Measuring Unloaded Q of Loop Antenna?

B

Bill Bowden

Guest
What is the best way to measure unloaded Q of a tuned LC circuit (AM
loop Antenna) without a Q meter?

I tried a scope and generator and loosly coupled the generator to the
loop, using a single turn of wire and 50 ohm resistor, and monitored
amplitude against frequency, but I get different results depending on
the setup. The scope probe is 10X. The frequency of interest is 500KHz
to 2 MHz.

How can I accurately measure the Q of the circuit without a Q meter?

-Bill
 
On Jul 12, 8:13 am, "john jardine" <john.jard...@idnet.co.uk> wrote:
"Bill Bowden" <wrongaddr...@att.net> wrote in message

news:f0885697-6c81-4c90-835e-5817c55e30ff@d77g2000hsb.googlegroups.com...

What is the best way to measure unloaded Q of a tuned LC circuit (AM
loop Antenna) without a Q meter?

I tried a scope and generator and loosly coupled the generator to the
loop, using a single turn of wire and 50 ohm resistor, and monitored
amplitude against frequency, but I get different results depending on
the setup. The scope probe is 10X. The frequency of interest is 500KHz
to 2 MHz.

How can I accurately measure the Q of the circuit without a Q meter?

-Bill

Yep, each measurement can turn out different. The pleasing upside is
acknowledging that the most accurate test setup was the one that returned
the highest Q value!.http://www.crystal-radio.eu/enqmeting.htm describes in great detail what
Mike has already noted. Also look at the series of "LC experiments".
Click to expand...
Yes, I'm getting a Q of around 100 at 1 MHz, but I think it's higher.
I used a 1 meg resistor in series with the scope probe and the half
power points are around 9KHz apart. Using a 100K resistor gives a much
lower Q, so it looks like a 100k load effects the circuit
significantly.

Thanks for the link.

-Bill
 
On Jul 13, 11:36 am, "amdx" <a...@knology.net> wrote:
"Bill Bowden" <wrongaddr...@att.net> wrote in message

news:0cc848c7-5b33-4790-951c-8cff1365720b@59g2000hsb.googlegroups.com...

On Jul 12, 8:13 am, "john jardine" <john.jard...@idnet.co.uk> wrote:
"Bill Bowden" <wrongaddr...@att.net> wrote in message

news:f0885697-6c81-4c90-835e-5817c55e30ff@d77g2000hsb.googlegroups.com...

What is the best way to measure unloaded Q of a tuned LC circuit (AM
loop Antenna) without a Q meter?

I tried a scope and generator and loosly coupled the generator to the
loop, using a single turn of wire and 50 ohm resistor, and monitored
amplitude against frequency, but I get different results depending on
the setup. The scope probe is 10X. The frequency of interest is 500KHz
to 2 MHz.

How can I accurately measure the Q of the circuit without a Q meter?

-Bill

Yep, each measurement can turn out different. The pleasing upside is
acknowledging that the most accurate test setup was the one that returned
the highest Q value!.http://www.crystal-radio.eu/enqmeting.htmdescribes
in great detail what
Mike has already noted. Also look at the series of "LC experiments".

Yes, I'm getting a Q of around 100 at 1 MHz, but I think it's higher.
I used a 1 meg resistor in series with the scope probe and the half
power points are around 9KHz apart. Using a 100K resistor gives a much
lower Q, so it looks like a 100k load effects the circuit
significantly.

Thanks for the link.

-Bill

Just a check, you are using half power point and not half voltage point,
right?
.707 vs. .5
Mike
Click to expand...
Yes, half power points, 0.707 voltage points. 0.707 squared is 0.5.

-Bill
 
On Jul 15, 1:53 pm, "john jardine" <john.jard...@idnet.co.uk> wrote:
"Bill Bowden" <wrongaddr...@att.net> wrote in message

news:cacfb357-b9f2-4235-a803-31f797ce8fd9@l42g2000hsc.googlegroups.com...

On Jul 13, 11:36 am, "amdx" <a...@knology.net> wrote:
"Bill Bowden" <wrongaddr...@att.net> wrote in message

news:0cc848c7-5b33-4790-951c-8cff1365720b@59g2000hsb.googlegroups.com...

On Jul 12, 8:13 am, "john jardine" <john.jard...@idnet.co.uk> wrote:
"Bill Bowden" <wrongaddr...@att.net> wrote in message

news:f0885697-6c81-4c90-835e-5817c55e30ff@d77g2000hsb.googlegroups.com...

What is the best way to measure unloaded Q of a tuned LC circuit
(AM
loop Antenna) without a Q meter?

I tried a scope and generator and loosly coupled the generator to
the
loop, using a single turn of wire and 50 ohm resistor, and
monitored
amplitude against frequency, but I get different results depending
on
the setup. The scope probe is 10X. The frequency of interest is
500KHz
to 2 MHz.

How can I accurately measure the Q of the circuit without a Q
meter?

-Bill

Yep, each measurement can turn out different. The pleasing upside is
acknowledging that the most accurate test setup was the one that
returned
the highest Q

value!.http://www.crystal-radio.eu/enqmeting.htmdescribes



in great detail what
Mike has already noted. Also look at the series of "LC experiments".

Yes, I'm getting a Q of around 100 at 1 MHz, but I think it's higher.
I used a 1 meg resistor in series with the scope probe and the half
power points are around 9KHz apart. Using a 100K resistor gives a much
lower Q, so it looks like a 100k load effects the circuit
significantly.

Thanks for the link.

-Bill

Just a check, you are using half power point and not half voltage
point,
right?
.707 vs. .5
Mike

Yes, half power points, 0.707 voltage points. 0.707 squared is 0.5.

-Bill

Loathe to do any paying work today, I spent a diverting hour measuring some
aerial coils kicking about.
A data-point ...http://img140.imageshack.us/img140/2631/ferriterodcoilqba6.jpg
Would seem Q's of a couple of hundred are not unreasonable using Litz wire.
(Q's were measured using an 'Advance C.M.1' Q meter)
Click to expand...
Yes, I found somebody with a Q meter and the uloaded Q measures 300 at
600KHz, 285 at 1 MHz, and 225 at 1700KHz. Quite a bit higher than
scope measurements. The Q meter is a Japaneese copy (MQ160B) of the
Booton 260A. The loop is a 15 inch square wooden cross frame with 14
turns of #18 solid copper wire. I added spreaders in the mid sections
to separate the odd and even windings to further reduce the
capacitance. Resistance is only about 0.5 ohms. Works pretty well and
I can hear a 2KW station about 30 miles away in the daytime. Going to
do some DX listening tonight to see what I can hear from other out of
town places.

-Bill
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top