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Message-ID: <9ADAE0806D17400EBA500453FF36AF90@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <260286.17766.qm@web28508.mail.ukl.yahoo.com> <8bf118410905040810m1fb41016pab9a1c0a7c79bf06@mail.gmail.com> <106017.93017.qm@web28508.mail.ukl.yahoo.com>
In-Reply-To: <106017.93017.qm@web28508.mail.ukl.yahoo.com>
Date: Tue, 5 May 2009 02:00:55 +0100
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Subject: LF: Re: Field Strength Meter - advice needed
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Dear Chris, LF Group,

...>>>>
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.
>>>>...

So that narrows it down to the ferrite rod antenna circuit or the FET 
preamp. Injecting the signal directly into the preamp input should tell you 
which it is.

...>>>>
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.
>>>...

At the moment, my bet would be on the ferrite rod antenna. The 
transconductance of a FET (JFET or MOSFET) certainly varies with 
temperature, but generally decreases with increasing temp, so the output 
level should fall as the temperature rises, and probably not as much as you 
are observing. Since the output of the ferrite rod ant depends mostly on the 
bulk properties of a fairly large chunk of ferrite, it would take an awful 
lot of blowing down a straw to have an effect ;-) But a change in ambient 
temp would give a slow change in output as the ferrite warmed or cooled.

Cheers, Jim Moritz
73 de M0BMU