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Date: Sun, 06 May 2007 15:09:55 +0200
To: LF-Group <rsgb_lf_group@blacksheep.org>
From: Dick Rollema <dickrollema@casema.nl>
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Message-Id: <20070506130953.EF06D406C@mta-fe.casema.nl>
Subject: LF: PA0SE FS meter
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To All from PA0SE

There seems to be some interest to modify my design for a portable 
field strength meter for use on 500 kHz.  The instrument is described 
in G3LDO's boek The LF Experimenters Handbook. But the circuit 
diagram has some errors.
The description can also be found at 
http://www.veron.nl/tech/lf/fsm/pa0se.htm. Better use that one.

I used a ferrite rod antenna from a transistor radio with its LW 
coil. I expect the coil is too large for tuning the antenna circuit to 500 kHz.
You could try a MW coil or a LW coil with turns removed.

The article also describes how the instrument can be calibrated using 
a pair of Helmholtz coils.  The coils are fed from a signal generator 
in series with a 50 ohms resistor. I found that my coils have an 
inductance of 1.2 microhenry.  At 500 kHz that gives a reactance of 2.7 ohms.
This can be completely  be neglected against the 50 ohm resistor that 
is in series with the reactance..
So the current through the coils is the generator output, divided by 
50 ohms. Note: the two coils are in parallel so each of them carries 
half the current.  The article describes how to handle this.

One has to be careful about what the meter on the signal generator 
indicates. On some models the meter indicates the EMF. When loaded 
with a resistor equal to the internal resistance of the generator the 
output voltage is half the EMF.
But on other generators the meter indicates the voltage at the 
terminals (potential difference) when the generator is loaded with a 
resistor equal to the internal resistance of the generator.  This PD 
is taken as half the EMF.

A simpler way to measure field strength is to use a single turn 
untuned loop, connected to a calibrated selective voltmeter. I did it 
that myself on 136 kHz after I was presented a Wandel & Goltermann 
SPM-12 selective level meter that can also be run off a car battery.
In a earlier e-mail to the reflector I  included the equation that 
gives the relation between the field strength and the voltage induced 
in the loop.
But my provider or the one of the reflector did not accept 
it  because the message "scored 5.8 spam points" as the Mail 
Administrated said.
I had not realised I had used dirty words.....

If you have a receiver that covers 500 kHz, has an S-meter and can be 
run off batteries you could use that with the single turn loop.
But the S-meter must be calibrated first using a signal generator.

73, Dick, PA0SE  
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<body>
To All from PA0SE<br><br>
There seems to be some interest to modify my design for a portable field
strength meter for use on 500 kHz.&nbsp; The instrument is described in
G3LDO's boek <i>The LF Experimenters Handbook</i>. But the circuit
diagram has some errors.<br>
The description can also be found at
<a href="http://www.veron.nl/tech/lf/fsm/pa0se.htm" eudora="autourl">
http://www.veron.nl/tech/lf/fsm/pa0se.htm</a>. Better use that
one.<br><br>
I used a ferrite rod antenna from a transistor radio with its LW coil. I
expect the coil is too large for tuning the antenna circuit to 500
kHz.<br>
You could try a MW coil or a LW coil with turns removed.<br><br>
The article also describes how the instrument can be calibrated using a
pair of Helmholtz coils.&nbsp; The coils are fed from a signal generator
in series with a 50 ohms resistor. I found that my coils have an
inductance of 1.2 microhenry.&nbsp; At 500 kHz that gives a reactance of
2.7 ohms.<br>
This can be completely&nbsp; be neglected against the 50 ohm resistor
that is in series with the reactance..<br>
So the current through the coils is the generator output, divided by 50
ohms. Note: the two coils are in parallel so each of them carries half
the current.&nbsp; The article describes how to handle this.<br><br>
One has to be careful about what the meter on the signal generator
indicates. On some models the meter indicates the EMF. When loaded with a
resistor equal to the internal resistance of the generator the output
voltage is half the EMF.<br>
But on other generators the meter indicates the voltage <b>at the
terminals</b> (potential difference) when the generator is loaded with a
resistor equal to the internal resistance of the generator.&nbsp; This PD
is taken as half the EMF.<br><br>
A simpler way to measure field strength is to use a single turn untuned
loop, connected to a calibrated selective voltmeter. I did it that myself
on 136 kHz after I was presented a Wandel &amp; Goltermann SPM-12
selective level meter that can also be run off a car battery.<br>
In a earlier e-mail to the reflector I&nbsp; included the equation that
gives the relation between the field strength and the voltage induced in
the loop.<br>
But my provider or the one of the reflector did not accept it&nbsp;
because the message &quot;scored 5.8 spam points&quot; as the Mail
Administrated said.<br>
I had not realised I had used dirty words.....<br><br>
If you have a receiver that covers 500 kHz, has an S-meter and can be run
off batteries you could use that with the single turn loop.<br>
But the S-meter must be calibrated first using a signal
generator.<br><br>
73, Dick, PA0SE </body>
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