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..." :-)