[Antennas] Measuring Q of a resonant circuit with MFJ66

Thu, 27 Nov 2003 09:21:24 -0500

Dear Juan,

The MFJ instructions for measuring Q are correct. The procedure and the
formula for Q are the same as have been used for as long as I can remember.

The problem is in the details. As is stated in their manual, the coupling
between the circuit under test and the probe must be minimal. Also, the
positioning of the probe in relation to the trap under test MUST be
maintained constant. The skill of the operator here is just as important as
the theory, if not more so.

If you want to make a more accurate measurement, it will require the use of
an RF impedance bridge, which is usually beyond the reach of most hams.

Another factor that may cause the Q to be lower than your expectation is the
proximity of conducting materials in the vicinity of the trap under test.
There should be NO metal objects within the surrounding area for several
feet.

The higher the intrinsic Q of the trap under test, the lower the tolerance
for any coupling to nearby objects.

Bob Lay (W9DMK) in Dahlgren, VA
http://www.qsl.net/w9dmk
----- Original Message -----
From: "JAB (EA5XQ)" <[email protected]>
To: <[email protected]>
Sent: Thursday, November 27, 2003 12:53 PM
Subject: [Antennas] Measuring Q of a resonant circuit with MFJ66

Hi,

I got the MFJ66 for measuring the Q of my traps. I built a RF probe and
followed the steps indicated by the manual to measure the Q however the
results are a little bit odd because quite different from the numbers
calculated theoretically by a program (when I mean quite different is
around half or one third). For instance, I'm trying to measure the Q of a
coax trap. According to the calculus it should around 50-60 however I'm
obtaining values around 30 and for a normal trap (with inductance +
external capacitor), the values calculated show Q=200 but in the measuring
process I do not get more than 100.

Is there any consideration in order to use the RF probe with the MFJ66 that
was missed in the manual?

Any help will be highly appreciated

73 Juan, EA5XQ

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