To All from PA0SE
Christer, SM6PXJ wrote:
Last weekend I made some field strength measurements on my own station.
http://home5.swipnet.se/~w-54761/fs.htm
I use an untuned loop and a selective level meter.
The output from the loop is calculated, so maybe there is some uncertainty
in the method.
I had a look at the website and Christer certainly made a beautiful job of
his loop antenna.
The differences between calculated and measured field strengths are much
smaller than found by Jim, M0MBU (see his e-mail of Febr. 25, 13.50 hrs) and
myself.
As I think Christer lives in relatively open country that seems to support
the view of Dr. J.H. Causebrook, as reported by Jim, that in an urban area
considerable power is absorbed by buildings, trees, lampposts, etc.
The loop made by Jim is very small, expressed in wavelengths. So it is
likely that the distributed capacitance and leakage inductance between the
turns are so small that the equation for the voltage induced in the loop
yields correct results ("Radio Engineer's Handbook" by Terman, First
Edition, p.813).
Nevertheless it may be useful to calibrate the measuring equipment.
Inserting a 1 Ohm resistor in series with the loop and putting a known
voltage over it - as suggested to Christer - is theoretically correct but
not easily done because even at 137 kHz it is difficult to make a resistor
that is really 1 ohm.
I met the same difficulty when trying to calibrate a home made FS-meter with
a one-turn loop antenna for the short wave bands.
The following procedure was adopted.
The loop was put through the hole of a ferrite toroid core; it so became the
one-turn secondary of a transformer. The primary winding consisted of ten
turns of wire, spread over the circumference of the toroid. The primary was
connected to the output of a signal generator. In order to properly load the
generator a 50 Ohm resistor was connected in parallel with the primary
winding. Now the voltage induced in the loop is 1/10 of the output voltage
of the signal generator. (Beware: some signal generators indicate the output
as the EMF, others as the voltage at the output terminals when connected to
a load equal to the internal resistance of the generator!)
I checked the transformer action by connecting a selective level meter to
points on the loop immediately left and right of the toroid. The value was
indeed exactly 1/10 of the signal generator output voltage.
It is important that the inductive reactance of the primary winding is at
least 5 times 50 ohm, otherwise the sig gen is not properly loaded and its
output voltage cannot be correctly read.
On 137 kHz this may be difficult to achieve with a ten turn primary. But
there is no objection to using more turns. If N turns are used the voltage
induced in the loop is simply the generator voltage divided by N.
The extra resistance introduced into the loop due to the transformer is
equal to
50 Ohms/2 * N^2. The factor 2 arises because the 50 Ohm resistor appears in
parallel with the generator output resistance of 50 Ohm. When N = 10 this
works out at
50/2 * 100 = 0.25 Ohm. The effect of this can be neglected.
Christer uses a loop with 47 turns. Only one turn must be put through the
hole in the toroid, preferably the centre one to avoid upsetting the
symmetry. But I don't think
putting the toroid over one of the wires connecting the loop to the
selective level meter will introduce a serious error.
73, Dick, PA0SE
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