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..." :-)

wrote:

Q alone won't determine sensitivity.

Very true. In fact the opposite is usually the case when it comes to
function. In a good hi-q xtal set one often has to sacrifice
sensitivity for selectivity...and vice-versa. Tighter coupling=more
sensitivity but less selectivity because of loading effects. If you
want sensitivity, go outdoors and hang wire or add sound-powered
headphones. If you want selectivity, tweak the ckt.

But still, by aiming for highest Q you at least have something to work
with when trading off.

Crystal radios are somewhat like a house of mirrors. Changes at one end
reflect at the other end of the ckt. The old razor-blade radio is an
example of everything being done the worst way but it still works. As
you improve on the scheme each component becomes more and more critical.

-Bill

-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