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