Jan_Martin and All,

You could use your loop aerial, together with a selective level meter, for field strength measurement on strong signals by using the coupling loop on its own with the tuned loop open circuit. The voltage induced in a loop is given by Terman , first edition, on page 813:

in which:
epsilon = field strength, volts per meter
N = number of turns in loop
A = area of loop, square meters
lambda = wave length, meters
phi = angle with respect to loop

Cos phi = 1 when loop is turned for maximum signal.

At the low frequency concerned I don't expect that signal pick up from the electric field by the unscreened loop will disturb the measurement.
To check this you could turn the loop 180 degrees. This reverses the polarity of the magnetically induced voltage.
If this produces a different reading then you could take the average of the two readings.

In the tuned loop, when open circuit, no current should flow that could affect the magnetic field. In practice the loop is not completely open circuit because of the distributed capacitance between the turns. But hopefully this effect is not too disturbing. You could rig a duplicate loop without the tuned winding to check this.

I use a square untuned single turn loop of 1 square meter. It is made of coax with the outer shield open at the point opposite the take off point. On DCF39 (500km distant) it produces a voltage of 4.9 microvolt, easily measured by the SPM-12. The impedance of the loop is so low that it makes no difference whether the 75 ohm loading resistor at the input of the SPM-12 is switched on or off.

I expect that at your location you will obtain a suitable reading as well.  Do it during daytime, because after dark the sky wave may wreck the measurement.
Once you know the FS of DCF39 you could use that to calibrate your other aerials. 

73, Dick, PA0SE
JO22GD
 
At 12:16 12-11-02 +0100, you wrote:

Hi

still experimenting with frame antenna for 136kHz. The attached drawing (20kB)
shows almost how it would have been made today, somewhat different from
the actual antenna.

Experienced that it was no need for amplifier for a loop for 80m reception
(used to avoid electrical noise from the open power lines),
and now also experienced that an amplifier did not improve reception for my
136kHz aerial,
the coupling has 1/100 area of the whole frame, seems to work for 50 ohm,
the tuning range seems to be somewhat too small, need somewhat increased
capacitance in cold weather, but it is low enough for rain.

The S/N ratio is almost the same with the coupling loop as for high impedance
amplifier connected to the end of capacitance.

The aerial has been tested in +10°C....-2°C

Signal strength last night for DCF39 was 80-150µV (-75...-80dBU)

Not sure if it is still a point to ground the center of the tuned loop.

73

Jan_Martin
[email protected] (WW.Loc: JO38XC)
N4623-Kristiansand (tel.: +47-380-87178)
http://www.laud.no/la6nca/la8ak/