver? Can you even make a regenerative radio out of a
varactor? (Actually, I'll try ghetto rigging a red LED for a varactor,
since I don't really care about it being nicely linear, I just want
something with a knob that does something!).

Can anyone recommend any type of reasonably simple radio (simple mostly
as in reasonably low parts count, i don't need an LCD readout) that
could possibly be done with parts that can be had at a reasonably
stocked electronics store. (Much more inventory than Fry's). A knob is
important, I don't want to build one of those ghetto radios with the
coil and a metal bead that I slide along the coil to 'tune' it. Any
other way to use a potentiometer to tune a radio?

SparkySKO wrote:
Hello All,

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]

I'm interested in getting my feet wet in building a receiver, but don't
want to buy a 'kit'. I'd like to build a radio that has a knob I can
somewhat tune it with. I've been really looking at regenerative radios
and superheterodynes, although a simple radio is more important than
how well it really performs.

One common theme in almost every 'simple' radio design I can find, is
that they *ALL* use Air variable capacitors. While I'm sure these were
everywhere 20 years ago, going to every major electronics supply house
in Portland, Oregon has yielded me only with small trimmer capacitors,
which AFAIK, will not do for a 'tuner'. I'm aware I can order online
for about 10-20$ and purchase an air variable, but I'd like to see if
any of you have any suggestions on what might work as an alternative?

Varactor diode seemed like a neat idea, although I guess they suffer
from temperature fading. Would this make it impratical for a shortwave
or AM receiver? Can you even make a regenerative radio out of a
varactor? (Actually, I'll try ghetto rigging a red LED for a varactor,
since I don't really care about it being nicely linear, I just want
something with a knob that does something!).

Can anyone recommend any type of reasonably simple radio (simple mostly
as in reasonably low parts count, i don't need an LCD readout) that
could possibly be done with parts that can be had at a reasonably
stocked electronics store. (Much more inventory than Fry's). A knob is
important, I don't want to build one of those ghetto radios with the
coil and a metal bead that I slide along the coil to 'tune' it. Any
other way to use a potentiometer to tune a radio?

If you want to build a radio circuit without much circuitry you really
have to put up with either a variable cap or a variable coil. I'd say
you either have to scrounge something or buy one -- they're really not
that spendy.

Yes, you can go the varactor route, and folks have made it work well.
But you have to add more components, it'll never be the same, and you'll
have to tinker with it to keep it from drifting all over the place. For
a 1st-time radio it may not be a bad thing, but after that I'd go with a
real variable element.

As far as your complaints about 'ghetto' radios -- your ability to build
a high-quality, good looking tunable element is a matter of the talent
that you were born with and the effort you take to use it. If you want
to build something without many components then you're going to need to
work at it anyway.

Having said that I wouldn't build -- I'd buy. Have you checked with
Cascade Surplus Electronics in North Portland? It's been ages since
I've been in there, so I can't guarantee anything. You may also want to
try R 5-D 3 Electronic Surplus out in Southeast. He's got lots of old
stuff there, if he doesn't have a variable cap he'll have some piece of
crud that you can buy and harvest the caps.

After those two, consider going to yard and garage sales, buying old
radios, and tearing out their capacitors. Stereo tuners from the 70's
and 80's are best -- if it's big and has a knob, chances are it has a
cap. If it receives FM and it's big and has a knob , chances are the
cap will be a good size for shortwave. You may even find that it has a
cap with a built-in reduction drive which is cool if you want to tune in CW.

Good luck.

--

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

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

"Tim Wescott" wrote in message
...

Yes, you can go the varactor route, and folks have made it work well. But
you have to add more components, it'll never be the same, and you'll have
to tinker with it to keep it from drifting all over the place.

Varactors are not that tough, and don't mistake the "never be the same".
Good varactor rigs are much more pleasant than anything with a variable cap.
(Although there are plenty of bad varactor designs out there!) Yes,
sometimes there are a couple more components, but not many more. It does
take some work to get a varactor oscillator close to drift-free, but five
minutes with a calculator will get you close enough for SWLing. Oh by the
way, getting a capacitor oscillator close to drift free can be a challenge,
especially if you don't have hard-to-get air inductors, and microphonics in
that case can be a challenge, too. The larger size of the variable cap/air
inductor makes mechanical construction details a lot more important. There's
really no free lunch.

Varactors do have some temperature coefficient, and they are often coupled
with toroid inductors, which also have some considerable temperature
coefficient. Most of the designs you see out there are for CW rigs in the
ham bands, where temperature stability is extremely important. The maze of
capacitors around the varactor are there to balance the temperature
coefficients. Usually there is a polystyrene capacitor which has a
temperature coefficient opposite to the toroid and varactor, but you can
never get exactly the right value for that, so it is a question of getting
the right combination of positive and negative temperature coefficients AND
the right value of capacitance. For AM in the broadcast band, you can
probably come close enough with one or two capacitors. For a rig with a 200
Hz CW filter at 15 meters, it can be a real bear keeping the frequency to
within the 0.0001% that you need for comfortable operation.

That being said, it is undoubtedly easier to use a variable cap, and caps
can be cannabalized from old radios easily and cheaply. Leon also mentions
the possibility of making a variable inductor. Actually, from time to time
articles have appeared about making variable caps, too. In either case,
though, you will need considerable mechanical skills.

One thing that makes projects hard, especially your first projects, is this
idea that you need exactly the right part. With a little help from your
trusty calculator, you can play pretty fast and loose with parts values in
most projects. In the case of a tuning cap, if you can find something on
the right planet it typically isn't too tough to make adjustments to
accomodate the different part. Study your circuit and understand what each
part is doing, especially around the tuning circuits. It is usually pretty
simple to make some adjustments, especially if, as you suggest, getting some
precise tuning range isn't critical.

...

xpyttl wrote:
. . .
Varactors do have some temperature coefficient, and they are often coupled
with toroid inductors, which also have some considerable temperature
coefficient. Most of the designs you see out there are for CW rigs in the
ham bands, where temperature stability is extremely important. The maze of
capacitors around the varactor are there to balance the temperature
coefficients. Usually there is a polystyrene capacitor which has a
temperature coefficient opposite to the toroid and varactor, but you can
never get exactly the right value for that, so it is a question of getting
the right combination of positive and negative temperature coefficients AND
the right value of capacitance. For AM in the broadcast band, you can
probably come close enough with one or two capacitors. For a rig with a 200
Hz CW filter at 15 meters, it can be a real bear keeping the frequency to
within the 0.0001% that you need for comfortable operation.
. . .

All toroid inductors aren't equal, and neither are capacitors.

I routinely build VFOs with no temperature compensation which have about
200 Hz total warmup drift on 40 meters. The trick is to use components
which have inherently low temperature coefficients rather than try to
make ones with high coefficients compensate each other. Polystyrene
capacitors have a fairly high temperature coefficient, but it's in the
opposite direction than a typical poor inductor. Sometimes people get
lucky and the combination works ok, but often they don't and it doesn't.

The other thing you have to do is design your oscillator so that its
frequency depends almost solely on the tank components and not the
active device.

I found that good quality NPO ceramic capacitors have the lowest
temperature coefficient of any commonly available parts, and inductors
wound on type 6 powdered iron cores were the best. It's the inductor
which dominates the drift in my VFOs, and that small amount can easily
be compensated if desired by replacing part of the tank C with a
capacitor with controlled temperature coefficient.

I described these techniques (except for compensation) in more detail in
"An Optimized QRP Transceiver", in August 1980 QST.

Roy Lewallen, W7EL

BTW, real air trimmers seem to be far better for temperature stability
than normal plastic dielectric trimmers! In an AM broadcast
transmitter using a VXO, a plastic trimmer in series with one of two
crystals(mixer setup) gave instability and a deep hetrodyne drone that
got worse over several days(unattended remote pirate rig) even though
the outside temperature was the same.

Replacing it with an air variable trimmer whose plates were apparently
cut from two solid pieces of brass mude for utter stability, with no
sign of drift over at least a 20 degree temperature change without
having to reset it.

For noncritical applications, those little plastic variable caps from
car stereos are pretty good. Have not used one in an ultra-critical
application in a role giving a lot of control over frequency, but when
I used one in the 2004 rig to shift a crystal maybe 2KHZ out of 16MHZ,
it didn't seem to drift.

Of course, a VXO(pulled crystal) is a hell of a lot more stable than
any VFO! A VXO and a similar but not the same frequency fixed crystal
can give a suprisng tuning range with stability. Put in a crystal oven
or even a heated/air conditioned room it would leave little to be
desired in stability.

For a VFO for any application, the better your parts, the better
your results. Wind coil on "air" or unity permeability cores such as
wood or ceramic, and epoxy the windings in place. Blow on an
oscillator's coil while listening to the beat note with and without the
epoxy, and hear the difference for yourself. Wood cores seem to work
fine with the epoxy covering.

With an air-core coil,a good tuning capacitor, and a circut that
minimizes active device contribution to drift, you end up with a lot
less chasing drift to do. Best active device for any VFO and probably
any VXO as well is a JFET. Almost no heat(unlike a tube), and no
junctions in the current path to change characteristics with
temperature(unlike a bipolar). If you must use a powdered iron core,
keep DC out of the windings as changes in the DC current change the
permeability opf any ferrite or powered-iron core. Ferrite cores of any
type have been named as an especially bad source of drift in
oscillators, so don't use modified IF transfomers as tuners in
oscillators expected to be stable. They are fine in tuned small-signal
amps, just not in oscillators.

In that VXO with the bad trimmer, I got lucky and found the bad part
first try, but this is unusual. If you have to track down drift expect
hours of work. That's why it takes less tiem to use the good stuff from
the start, unless it takes hours of driving or biking to obtain it, of
course.

SparkySKO wrote:
Hello All,

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]

I'm interested in getting my feet wet in building a receiver, but don't
want to buy a 'kit'. I'd like to build a radio that has a knob I can
somewhat tune it with. I've been really looking at regenerative radios
and superheterodynes, although a simple radio is more important than
how well it really performs.

One common theme in almost every 'simple' radio design I can find, is
that they *ALL* use Air variable capacitors. While I'm sure these were
everywhere 20 years ago, going to every major electronics supply house
in Portland, Oregon has yielded me only with small trimmer capacitors,
which AFAIK, will not do for a 'tuner'...

I'd try R5D3 first, then see if I could find something worth
cannibalizing at Cascade (note new address) or Wacky
Willy's (lots of junk, but you never know...). Then I'd hit the
various thrift stores and see if I could find an old FM tuner.

Are there any ham radio swap meets coming up?

If you go to R5D3, they really are on a residential side street
out in the middle of nowhere.

The parts are out there. Go get them!

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89mg pied a terre..."
ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte

laura halliday wrote:
. . .
I'd try R5D3 first, then see if I could find something worth
cannibalizing at Cascade (note new address) or Wacky
Willy's (lots of junk, but you never know...). Then I'd hit the
various thrift stores and see if I could find an old FM tuner.

At Sea-Pac this year I found out that Wacky Willy's has also moved from
its long time location across from the Reedville Cafe.

Are there any ham radio swap meets coming up?

There's the Salem club swap meet at Rickreall in February, and Sea-Pac
at Seaside in June, but no other major local ones I know of. I'm sure
that a query on a local repeater would bring full details of any others.

. . .

Roy Lewallen, W7EL

"SparkySKO" wrote in message
oups.com...
Hello All,

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]

Go he

http://www.tubesandmore.com/

and check under Capacitors -- variable.

"SparkySKO" writes:

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]
....[snip]....

For TWO BUCKS I bought a whole box full of cheapie "transistor radios"
from a church rummage sale a few weeks ago (most were about the size
of a man's wallet, but a few were about the size of a thick book), and

ALL OF THEM HAD AN AIR-VARIABLE TUNING CAPACITOR!

Don't fight a battle you don't need to fight; just tear up an old
"transistor radio"!

(Now the NEWER transistor radios -- the kind you buy at WalMart for
a buck each that have a "push to tune to the next station" button --
do NOT have air-variable capacitors, but the OLDER ones do!)

--
--Myron A. Calhoun, W0PBV.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license

SparkySKO wrote:
Hello All,

[Basically this says, are there any practical alternatives to an air
variable capacitor that a beginner can use in building his first
receiver radio?]

I'm interested in getting my feet wet in building a receiver, but don't
want to buy a 'kit'. I'd like to build a radio that has a knob I can
somewhat tune it with. I've been really looking at regenerative radios
and superheterodynes, although a simple radio is more important than
how well it really performs.

One common theme in almost every 'simple' radio design I can find, is
that they *ALL* use Air variable capacitors. While I'm sure these were
everywhere 20 years ago, going to every major electronics supply house
in Portland, Oregon has yielded me only with small trimmer capacitors,
which AFAIK, will not do for a 'tuner'. I'm aware I can order online
for about 10-20$ and purchase an air variable, but I'd like to see if
any of you have any suggestions on what might work as an alternative?

Varactor diode seemed like a neat idea, although I guess they suffer
from temperature fading. Would this make it impratical for a shortwave
or AM receiver? Can you even make a regenerative radio out of a
varactor? (Actually, I'll try ghetto rigging a red LED for a varactor,
since I don't really care about it being nicely linear, I just want
something with a knob that does something!).

Can anyone recommend any type of reasonably simple radio (simple mostly
as in reasonably low parts count, i don't need an LCD readout) that
could possibly be done with parts that can be had at a reasonably
stocked electronics store. (Much more inventory than Fry's). A knob is
important, I don't want to build one of those ghetto radios with the
coil and a metal bead that I slide along the coil to 'tune' it. Any
other way to use a potentiometer to tune a radio?

Varying the inductance by moving a core in and out of the coil works
just as well, and is quite easy to make at home.

Leon