LF: More on measuring field strength]
Andre' Kesteloot
akestelo@bellatlantic.net
Sun, 18 Jul 1999 10:07:10 -0400
> Dick Rollema wrote:
>
> To All from PA0SE
>
> Using my homemade field strength meter I measured the signal of DCF39 at an open field near my location at 2.12 mV/m. (It almost doubles after dark!) Using a 1,34 diameter circular loop connected to a selective level meter - a basic system not requiring calibration - I found a field of 2 mV/m in my shack on an earlier occasion. So I feel pretty certain the measurement is correct.
>
> This permits some speculation as to the power of DCF39.
>
> I used the graphs in CCIR Recommendation 368-7: GROUND-WAVE PROPAGATION CURVES FOR FREQUENCIES BETWEEN 10 kHz AND 30 MHz.
> There are no curves for 137 kHz so I used the ones for 150 kHz as being the nearest ones.
>
> Gamal Soegiono in an e-mail of some time ago gave the locator of DCF39 as JO52XH (accurate to 10 km).
> With my own locator JO22GD this yields 505 km for the distance between DCF39 and PA0SE.
> Measuring the distance on a road map produced 495 km. I therefore entered the graphs at 500 km.
>
> We find for the power of DCF39 the following figures:
>
> Type of terrain Conductivity Dielectric Constant Power radiated
> S/m epsilon kW
>
> Perfect ground 12.2
>
> Sea water (low salinity) 1 80 45
>
> Sea water (avarage salinity) 5 78 45
>
> Fresh water 0,003 80 282
>
> Land 0.03 40 45
>
> Wet ground 0.01 30 56
>
> Land 0.003 22 224
>
> Medium dry ground 0.001 15 2818
>
> The power radiated must be muliplied by 1.83 to obtain ERP.
>
> Gamal Soegiono in the same e-mail said the transmitter is designed for nominal 200 kW but presently operated with 50 kW out.
> What are the conclusions?
> Perfect ground, sea water and fresh water are not applicable and were only included for comparison purposes.
> Land with 0.003 S/m conductivity and epsilon 22 as well as medium dry ground produce unrealistic figures for the power.
> Wet ground is also not very likely. That leaves us with Land (0.03 S/m; epsilon = 40). (Apparantly just as good as sea water!)
> When the transmitter really has an output of 50 kW and 45 kW is radiated the efficiency of the antenna/earth system must be unlikely high.
> Could it be that the output power has been increased over 50 kW?
> It would be nice if someone could find out.
>
> A field strength meter must be used at such a distance from the transmitter that the near field is sufficiently weak not to invalidate the measurement.
> In the description of my FSM I say that 1 km is probably sufficient but 2 km is safe. This was based on the following. When I had just finished the instrument I measured the field of my transmitter at 1 and 2 km and found at 2 km exactly half the value at 1 km. This satisfies the inverse distance relation which is only present in the far field.
> Later I found that the location used at 1 km was not a good one; the reading varied considerably when moving around a bit.
> So that leaves us again with the question whether 2 km (about one wavelength) is sufficient?
>
> Back to Recommendation 368-7. It says: The curves give the total field at distance, r, with an error less than 1 dB when kr is greater than about 10, where k = 2 * pi/lambda (Lambda is the wavelength in km).
> To make kr > 10 at 137 kHz r must be greater than 3.5 km.
>
> As a test I measured the field of my transmitter at 1.52 km; 2.0 km and 3.1 km from the station. (3.5 km fell outside my
> 1 : 10 000 map.)
> To check whether the field at the locations was sufficiently undistorted I first measured DCF39. The values found at the three locations differed no more than 0.17 dB.
>
> For the power radiated by my station I found:
>
> Distance in km Power radiated in mW
>
> 1.52 62
>
> 2.0 57
>
> 3.1 40
>
> To be multipied by 1.83 for ERP
>
> A difference of 1.9 dB between maximum and minimum value.
>
> So it seems advisable to measure at a distance of at least 3.5 km.
>
> For the relation between power radiated and field strength I gave the following equation:
>
> P = 0.0111(E*D)^2 in which:
>
> P in watt
> E in mV/m
> D in km
> (* means multiplication; ^2 means squared)
>
> This equation is theoretically only valid for a perfect earth. The CCIR curves show that it can also be used up to at least 3.5 km with sufficient accuracy for our purpose in case of Sea water, Fresh water, Wet ground, the two types of Land, Medium dry ground, and Dry ground. Only in case of Very dry ground (0.0001 S/m, epsilon = 3) the CCIR curves should be used.
>
> As mentioned earlier DCF39 has a field of 2.12 mV/m at my location. The antenna system at PA0SE produces 6 mV over 50 ohm when tuned to DCF39. This relation makes it possible to find the field strength of a station using equipment in the shack. The signal over 50 ohm can be found with my Wandel & Goltermann PSM-5 selective level meter or the combination of LF receiver and signal generator (substitution method). Multiplication by 6/2.12 = 2.83 yields the field strength in mV/m.
> That can be useful for future experiments.
>
> 73, Dick, PA0SE
> JO22GD
> D.W. Rollema
> V.d. Marckstraat 5
> 2352 RA Leiderdorp
> The Netherlands
> Tel. +31 71 589 27 34
> E-mail: d.w.rollema@gironet.nl
> or
> pa0se@amsat.org