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Subject: Re: LF: Measuring Q
From: "Hans-Joachim Brandt" <hajo.brandt.dj1zb@t-online.de>
To: rsgb_lf_group@blacksheep.org
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Date: 21 Nov 2001 14:19 GMT
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Dear all,

I have measured the loss of my aerial variometer with the same resistive bridge which I use to measure the feedpoint resistance of my LF arial (two resistors 150 ohms from the feed point to the "measuring diagonale" (containing a diode circuit), below that one 220 ohms potentiometer and the "unknown" terminal). The bridge is fed by a 3 watts lf transmitter (PA = TBA810AS). The "unknown" terminal is connnected to the variometer and an 500 pF air variable capacitor in series.

At various settings of the variable capacitor (to see the influence of the inner turnable coil on Q) the bridge is balanced by tuning the variometer and the 220 ohms potentiometer. The potentiometer reading then gives the series resonant loss R of the variometer (assuming that the Q of the air variable capacitor is much higher). Then the L of the variometer is measured, and then Q = XL/R.

(in my case 3160 ohms/18 ohms = 175). The main coil of my variometer has a diameter of 160 mm, length 180 mm, wire 1 mm dia enameled wire, close wound). The inner turnable tuning coil is much smaller. As my ground loss is still around 70 ohms the variometer loss is tolerable.

With this procedure there is no need to determine a 3 dB bandwidth or care for any form of loose coupling. If the loss resistance of the variometer is expected to be lower, the resistors of the bridge should also be lowered correspondingly.

HW?

73 Ha-Jo, DJ1ZB 


"g3ldo" <g3ldo@ukonline.co.uk> schrieb:
>Recently I described some experiments with multilayer coils based on a Decca
>coil.
>The resultant coil is now similar to that shown in the LF book, page 13 (See
>the second coil back) except my coil has two layers, double wound. I had
>been advised by some of you that the Q of a multi-layer coil is low compared
>with a single layer solenoid. The question is just how much lower is the Q
>and how significant is it in practice.
>The coil is at this moment being tested with a full kW on a T/A test and I
>get just as good results as I did with the single solenoid although the
>single solenoid was a much smaller coil wound with thinner wire.
>To progress I need to measure Q.
>
>It is quite surprising how little  there is in general Ham literature on
>measuring
> Q. I found something in the 'ARRL Electronics Data Book' and Wes Hayward's
> 'Radio Frequency Design', although the latter is related to filters and
> measuring loaded Q.
>
> I made a test rig up using a signal generator (with frequency counter) and
> the Pegelmesser D2155 level meter set on wide band.
> I connected the coil under test with a parallel air-spaced capacitor to the
> signal generator via a variable capacitor. I coupled the coil to the level
> meter with with just a clip on the insulation of the coil. I reduced the
>coupling to the signal generator with the variable capacitor to minimum
>while maintaining a good signal on the level meter.
> The parallel capacitor caused the coil to resonate at 182kHz . Using the
>3dB B/W points (down from resonance) on the meter I measured the Q as 185.
>With the variometer in the coil set to minimum the coil resonated at 209kHz
>and the measured Q was 167.
>
>Is this the right way of going about it? If I use a larger capacitor across
>the coil won't this give a sharper selectivity and improved apparent Q?
>
>Regards,
>Peter, G3LDO
>
>e-mail <g3ldo@ukonline.co.uk>
>
>Web <http://web.ukonline.co.uk/g3ldo>
>
>
>
>