Simple questions on receivers
 

I had a couple of questions regarding recievers that I haven't been
able to figure out. I'd appreciate it if anyone could give me some
insight...

How well-defined is the gain for a cap-coil loop, like in an AM radio?
(i.e., how fast does the gain droppoff as you move up or down from the
'tuned' frequency?) Is it a function of L and C? Or just frequency?
(different combinations of L and C will tune to the same frequency, but
is the gain the same?)

How come the coils on many of the CR schematics I've seen have multiple
tap locations? It seems that with a variable cap, you should be able
to tune to whatever frequency that's in your range. Is it to increase
the range of your radio's coverage? Or because the gain at certain
frequencies is better with different C/L combinations?

Thanks in advance for any insight!
Dave

"dave.harper" wrote in message
ups.com...
How well-defined is the gain for a cap-coil loop, like in an AM radio?
(i.e., how fast does the gain droppoff as you move up or down from the
'tuned' frequency?) Is it a function of L and C? Or just frequency?

For the L-C resonator itself, the falloff will be 6dB per octave (doubling of
frequency) once you're well outside of the center (tuned) frequency. The 3dB
bandwidth will be determined by the Q of the circuit, and this is often not
particularly well defined during the design stage -- often a well-defined
bandpass filter somewhere "down the line" (e.g., at an IF stage) will
ultimately define what the radio receives.

The 6dB/octave drop comes from just looking at the impedance or transfer
function of an LC(R) resonator -- you'll end up with an s^2 (frequency
squared) term in the denominator of the equation.

(different combinations of L and C will tune to the same frequency, but
is the gain the same?)

No, although for low Q resonators, it's often pretty close.

How come the coils on many of the CR schematics I've seen have multiple
tap locations? It seems that with a variable cap, you should be able
to tune to whatever frequency that's in your range. Is it to increase
the range of your radio's coverage? Or because the gain at certain
frequencies is better with different C/L combinations?

They're usually trying to match the impedance of the LC circuit to the input
impedance of "the next stage" (i.e., a transistor amplifier) to maximize power
transfer. So, while it's not really "gain" (we haven't amplified anything
yet -- this is more like a resistive divider), the output will be higher with
different C/L combinations.

Generally speaking, most relatively simple AM receivers are really only
intended to pick up relatively strong, nearby transmitters. As such, you can
get away with an awful lot of "cut and try" when it comes to designing the
circuit (largely ignore matching impedances, Q's, etc.) and still obtain
acceptable results.

---Joel

Joel Kolstad wrote:

No, although for low Q resonators, it's often pretty close.

Generally speaking, most relatively simple AM receivers are really only
intended to pick up relatively strong, nearby transmitters. As such, you can
get away with an awful lot of "cut and try" when it comes to designing the
circuit (largely ignore matching impedances, Q's, etc.) and still obtain
acceptable results.

Thanks for the response Joel. So if I understand correctly, Q is
basically an indicator of how well the LC circuit resonates? Could you
think of Q as the inverse of a dampening coefficient?

If so, I guess ideally you'd get the best Q with an iron-core inductor,
thick windings, and as few windings as possible?

Dave

dave.harper wrote:

Joel Kolstad wrote:


No, although for low Q resonators, it's often pretty close.

Generally speaking, most relatively simple AM receivers are really only
intended to pick up relatively strong, nearby transmitters. As such, you can
get away with an awful lot of "cut and try" when it comes to designing the
circuit (largely ignore matching impedances, Q's, etc.) and still obtain
acceptable results.


Thanks for the response Joel. So if I understand correctly, Q is
basically an indicator of how well the LC circuit resonates? Could you
think of Q as the inverse of a dampening coefficient?

If so, I guess ideally you'd get the best Q with an iron-core inductor,
thick windings, and as few windings as possible?

Dave

If I may jump in....
The case of crystal receivers is somewhat different than a "powered"
radio in that you want to keep things at as high a Q as possible to
match the high impedance of the circuit. High Q is desirable in any
case but moreso in a crystal set.

After having established a good high Q with the LC configuration you can
count on the circuit/antenna loading it down somewhat so then it becomes
a matter of selecting appropriate diodes, decoupling the ant, etc. so it
always helps to start out with as much as possible.

Textbook Q of more C/less L is dictated simply by resistance. In
practice, at least for BCB xtal radios, the dielectric of the coil, skin
effects of the wire, interwinding capacitance are the key players.
Thats why certain coil dimensions, use of litz wire and certain winding
techniques can generally be counted on for the highest Q.

The Q of a coil, and/or complete circuit, will have a curve of its own.
With BCB, what is good at 600kc may be better at 800kc and
(relatively) terrible at 1600kc.

I've had good success with ferrite toroids approaching Q=400, although
ferrites are by nature very unpredictable Q-wise. This is as good as
one can expect with something like a 3-4" diameter coil of #18 wire on a
good coil form. OTOH, the toroid stops there. That same 4" coil wound
with 660-strand litz can get up into the Q=800 stratosphere with a
basket-wind technique.

There's always a downside. A big, hi-Q coil needs to be kept well away
from ANYTHING or else the Q will take a nosedive and then lead
capacitance starts biting you from the backside.

I find xtal sets fascinating. I've been radioing for 35-40 years and
never gained a full appreciation for L, C and Q until I got into DXing
with xtal sets. Logged 105 BCB stations in the competition earlier this
year including two in Brazil!

-Bill

From: "dave.harper" on Thurs 14 Jul 2005 18:55

Joel Kolstad wrote:

No, although for low Q resonators, it's often pretty close.

Generally speaking, most relatively simple AM receivers are really only
intended to pick up relatively strong, nearby transmitters. As such, you can
get away with an awful lot of "cut and try" when it comes to designing the
circuit (largely ignore matching impedances, Q's, etc.) and still obtain
acceptable results.

Thanks for the response Joel. So if I understand correctly, Q is
basically an indicator of how well the LC circuit resonates? Could you
think of Q as the inverse of a dampening coefficient?

In a way, you might think that. For going to more advanced things
besides "simple AM receivers," I'd suggest thinking of Q as a
built-in LOSS element.

For parallel-tuned circuits, the loss can be modeled as a resistor
in parallel with the L and C. This equivalent resistor value is
the reactance of either L or C (they are equal at resonance)
multiplied by Q. A high Q indicates least loss in a parallel
circuit, a high value of equivalent parallel resistance.

But, for series-tuned circuits, the loss is equal to a resistor
in series with L and C. That resistor value is equal to the
reactance of either L or C divided by Q. A high Q in a series-
tuned resonance would have the lesser value of series resistance.

If so, I guess ideally you'd get the best Q with an iron-core inductor,
thick windings, and as few windings as possible?

Yes and NO. Q will vary by MANY things. Generally, physically
big coils will have higher Q, physically big windings will have
higher Q. Shape factor, like length versus diameter of a
solenoidal winding has an optimum value. Nearby shielding will
tend to reduce Q; one reason why toroidal forms have higher Q
than solenoidal or cylindrical windings.

CORE MATERIAL IS FREQUENCY SENSITIVE! "Iron core" has to be
defined. Power transformer laminations are okay at up to
about 10 KHz and then become more lossy with increasing
frequency. Special iron (tape shape, usually) is used for
higher frequencies in the supersonic range. At LF and higher,
various kinds of iron POWDER are used to enhance Q (within
their specified frequency range).

Q applies to capacitors also...and is affected by things like
plate area, plate material, dielectric if other than air, and
(to some degree) physical shape factors. Generally, though,
the Q of most resonating capacitors is 10 to 100 times larger
than inductors and can usually be neglected in most
calculations of tuned circuits. Inductor Q rules! :-)

For self-education, I'd suggest spending some time with a good
Q Meter and trying out measurements on various kinds of
inductors. That will probably give you the best Q picture in
your mind.

From: dave.harper on Jul 12, 4:12 pm

I had a couple of questions regarding recievers that I haven't been
able to figure out. I'd appreciate it if anyone could give me some
insight...

How well-defined is the gain for a cap-coil loop, like in an AM radio?
(i.e., how fast does the gain droppoff as you move up or down from the
'tuned' frequency?) Is it a function of L and C? Or just frequency?
(different combinations of L and C will tune to the same frequency, but
is the gain the same?)

"Gain" of a crystal radio depends on the bigness of the antenna.

If you are talking about a loop antenna on an AM [BC band]
radio, then it's a different story. The loop antenna on an
AM receiver is small/tiny/micro-stuff relative to the 200+
meters of AM BC wavelengths. The received signal VOLTAGE
is directly dependent on the number of turns in that loop and
the physical size of the loop.

A loop antenna is into what some folks call a "magetic antenna";
i.e., very small relative to wavelength, therefore it intercepts
only the magnetic part of the electro-magnetic wavefront radiated
by a transmitter. The more turns in that loop, the greater the
voltage induced in the loop.

A humungous-long wire is going to supply the greatest amount of
POWER to a crystal receiver. POWER drives the headphones. But,
the amount of power coupled in involves IMPEDANCE and that, right
away, gets into a complicated mess of more electrical rules.

Simple crystal receivers want to keep impedances very high at
both input, middle, and output. ["crystal" or piezo-electric
headphones are the best for that, next best is the highest
impedance magnetic headphones (2000 Ohms or higher) you can get]

For the typical parallel-tuned L-C input to a crystal set, the
inductor Q will make a difference. It must be as high as is
practical; Qs of 200 to 300 have been done. But, the Q of the
coil is dependent on a LOT of different factors which I noted
in the other message.


How come the coils on many of the CR schematics I've seen have multiple
tap locations? It seems that with a variable cap, you should be able
to tune to whatever frequency that's in your range.

Mostly, that is just old-time tradition! :-) [I kid you not]

The formula for resonance is: F^2 = 1 / (39.478 * L * C)

With F being frequency in Hz, L in Hy, C in Fd.

To check this out, a 2.5 mHy inductor and 1000 pFd capacitor
will be resonant very close to 100 KHz.

The maximum to minimum variable capacitance ratio is equal to
the square of the maximum to minimum frequency tuning ratio
desired. That's about IT.

"Taps" on a coil can be to select different inductance values
for resonance with limited-range variable tuning capacitors.
Note: Back in the prehistory of radio, like around the 1920s,
variable capacitors were expensive and not so easy to get. A
few old-time crystal sets "tuned" via lots of coil taps using
a fixed parallel capacitor. I had a Philmore crystal radio kit
back in 1946 that did that. Very cheap kit. It worked, so-so.

Presupposing a loop antenna that is resonated by a variable
capacitor, its "gain" is going to be greatly influenced by
its Q or Quality factor. The higher the Q, the greater the
voltage into the headphones. However, the Q may NOT be the
same over the approximate 3:1 frequency span of the AM BC
band. [again, too many variables as noted in other message]

The Q of that L-C circuit is going to be "spoiled" by the
impedance/resistance of the headphones. Those headphones are
in parallel with the parallel-tuned L-C circuit. The higher
the impedance/resistance of the headphones, the least effect
it will have on the Q of the L-C resonant circuit.

Somehow my browser failed to pick up your initial message so
this is a reverse-order answer. Sorry about that.

dave.harper wrote:

Thanks for the response Joel. So if I understand correctly, Q is
basically an indicator of how well the LC circuit resonates? Could you
think of Q as the inverse of a dampening coefficient?

Just about exactly. You'll find the term "damping factor" (sometimes
"damping ratio" or "damping coefficient") often used in many situations
involving network analysis and control systems, usually represented by
the lowercase Greek letter zeta. And it's numerically equal to 1/(2Q),
so Q is exactly 1/2 the inverse of the damping factor.

When the damping factor is 1 (Q = 0.5), a second order circuit is said
to be critically damped.

Roy Lewallen, W7EL

-ex- wrote:
Q in excess of 1000 is readily achievable. 200-300 is a starting point
on a decent dx set.

So what's a good inductance to DC resistance ratio for an inductor on
an xtal set? The one I wound is about 500uH, and I get a resistance of
3.2 Ohms.

Using the formula Q=2*pi*f*L/R, I get a Q for my coil of 981 (@1MHz).
But based on its performance, I KNOW it's not that good. I'm picking
up a couple stations at night, but just barely.

Dave

dave.harper wrote:


-ex- wrote:

Q in excess of 1000 is readily achievable. 200-300 is a starting point
on a decent dx set.


So what's a good inductance to DC resistance ratio for an inductor on
an xtal set? The one I wound is about 500uH, and I get a resistance of
3.2 Ohms.

Using the formula Q=2*pi*f*L/R, I get a Q for my coil of 981 (@1MHz).
But based on its performance, I KNOW it's not that good. I'm picking
up a couple stations at night, but just barely.

Dave

For BCB work the 'standard' is in the 220-240uh range for tuning with a
~365-400 pf cap. There's an (almost) infinite number of combinations
you can use if you want to split the band into segments which sometimes
has an advantage. But switches and tapped coils can also be Q-killers
once you get into the Q stratosphere.

In practice the coil Q is determined primarily by the form dielectric,
wire size, wire spacing, diameter/length ratio/neary coupling effects,
etc. R is far enough down the list that its generally not even
considered. When you do a DC measurement of coil R thats not
representative of the skin effects and true RF resistance, thats why the
textbook formula doesn't pan out.

If you want to make a fairly nice coil without getting into the expense
of litz, check out spider-web coils and rook coils. When done with say
16-18 ga wire, and diameters in the 4" range you can get a pretty nice
coil. With 166-strand litz (30-35c/ft) you'll note an improvement but
by that time its time to start thinking about a good hi-q ceramic
capacitor and circuit loading concerns.

The Rap-n-Tap forum is where to get some good info.
http://www.midnightscience.com/rapntap/ "Best coil" is a common topic!

-Bill

From: dave.harper on Jul 17, 6:42 pm


-ex- wrote:
Q in excess of 1000 is readily achievable. 200-300 is a starting point
on a decent dx set.

So what's a good inductance to DC resistance ratio for an inductor on
an xtal set? The one I wound is about 500uH, and I get a resistance of
3.2 Ohms.

Using the formula Q=2*pi*f*L/R, I get a Q for my coil of 981 (@1MHz).
But based on its performance, I KNOW it's not that good. I'm picking
up a couple stations at night, but just barely.

The "R" in the Q formula is an equivalent resistance at frequency,
not just the DC resistance. That equivalent resistance is made
up of many things: winding form factor, wire size, and the DC
resistance to name the major factors.

Q alone won't determine sensitivity. Sensitivity, without some
accurate numbers such as transmitter power output, distance to
transmitter, antenna gain/loss, is going to be a very subjective
item. Even with them available the numbers can turn out to be
rather off when listened to.

A couple of years ago now, I wound a loop for 60 KHz (WWVB
reception) using #14 electrical wire. It was rather cheap
at Home Depot compared to enameled "magnet" wire for a 500
foot length. Inductance came out roughly according to formula
but the low DC resistance didn't do much for the Q. At 60 KHz
the Q was only about 68. :-) Dimensions were about 2 1/2 feet
diameter, circular, with an aluminum foil electrostatic shield
over the top of 57 turns. In retrospect I should have used many
more turns of smaller wire, such as #26 AWG, since signal
strength is proportional to the number of turns for the same
size loop.

It could have been the insulation on the electrical wire that
reduced the Q. Unknown. Would have to wind a similar one in
"magnet" wire to find out. It was measured for Q and inductance
without and with the foil electrostatic shield with no discernable
changes in Q, only slight in inductance. As it is, it works well
enough, is presently in the attic above the interior workshop.
[size dictated by trap door access to that part of attic]

Years and years ago I fooled around trying to make an AM BC
loop according to "expert instructions" from some magazine.
Spent a lot of time cutting the "blades" of the former to allow
zig-zag winding of some Litz wire someone gave me. Former was
3/32" phenolic laminate, cutting via a jig-saw. About 14 inches
wide by 6 inches high. Q measured out to only about 120 at mid-
band (using an old Booton Q Meter). Low enough distributed
capacity but not near the Q claimed in the article, supposedly
about 300. shrug Maybe ordinary cardboard would have worked
better as the former? :-)

If you have some RF source of known frequency at the AM BC band,
you can get a fair handle on the Q by using a high series
resistor between RF source and the L-C parallel-tuned circuit.
Observe the voltage across the L-C tank and de-tune the RF
source frequency to the 71% amplitude, note the two frequencies
on each side of resonance and take their difference. That's
the delta-F "Q bandwidth" that, when divided into peak resonance
frequency, will get you the approximate Q. The high resistance
source-to-tank should be around 100 KOhms or so (higher the
better) at 1 MHz to avoid introducing too much error. That
resistor forms a "quasi-constant-current" stimulus...not ideal
but good enough for an approximation when observing the RF
voltage across the L-C tank.

"Ours is not to reason why, ours is but to cut and try..." :-)

-ex- wrote:

I've had good success with ferrite toroids approaching Q=400, although
ferrites are by nature very unpredictable Q-wise.

Is this the reason a lot of coils are air coils? Easy of calculations?
I assume you can get higher performance from ferrite coils than
air-core coils, right?

Thanks again!
Dave

On Mon, 18 Jul 2005, dave.harper wrote:

Date: 18 Jul 2005 14:18:03 -0700
From: dave.harper
Newsgroups: rec.radio.amateur.homebrew, rec.radio.amateur.misc
Subject: Simple questions on receivers

-ex- wrote:

I've had good success with ferrite toroids approaching Q=400, although
ferrites are by nature very unpredictable Q-wise.

Is this the reason a lot of coils are air coils?

1. Air core coils will be cheaper, lighter in weight, easier to make.

Easy of calculations?

2. Definitely.

I assume you can get higher performance from ferrite coils than
air-core coils, right?

You don't need as much ampere-turns to get a given amount of inductance
and thus, ohmic resistance will be less, therefore higher Q (in theory).

caveat: the magnetizable material you use for the core (i.e. iron,
ferrite, and other stuff that I think other guys here surely know better
than I) will have a big effect on useable frequency on up to some cut-off
threshold that may be sharp or spread out. Pure solid sheet iron, for
example, might be good at audio frequencies and maybe up to x00,000 Hertz,
but you need powdered iron to go into the megacycle range. There are other
core substances that get you up higher. Anyone else care to add to this?

Don't forget that winding a torroidal coil is not so easy. Some cores are
available in halves so you can make "pies", otherwise the "bobbin"
carrying the wire has to pass through the hole of the doughnut many times.

Thanks again!
Dave




Art, W4PON

dave.harper wrote:
-ex- wrote:


I've had good success with ferrite toroids approaching Q=400, although
ferrites are by nature very unpredictable Q-wise.


Is this the reason a lot of coils are air coils? Easy of calculations?
I assume you can get higher performance from ferrite coils than
air-core coils, right?

Thanks again!
Dave

Most crystal radio builders go with air core coils...partly because
thats the way it has been always done and such plans are available and
thats the way its supposed to 'look' :)
And you CAN make a better Q air coil than what is attainable with
ferrite.

When I say unpredictable about the toroids, the calculated
turns/inductance comes out the same but not the Q. I've wound the same
coil on half-dozen 'exact same' ferrite cores and gotten Q ranging from
below 200 to nearly 400. Those numbers (in my set) cover the range of
not-so-good to pretty-darn-good. I'm not sure why that is other than
its not something intended to be. You won't readily find Q charts for
ferrites like you do for iron powder cores. As a consequence of this
you don't have any guarantee that the cores you get are going to make it
to the p-d-g range.

There's some advantages and disadvantages with using ferrite cores. The
size is the most obvious advantage. Among other advantages - they are
not affected by nearby components and do not pickup signals from the air
(self-shielding). Thats why I got started with them...I have a 5kw BCB
tower about 1600 feet from my QTH! With air core coils the band is
totally swamped.

On the negative side...the ultimate Q-limitation seems to be about 400,
inability to have variable loose-coupling to traps and other stages are
most notable. They also don't have the "looks cool" factor like a big
coil :)

Now back down to earth. A random ferrite core inductor is going have a
MUCH higher Q than the average coil wrapped around a toilet tissue tube.
You're lucky to get 80-100 with that type of coil. It may not play out
as being important in a very simple circuit that has other shortcomings
but as mentioned before it also helps to have a high starting point on
as many of the components as possible.

Anyhow...its something else to tinker with!

-Bill

My comments are interspersed.
-Bill

straydog wrote:



On Mon, 18 Jul 2005, dave.harper wrote:


Is this the reason a lot of coils are air coils?


1. Air core coils will be cheaper, lighter in weight, easier to make.

A typical medium-sized ferrite toroid coil for BCB use, FT-82-61 for
instance, costs about US$1 and can't weigh more than an ounce and uses
up about 4-5 feet of wire.


Easy of calculations?


2. Definitely.

Different calculation but one is as easy as the other. Just look for an
online calculator :)

iron, for example, might be good at audio frequencies and maybe up to
x00,000 Hertz, but you need powdered iron to go into the megacycle
range. There are other core substances that get you up higher. Anyone
else care to add to this?

True. There are two main substances used in ferrite toroids - and I
can't quote either name - and they have vastly different permeability
characteristics. I think the CWS-Bytemark website goes into some of
these details.


Don't forget that winding a torroidal coil is not so easy. Some cores
are available in halves so you can make "pies", otherwise the "bobbin"
carrying the wire has to pass through the hole of the doughnut many times.

I've been referring to the simple doughnut cores. A BCB coil takes
around 50 turns on -61 material. Even with my fat fingers it only takes
about 10 minutes.

-Bill

-ex- wrote:

In practice the coil Q is determined primarily by the form dielectric,
wire size, wire spacing, diameter/length ratio/neary coupling effects,
etc. R is far enough down the list that its generally not even
considered.

Ah, so the voltage drop across the coil (due to the small internal
resistance) and the close proximity of the wires give it some
capacitance? Does this affect performance or just screw add unwanted
capacitance?

Wouldn't adding space between wires cause some eddy currents and lower
the L of the coil?

Does wire coating make a difference regarding the dielectric? Or is it
another capacitance-altering effect?

Thanks again!
Dave

Comments interspersed, and staying with the BCB range scenario...


dave.harper wrote:

-ex- wrote:


In practice the coil Q is determined primarily by the form dielectric,
wire size, wire spacing, diameter/length ratio/neary coupling effects,
etc. R is far enough down the list that its generally not even
considered.


Ah, so the voltage drop across the coil (due to the small internal
resistance) and the close proximity of the wires give it some
capacitance? Does this affect performance or just screw add unwanted
capacitance?

The internal capacitance of the turns isn't enough to radically change
the basic LC resonance. Instead it tends to result more like dielectric
leakage

Wouldn't adding space between wires cause some eddy currents and lower
the L of the coil?

Again, not significantly in the BCB example. Take for instance, a 4"
diameter coil wound with #18 wire, however many turns it takes. Lets
say 60. Winding the coil close-spaced as opposed to about
one-wire-diameter spacing will require a few less turns (maybe 10%) to
get the same L. But the close spacing WILL result in lower Q once you
re-establish the same inductance. There can be more than one reason for
this...is it the winding spacing or the length/diameter ratio or more
dielectric loss that causes this? (Its certainly not the R). Its
impossible to say because you can't have one without the other!

Smaller coils, say toilet-paper tube size, don't exhibit this effect -
or at least not to the same degree. But there's a whole different
geometry there and its not optimum.

Nobody really knows exactly what goes on here other than trial-and-error
experiments to see how they behave.


Does wire coating make a difference regarding the dielectric? Or is it
another capacitance-altering effect?

Yes it does. One of the tests on a good high Q coil is to set it up on
a Q-meter then touch a piece of your coil-form material (or
wire-insulation) to the coil and see how it behaves on the Q-meter. It
shouldn't move. Lossy core material/insulation will cause a visible
effect with this test. I'll reiterate in case someone jumps in and
reads this without reading the earlier parts of the thread...you won't
see this happen with a low-q coil but as you get higher in Q it becomes
more and more evident. In fact, with a big solenoid coil and Q500 you
pretty much have to tie the sample material onto the end of a stick to
do this test because of hand effects. Not to be confused with resonance
detuning effects.

There's no good rule of thumb for insulated wire other than a test like
this. There's quite a bit of insulation material in 660-strand litz and
thats darn good wire. No way to make a comparison because BARE litz
can't exist! I don't think I've ever heard a comparison made between
say bare 16-18 wire vs enamelled. I tend to think any difference would
approach the 'too difficult to evaluate' range.

-Bill

-ex- wrote:
Comments interspersed, and staying with the BCB range scenario...


dave.harper wrote:

-ex- wrote:


In practice the coil Q is determined primarily by the form dielectric,
wire size, wire spacing, diameter/length ratio/neary coupling effects,
etc. R is far enough down the list that its generally not even
considered.


Ah, so the voltage drop across the coil (due to the small internal
resistance) and the close proximity of the wires give it some
capacitance? Does this affect performance or just screw add unwanted
capacitance?

The internal capacitance of the turns isn't enough to radically change
the basic LC resonance. Instead it tends to result more like dielectric
leakage

Wouldn't adding space between wires cause some eddy currents and lower
the L of the coil?

Again, not significantly in the BCB example. Take for instance, a 4"
diameter coil wound with #18 wire, however many turns it takes. Lets
say 60. Winding the coil close-spaced as opposed to about
one-wire-diameter spacing will require a few less turns (maybe 10%) to
get the same L. But the close spacing WILL result in lower Q once you
re-establish the same inductance. There can be more than one reason for
this...is it the winding spacing or the length/diameter ratio or more
dielectric loss that causes this? (Its certainly not the R). Its
impossible to say because you can't have one without the other!

Smaller coils, say toilet-paper tube size, don't exhibit this effect -
or at least not to the same degree. But there's a whole different
geometry there and its not optimum.

Nobody really knows exactly what goes on here other than trial-and-error
experiments to see how they behave.

Thanks again for the reply.

From what I've read here and elsewhere, I'm debating either making a
spider coil or a tight wound 4" cylinder ( about 1" length). Is there
any significant advantage to either?

I can see with a powered ferrite core how the spacing would make less
of a difference... but if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?

Thanks!
Dave

Huh? You wrote, "if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?" Do you not want a higher Q? Generally, people try for the
highest unloaded Q they can get, under some set of constraints.

Close spacing lowers the Q mainly because the current in the wire is no
longer radially symmetrical, if you look at a cross-section of the
(round) wire. That raises the RF resistance of the wire. For decent
(low-loss) form material, it's mainly the RF resistance of the wire
that determines the loss and therefore the Q. Generally, highest Q for
a given diameter and length is obtained by spacing the wire about two
wire diameters, center to center, at least for high frequency work. If
you want to use Litz wire, there's an optimum stranding...more, finer
strands are not necessarily better as you get to either lower or higher
frequencies. You should be able to find info on that, if you do some
searching.

There is such a thing as TOO HIGH a loaded Q. Let's say you start off
with a coil with unloaded Q of 500, and couple lightly to it with your
circuit (antenna and detector), so the loaded Q is 250. That means the
bandwidth at 3dB points, if you tune a station at 1MHz, is 4000Hz. If
you've tuned to the center of the station, your demodulated bandwidth
will be only 2kHz. Since the rolloff is gradual with a single-tuned
circuit, voice should be OK, but you'll be missing out on a lot of the
highs. (Mind you, it's not easy at all to get an unloaded Q of 500 at
1MHz!)

Using a very hack crystal radio--coil of about 3" diameter, antenna
just ten feet or so of wire, and an HP zero-bias schottky detector
diode--but into a low-noise audio amplifier--I've been able to listen
to standard broadcast stations a thousand miles away in the evening.
Biggest problem is getting rid of local stations...I'd use probably a
carefully designed three-resonator filter and a much better wire
antenna if I was serious about it.

Cheers,
Tom

K7ITM wrote:
Huh? You wrote, "if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?" Do you not want a higher Q? Generally, people try for the
highest unloaded Q they can get, under some set of constraints.

Sorry, I made a typo. Rather, why do people tight-wrap coils, period?
Just ease of construction?

Close spacing lowers the Q mainly because the current in the wire is no
longer radially symmetrical, if you look at a cross-section of the
(round) wire. That raises the RF resistance of the wire. For decent
(low-loss) form material, it's mainly the RF resistance of the wire
that determines the loss and therefore the Q. Generally, highest Q for
a given diameter and length is obtained by spacing the wire about two
wire diameters, center to center, at least for high frequency work. If
you want to use Litz wire, there's an optimum stranding...more, finer
strands are not necessarily better as you get to either lower or higher
frequencies. You should be able to find info on that, if you do some
searching.
SNIP
Cheers,
Tom

Thanks for the information! Is there an advantage to wrapping cylinder
coils as opposed to spider or torroid, other than ease of construction?
I'm debating which one I'd likely get the best result with... I'll
probably make both to try it out, but I'd like to know which one would
'probably' work best.

Thanks again,
Dave

dave.harper wrote:
-ex- wrote:



From what I've read here and elsewhere, I'm debating either making a
spider coil or a tight wound 4" cylinder ( about 1" length). Is there
any significant advantage to either?

They're going to be very similar in performance. If you do the solenoid
(cylinder), go with one wire spacing between turns instead of tight
wound. Thats been pretty well proven to give a bit better Q. And 16-18
ga wire is also in the 'best' range for both the spider-web and
solenoid. Next step up would be some serious litz wire.


I can see with a powered ferrite core how the spacing would make less
of a difference... but if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?

I'm not 100% sure I understand the question....On a
rook/basketweave/spider coil there's inherent spacing already. And a
solenoid coil will also do better that way...on larger coils. The way I
understand it is that there are several factors at work - primarily
interwinding capacitance and overall l/d ratio. In practice, as you
change one you also change the other. You can compensate one for the
other somewhat with a different diameter coil, different gauge wire, etc
but the general concensus is that 4"/16-18 wire/~1 wire diameter spacing
is pretty close to the best you can squeeze out of that class of coil.

HTH.

-Bill

I s'pose Reg, the local expert on proximity effect, etc., should pop in
here and 'splain it all. Seems, though, like it's wrapped up in
practicalities. For low-frequency work you typically want a lot of
inductance, so you use fine wire so you can get a lot of turns in a
relatively small volume. The wire diameter is small enough that, at
low frequencies, the skin depth is large compared with the wire size.
I believe you will then find that the proximity effect won't have as
much influence on the Q as in the case where the skin depth is a small
fraction of the wire diameter. So for a 50Hz/60Hz power transformer,
you won't find the turns spaced apart any more than needed for
insulation.

You can do a Google search for conductor proximity effect and find a
bunch of references. The stuff at
http://www.national.com/nationaledge...c_article.html has some
nice pix to show the effect in a bit different environment than we're
talking about here.

The Q you actually obtain may depend on so many other things than just
the shape of the windings that it's not possible to tell you the "best"
geometry. But I can tell you that if you make a large coil of good
design, you should be able to get to a high enough unloaded Q that
doing better with a different geometry about the same size will give
you only small returns on the performance in the circuit. That is, if
you do manage to make a solenoid coil say 5 inches long and 5 inches
diameter, maybe getting the Q up near 500 if you're careful, then
operating it at a loaded Q of 100 (for a 10kHz bandwidth at 1MHz), the
loss in the coil compared with an INFINITE unloaded Q is so small as to
be nearly unnoticable. If my mental arithmetic is right, it would be
about a 1dB difference, just barely audible. And of course, you won't
get anything like that much improvement in Q with a different shape.
Plus--the standard solenoid shape is easy to construct! (There ARE
reasons for wanting higher unloaded Q, if you want to operate at a
higher loaded Q and if you want to build a multiple-resonator tuner,
but my impression is you are not there yet!)

Cheers,
Tom

dave.harper wrote:


K7ITM wrote:

Huh? You wrote, "if tight winding results in a lower Q/other
effects, why space the windings for air-core, crystal radio coils,
period?" Do you not want a higher Q? Generally, people try for the
highest unloaded Q they can get, under some set of constraints.


Sorry, I made a typo. Rather, why do people tight-wrap coils, period?
Just ease of construction?


Close spacing lowers the Q mainly because the current in the wire is no
longer radially symmetrical, if you look at a cross-section of the
(round) wire. That raises the RF resistance of the wire. For decent
(low-loss) form material, it's mainly the RF resistance of the wire
that determines the loss and therefore the Q. Generally, highest Q for
a given diameter and length is obtained by spacing the wire about two
wire diameters, center to center, at least for high frequency work. If
you want to use Litz wire, there's an optimum stranding...more, finer
strands are not necessarily better as you get to either lower or higher
frequencies. You should be able to find info on that, if you do some
searching.

SNIP

Cheers,
Tom


Thanks for the information! Is there an advantage to wrapping cylinder
coils as opposed to spider or torroid, other than ease of construction?
I'm debating which one I'd likely get the best result with... I'll
probably make both to try it out, but I'd like to know which one would
'probably' work best.

Thanks again,
Dave


Me again...I should have read the later threads before my earlier reply.

Tom is correct about the Q using litz. Some guys have tried 48 ga litz
and said it nosedived in performance from the more-common 46 ga litz.
I've seen that explained with a critique of skin depth in that the rf
resistance of 48 is considerably higher at those freqs. Strand count
seems to still be in the 'more-is-better' range at BCB. 660-strand is
commonly used in DX sets...although I haven't graduated to that level of
expenditure myself :)

As to which to try....in a single-tuned set you won't notice the
difference. If you have a strong local BCB station the toroid will do a
very effective job of decreasing direct pickup by the coil. You might
still want a trap inline, though. Guess which type of coil makes the
best trap in this scenario!

My own tests gave a slight nod to the spider web coil. Not enough to be
noticeable in reception but enough for "spec-talk". The spider-web is
also less prone to proximity effects and even direct pickup because it
is 'directional'.

My own dx set which is admittedly tailored for my particular environment
uses a toroid on the first tuned stage, a toroid inline trap, then a
spiderweb on the detector stage with a loose coupled trap made with a
loopstick ferrite. A little of each, huh? I can receive stations
within 80-100 kc of the 5kw local that is 1/4 mile away on 1370.

My best recommendation would be to seriously consider a double-tuned
set. Its a whole different world than a single-tuned one.

-Bill

K7ITM wrote:

There is such a thing as TOO HIGH a loaded Q. Let's say you start off
with a coil with unloaded Q of 500, and couple lightly to it with your
circuit (antenna and detector), so the loaded Q is 250. That means the
bandwidth at 3dB points, if you tune a station at 1MHz, is 4000Hz. If
you've tuned to the center of the station, your demodulated bandwidth
will be only 2kHz. Since the rolloff is gradual with a single-tuned
circuit, voice should be OK, but you'll be missing out on a lot of the
highs. (Mind you, it's not easy at all to get an unloaded Q of 500 at
1MHz!)

Ok, I'm back with a couple more questions, since y'all are so
informative... I know it's not a true xtal set if I add an amp, but
wouldn't the best way to minimize the drop between the unloaded and
loaded Q be to add an amp? With a transistor/op amp, it seems that you
could tailor the load imposed on the tank circuit so that it's minimal,
while it seems a headphone load would be pretty significant (depending
on the headphone)...?

Also, I've seen some schematics with transistor-based and Op amp-base
amplifiers. Generally speaking, are there advantages/disadvantages to
either transistors or op-amps as the first-stage RF amplifier?

Thanks again for all the information!
Dave

dave.harper wrote:

K7ITM wrote:


There is such a thing as TOO HIGH a loaded Q. Let's say you start off
with a coil with unloaded Q of 500, and couple lightly to it with your
circuit (antenna and detector), so the loaded Q is 250. That means the
bandwidth at 3dB points, if you tune a station at 1MHz, is 4000Hz. If
you've tuned to the center of the station, your demodulated bandwidth
will be only 2kHz. Since the rolloff is gradual with a single-tuned
circuit, voice should be OK, but you'll be missing out on a lot of the
highs. (Mind you, it's not easy at all to get an unloaded Q of 500 at
1MHz!)


Ok, I'm back with a couple more questions, since y'all are so
informative... I know it's not a true xtal set if I add an amp, but
wouldn't the best way to minimize the drop between the unloaded and
loaded Q be to add an amp? With a transistor/op amp, it seems that you
could tailor the load imposed on the tank circuit so that it's minimal,
while it seems a headphone load would be pretty significant (depending
on the headphone)...?

In my set I simply use an audio matching xfmr. I'm using sound-powered
phones which are in the 200 ohm impedance range, and around 50 ohms DC.
A little xfmr like the Calrad 45-700 is a good choice. Some of the
guys use a switchable matchbox using a tapped xfmr like the Bogen. You
can see some of these at http://www.crystalradio.net/

As for an amp...mine plays great into my computer's sound card! I tried
a little one chip amp but it only had about 20db of gain which isn't
really enough to do a lot of good.

-Bill

Yes, by using an amplifier, you can lower the loading caused by the
detector. There is an optimum load impedance for the detector output
in terms of best output power for a given input signal, and for small
signals it's a pretty high impedance (resistance), because the diode's
dynamic impedance is quite high for very small signals. You can find
info on this at the Agilent web site (at least till Agilent sells off
their semiconductor business...). Look for ap notes and data sheets
covering zero-bias detector diodes. It gets a bit technical. But the
optimum load is, as a rule, rather high resistance. An FET-input
amplifier chosen for low input voltage noise is probably ideal.
HOWEVER, the crystal radio purists would probably complain that it's
not a crystal radio then. As Bill says, a matching transformer can
help you out a lot. I've done some work using zero-bias Schottky
detector diodes driving DC amplifiers to look for small signals, and
can detect signals down in the few tens of microvolts---but the output
is in the vicinity of a microvolt at picoamp currents.

The other thing that lowers the loaded Q of the coil is coupling to the
antenna. Remember, the antenna looks like some impedance. A resonant
antenna looks like a resistance, and an antenna coupler or tuner will
make a non-resonant antenna look resistive also. And that resistance,
coupled to the tank coil in your crystal radio, will lower the Q. If
you couple too lightly, you won't get all the signal you can, and if
you couple too heavily, you will lower the Q so much that you won't get
the desired selectivity. It's a balancing act. In fact, in a
multiple-resonator tuner, the bandpass shape is adjusted by changing
the coupling from one resonator to the next, which changes the loaded Q
of each resonator. When you have but one resonator, you just change
the bandwidth (and signal level) as you change the coupling and loaded
Q. Coupling that's too light mostly just changes the signal level,
with minimal change in bandwidth. Coupling that's too tight mostly
changes the bandwidth, with minimal change in signal level.

Hope these thoughts help some...

Cheers,
Tom

In theory, one could also use a synthetic inductance, aka 'gyrator". I
took a gyrator based audio oscillator that used 741's and on LTSpice
rebuilt it using 1000 Mhz GBW op-amps. Using an FFT of a transient
analysis I had a nice narrow adjustable center frequency peak of about
10 Mhz, But I never did have a chance to wire it up, as I have a baby
to take care of.

The Eternal Squire