Improvised dummy load.


I had to adjust the PA of my mcHF and did not have a phantom charge at hand. I looked in "the trunk of the junk memories" and found an old instrument of 50 μA DC full scale that was never used. I only had to buy 29 carbon resistors of 1,500 Ω, 2 Watts, which were connected in parallel to obtain a value of 51.72 Ω. Everything was put in an ad hoc box. A PL259 female connector was located on the rear and another female BNC connector on the front. The idea was to obtain in the BNC connector a sample of the attenuated input signal ten or one hundred times, to analyze it in an oscilloscope.




Since the original idea was to use it up to 30 Mhz, no precautions were taken to arrange the resistances around a radial conductor, it was enough to connect them in parallel as close together as possible and keep short terminals. With copper ribbon, all terminals were joined on each side. The signal present in the load resistor is rectified with a diode bridge 1N4148, then filtered with a 100 nF capacitor and carried to the microammeter, which has previously been converted to a voltmeter of 50 Volts at full scale, this was made by puting in series a 1 MΩ, 2% tolerance resistor. 

The 100 nF capacitor is charged to the peak value of the RF voltage present in the 29 resistors. This is a data that must be taken into account when calculating the power dissipating in the load, since we will be interested in the RMS value, that is, the voltage in direct current that would cause an equivalent dissipation in the load


The 22 KΩ potentiometer will adjust the voltage divider so that on the 1 KΩ resistor - the output of the BNC connector - there is a voltage value 10 times lower than what exists in the load resistors. When the microswitch is closed, a resistance of 100Ω is added in parallel with the one of 1,000Ω, which will decrease the voltage in the BNC connector 10 times more, that is, it will divide it by approximately one hundred. The value of 100 nF of the filter capacitor has been chosen so that the instrument shows the peak voltage and at the same time is able to follow the variations of the signal, for example, the modulation in SSB.


To calculate the power that dissipates the load, we must divide the voltage shown in the instrument (remember that the microamp scale is equivalent to volts with a series resistance of 1 MΩ) by 0.707 (same as divide by √2) because it is a sine wave form. Once the effective value of the voltage is known, it multiplies itself to be squared and divided by 52, which is the approximate value of the load resistance.

P = E² / R

It must be borne in mind that silicon diodes have an approximate voltage drop of 0.5 Volts in conduction, so we would have to add a volt to the peak voltage before calculating the RMS value. Obviously, this instrument only has accuracy from two or three volts in the load resistance. Since each 1,500 Ω resistor can dissipate 2 Watts, this phantom load could dissipate continuously 58 Watts without any problem.

58 Watts, by the way, equals a RMS voltage of about 55 Volts E² = 58W x 52Ω then E = sqr (58W x 52Ω). It would go out scale by little.



Equivalence of the most common power values distributed on the scale of 1 ... 50 volts



Tabulated values of PEP and RMS measured voltage, translated into power over a load of 52 Ohms.

Voltss PEP
Volts RMS
 PEP * PEP
RMS * RMS
PEP power
RMS power
1 0,71 1 0,50 0,02 0,01
2 1,41 4 2,00 0,08 0,04
3 2,12 9 4,50 0,17 0,09
4 2,83 16 8,00 0,31 0,15
5 3,54 25 12,50 0,48 0,24
6 4,24 36 17,99 0,69 0,35
7 4,95 49 24,49 0,94 0,47
8 5,66 64 31,99 1,23 0,62
9 6,36 81 40,49 1,56 0,78
10 7,07 100 49,98 1,92 0,96
11 7,78 121 60,48 2,33 1,16
12 8,48 144 71,98 2,77 1,38
13 9,19 169 84,47 3,25 1,62
14 9,90 196 97,97 3,77 1,88
15 10,61 225 112,47 4,33 2,16
16 11,31 256 127,96 4,92 2,46
17 12,02 289 144,46 5,56 2,78
18 12,73 324 161,95 6,23 3,11
19 13,43 361 180,45 6,94 3,47
20 14,14 400 199,94 7,69 3,84
21 14,85 441 220,43 8,48 4,24
22 15,55 484 241,93 9,31 4,65
23 16,26 529 264,42 10,17 5,09
24 16,97 576 287,91 11,08 5,54
25 17,68 625 312,41 12,02 6,01
26 18,38 676 337,90 13,00 6,50
27 19,09 729 364,39 14,02 7,01
28 19,80 784 391,88 15,08 7,54
29 20,50 841 420,37 16,17 8,08
30 21,21 900 449,86 17,31 8,65
32 22,62 1024 511,85 19,69 9,84
32 22,62 1024 511,85 19,69 9,84
33 23,33 1089 544,34 20,94 10,47
34 24,04 1156 577,83 22,23 11,11
35 24,75 1225 612,32 23,56 11,78
36 25,45 1296 647,80 24,92 12,46
37 26,16 1369 684,29 26,33 13,16
38 26,87 1444 721,78 27,77 13,88
39 27,57 1521 760,27 29,25 14,62
40 28,28 1600 799,76 30,77 15,38
41 28,99 1681 840,25 32,33 16,16
42 29,69 1764 881,73 33,92 16,96
43 30,40 1849 924,22 35,56 17,77
44 31,11 1936 967,71 37,23 18,61
45 31,82 2025 1012,19 38,94 19,47
46 32,52 2116 1057,68 40,69 20,34
47 33,23 2209 1104,17 42,48 21,23
48 33,94 2304 1151,65 44,31 22,15
49 34,64 2401 1200,14 46,17 23,08
50 35,35 2500 1249,62 48,08 24,03


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