Artificial load on VHF

The designer of this artificial load is Mak, SV1BSX. Several interesting constructions, but not only can you find it on his site: http://www.qsl.net/sv1bsx/. TNX Mak!

Home made artificial load is usually made by connecting several carbon resistors in parallel like this, to achieve resistance 50 ohm. Such a construction will serve well at KV, but not on VKV, or UKV (430 – 440 MHz), where suddenly a high PSV appears when measuring. Such an artificial load is then unusable for measurement purposes.

The reason for the high PSV is in the parasitic capacitance of the resistors. Figure 1 shows a carbon resistor. The two caps at the ends of the ceramic body act like two plates of a small capacitor. By connecting resistors in parallel, this capacity “multiplies” by the number of resistors. A disaster!

If we want to eliminate this unwanted capacity, it is possible and easy with little inductance, by which we download PSV to 1:1. Picture no. 2 shows a diagram of such an artificial load.

Resistors are 560 ohm/2W. I used eleven resistors connected in parallel, to achieve value 50 (50,9) ohms. The total load is 11x2W = 22W, but practically such a load can be handled by 100W for a few seconds. This is sufficient for most measurements with commercial equipment, whose output power is between 5 a 35 watts.

Picture No. 3 shows the practical design of the artificial load. A panel N-connector is used as a connector (female). Inductance “L” consists of four turns of 0.8 mm wire with a diameter of 3 mm in a winding length of 15 mm. If PSV is not optimal, the number of turns can be increased or decreased.

One last note - the artificial load made in this way is only suitable for UKV (it will again have a higher PSV on VKV, someone could place a dipole at a height 2:1).

Makis SV1BSX, http://www.qsl.net/sv1bsx/