----- Original Message -----

From: Dick Rollema

To: LF-Group

Sent: Sunday, May 06, 2007 1:09 PM

Subject: LF: PA0SE FS meter

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.

 

Try again and leave certain words out hi

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

Hi Dick

This is the method that I use, the tricky part is getting the loop antenna calibrated accurately, more complicated than you think, for absolute accuracy.