[Antennas] Measuring Q of a resonant circuit with MFJ66

Thu, 27 Nov 2003 16:32:50 +0100

Dear Bob,

Many thanks for your comments. Because what I did was to put the MFJ66=20
longest coil inside the PVC form to get the maximum deflection and maximum=
=20
voltage. So following your words is in the other way around, I should=20
separate the coil from the trap until the dip is barely detected, therefore=
=20
in such situation I will never get SWR 1:1 in the frequency related to max=
=20
voltage, am I correct? am I in the right way?

Thanks

Juan
At 27/11/2003, Robert Lay wrote:
>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=3D200 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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>-------------------------------------------------
>EA5XQ, QRA:Juan, QTH:Almazora, LOC:IM99XW
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>CLUBS: G-QRP #9805, QRP-L #1461, EA-QRP #471, EACW #309
>E-MAIL: [email protected]
>MSMESSENGER: [email protected]
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----------------------------------------------------------------------------=
-------------------------------------------------
EA5XQ, QRA:Juan, QTH:Almazora, LOC:IM99XW
MEMBER OF/MIEMBRO DE:  URE (www.ure.es) y/and SLUREVILL (Secci=F3n Local URE=
=20
Villarreal - http://slurevill.iespana.es/slurevill)
CLUBS: G-QRP #9805, QRP-L #1461, EA-QRP #471, EACW #309
E-MAIL: [email protected]
MSMESSENGER: [email protected]
WEB SITE:  http://www.qsl.net/ea5xq
(english version in http://www.qsl.net/ea5xq/index_eng.html )
----------------------------------------------------------------------------=
------------------------------------------------- =20

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