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Measuring Unloaded Q of Loop Antenna?

B

Bill Bowden

Jan 1, 1970
0
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
 
A

amdx

Jan 1, 1970
0
Bill Bowden said:
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
You say unloaded, so you need to get a setup that is stable and away from
anything that will cause losses. Get your signal loop as far away from the
AM loop as possible, but close enough to couple in a signal large enough to
see on the scope.
I often put a resistor in series with the scope probe it help isolate the
15pf of scope capacitance. 100k should work.
It has been a while since I've done this work, but I think I used a couple
of clip leads
as a dipole on the end of my signal generator cable. See which gives you the
most signal, loop or dipole.
I like to peak the signal on the scope at 7 units, by adjusting the output
level of the sig gen. Then move the frequency until you have 5 units (the
3db point) then go back the other way to 5 units (the other 3 db point). The
math, .707 x 7units = 4.949units
This is 5 units to my eye.
Hope I explained that ok, I have a writeup, but it's on a different
computer I don't have access to.
Mike
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
amdx said:
"Bill Bowden" <[email protected]> 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

You say unloaded, so you need to get a setup that is stable and away from
anything that will cause losses. Get your signal loop as far away from the
AM loop as possible, but close enough to couple in a signal large enough to
see on the scope.
I often put a resistor in series with the scope probe it help isolate the
15pf of scope capacitance. 100k should work.
It has been a while since I've done this work, but I think I used a couple
of clip leads
as a dipole on the end of my signal generator cable. See which gives you the
most signal, loop or dipole.
I like to peak the signal on the scope at 7 units, by adjusting the output
level of the sig gen. Then move the frequency until you have 5 units (the
3db point) then go back the other way to 5 units (the other 3 db point). The
math, .707 x 7units = 4.949units
This is 5 units to my eye.
Hope I explained that ok, I have a writeup, but it's on a different
computer I don't have access to.
Mike
-3db in my book is 1/2 amplitude
 
B

Bill Bowden

Jan 1, 1970
0
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".

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
 
A

amdx

Jan 1, 1970
0
Bill Bowden said:
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
 
B

Bill Bowden

Jan 1, 1970
0
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
 
B

Bill Bowden

Jan 1, 1970
0
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)

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
 
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