Hi Chris Mmmmm it seems that you might have a problem with the tuning caps,
I wonder what sort you used. You have to be a bit careful with cap types in
critical circuits. The other though is to try to make the L/C ratio on the
ferrite areal the same as that at 136kHz. If you have too little inductance
and more capacitance that will make the tuning sharp and also maybe be more
critical on cap temp coefficient. Just I ideas, It sounds like you are
making progress, even if it by deciding what is not a problem.
Dick, PA0SE, may be around he does monitor the group else drop him an email
he may have some ideas.
Best wishes
Alan G3NYK
----- Original Message -----
From: "Chris Osborn" <[email protected]>
To: <[email protected]>
Sent: Monday, May 04, 2009 8:23 AM
Subject: Re: LF: Re: Field Strength Meter - advice needed
Many thanks for the prompt suggestions Alan.
I've spent ages trying to sort the problem and have now run out of ideas.
I completely rebuilt the unit, even using a new ferrite rod.
The front end tuning was very sharp and I subsequently added another
RF tuning control (varicap + pot) to peak it up as it drifted.
Also I've placed a 220 k resistor across the tuned circuit to lower the Q
and flatten the response.
I did the usual trick of blowing with a drinking straw on various components
and the front end capacitors seemed to be the culprits hence the added
RF amp tuning control.
I'll try a BF981 as you suggest and also disconnect the front end and inject
a signal.
I'll hunt around for a BF960 as shown in the original circuit.
Thanks again es 73
Chris XIZ
________________________________
From: Alan Melia <[email protected]>
To: [email protected]
Sent: Sunday, 3 May, 2009 21:00:26
Subject: LF: Re: Field Strength Meter - advice needed
Hi Chris the following things occur to me to try
1 the 2N3819 is a juction fet not a mosfet (as with Dick original design) so
is probably more likely to show changes in input (gate-source) current with
temperature. The input current of a mostfet will be negligable. I cant
remember what the original device was but if a 40673 for instance try a
BF981 which are readily available.
2 At a lower level of probability the permiability of ferrite changes with
temperature. I would not that to be anything like so great as problem
though.
I hope that helps to solve the problem.
Alan G3NYK
----- Original Message -----
From: "Chris Osborn" <[email protected]>
To: "LF Group" <[email protected]>
Sent: Sunday, May 03, 2009 6:13 PM
Subject: LF: Field Strength Meter - advice needed
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
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