50 Ohm Dummy Load with Sample Port (a Homebrew Project by our member JAYAR)

This simple project illustrates the assembly of a 50ohm dummy load rated at 100watt.  

Provision of a -40dB, 50ohm sample port permits monitoring of the transmitter signal. Attenuation for the sampling port is achieved using a pi network resistor arrangement.  

Unlike conventional dummy loads, which normally feature cooling fins, this unit relies upon a thermal mass to handle, for a reasonable time period, the 100watt rating.

To minimise any inductive effect, normally associated with wire-wound power resistors, thick film resistors were used.  These were rated at 100ohms, 100watt.  Using a pair connected in parallel provides the required 50ohm load rated at 200watt.  Limiting the combined pair to 100watt ensures the resistors operate well below their stated rating. The resistors are TO247 dimensions and are clamped tight to the thermal mass. The thermal mass is a solid block of aluminium that occupies the whole enclosure volume.

The whole was built into a diecast metal enclosure measuring 11 x 9 x 3 cms.  Power input port is an SO239 bulkhead connector and the sample port is a female SMA bulkhead unit.

Using an NVA to determine frequency response it confirms good response up to VHF.  As such it is suitable for both HF and VHF test loading.

https://chemandy.com/calculators/matching-pi-attenuator-calculator.htm provided an interactive means of determining the ideal resistor arrangement and, more importantly, the effect of using the nearest available resistor values as shown in the orange box. 

Using the pair of 100ohm input resistors, 51ohm output resistor and a 2.5kohm coupling resistor it is noted that the calculated input impedance drops to 49.03ohm.

Pictures of Dummy Load.jpg

The above diagram, taken from a NVA, illustrates the SWR with respect to system frequency

                                        Vertical scale is 1 SWR per division ie 1 to 9

                  Horizontal scale is 100Mhz per division and is set 100kHz to 474.5MHz

 Note that the SWR rises to around 5.7 at 450MHz, however at HF frequencies it remains around 1, rising to about 1.6 at normal VHF frequencies.