LF: Field Strength Meter - advice needed

[Chris Osborn <g3xiz@yahoo.co.uk>]

No Graham,

I can swap a flat battery with a new one and the output hardly moves.
Yes the oscillator and RF stages are regulated and holding constant.

The problem's before the mixer.
As advised,  I disconnected the RF stage and injected a signal into the
mixer and it gives a constant output even with a blow heater playing on the insides.

73 Chris XIZ

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From: Graham <g8fzk@g8fzk.fsnet.co.uk>
To: rsgb_lf_group@blacksheep.org
Sent: Monday, 4 May, 2009 21:31:37
Subject: Re: LF: Field Strength Meter - advice needed

Chris

 

Its not the  battery is it ... assume the  supply  voltage / regulated voltage  is holding  constant ?

 

G ..

 

From: Chris Osborn

Sent: Monday, May 04, 2009 9:23 PM

To: rsgb_lf_group@blacksheep.org

Subject: LF: Field Strength Meter - advice needed

Thanks again Dave and Alan,

I've spent a few more hours on the F/S meter today and still have not found the problem.
I can blow with a drinking straw on any component and there's very little
output change yet with time there's a slow drift with temperature - the output goes up
with increased temperature.

I thought I'd found the problem as by squeezing the ferrite rod 's screening tube
I could get a fair change in the output (about 10 %).
The tube's metal is not aluminium but I guess an aluminium alloy.
In any case I changed it for a proper aluminium tube - no difference !

From the shack's temperature of  23 degrees and taking the F/S meter outside to an ambient
13 degrees the meter's output changed from  0.48 volts to 0.35 volts !!

I have run out of ideas now and am thinking maybe it's even the permeability of the die cast box
that's changing or maybe the perspex end caps of the ferrite rod. . . . I'm getting desperate !

73 Chris G3XIZ

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From: Dave Pick <daveyxm@googlemail.com>
To: rsgb_lf_group@blacksheep.org
Sent: Monday, 4 May, 2009 16:10:26
Subject: Re: LF: Field Strength Meter - advice needed

Chris

Have you tried a can of "freezer" spray to identify where the problem lies? Is it the ferrite aerial or is it the FET amplifier? If you have enough gain then a little negative feedback on the FET stage may help, add some un-decoupled source resistor.

Dave 'YXM

2009/5/3 Chris Osborn <g3xiz@yahoo.co.uk>

LF,

I've just rebuilt PA0SE's field strength meter (LF Experimenters' Handbook),
modified for use on the 500 kHz band.

I've followed the published circuit as near as possible considering available components,
the differences being:
a) I have used a small variable capacitor instead of the published varicaps.
b) I used a JFET 2N3819 in the front end instead of dual gate mosfets.
c) Obviously I now use the MW coil of the ferrite rod instead of the published LW coil.

I have made a decent Helmholtz coil which gives a stable magnetic field yet the
field strength meter's output voltage is very dependent on temperature and can vary up to
30 %.

I would be most grateful for any advice on curing this problem as I've spent hours trying
to correct it.

73 Chris G3XIZ

--
G3YXM IO92BK Birmingham UK

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