QRP WATT METER

Here's a very simple but very accurate wattmeter, combined with a tap for sampling. The sampling port could be used for to connect to an external frequency counter, oscilloscope or spectrum analyser. Brief description. It's a 50 Ohm dummy load which uses a non inductive planar resistor capable of handling 5 watts (or search for 50 Ohm thick film.)   No need to pay silly money £/$5 is enough. A BAS70 diode acting as a peak voltage detector. The BAS70 was chosen over other more common diodes due to it's lower forward voltage drop and very low capacitance. This makes it suitable for lower power measurement down to 1mw, and has a typically flat response from 0-70Mhz. A simple tap off point via a resistor provides a safe low level output for feeding into external measurement equipment (not frequency dependent so the 70Mhz limit does not apply). The value of the resistor determines the safe output level. The circuit diagram shows some sample resistor values to choose from.  An analogue moving coil type voltmeter used since a digital one is badly effected in it's operation in the presence of RF. 

The uses of a simple tap off is much cheaper than acquiring a high powered power reducer, or other forms of RF sample equipment that can have limited frequency response.  One potential addition to this system maybe to add a series capacitance in line with the tap resistor just n case there is a DC offset present on the equipment being tested. NOTE - If you are using high power and feeding a spectrum analyzer then you MUST use a much higher level of attenuation !!

1K = 27db attenuation

2K = 33db attenuation

3K = 37db attenuation

10K = 47db attenuation

etc

Assumes 50R system impedance to spectrum analyzer, frequency counter or whatever external equipment you are feeding. External equipment that is being fed by the tap off point will obviously have it's OWN maximum input signal level for which you need to account for, and then select a suitable resistor. You might also be interested in another webpage here about RF Taps.

Here's an image of the overall system in use with an analogue voltmeter, and Yaege FC1+   frequency counter.

And here's the set up, which indicated the attenuated power output fed to the FC1+ in signal level mode.

5w output from the VX7R transmitter gives at the tap off point 10dbm.

The wattmeter is flat from 0-70Mhz. If you try to use it outside this frequency range then you'll typically read 20% low at 145Mhz and 50% low at 440Mhz. The formula used for this PEAK voltage reading meter is  Power = ( (Vpeak + volt drop of diode) ^ 2 / 100)). Note that the100 is 2x the load resistance.  0.35v is typical of the BAS70 diode forward voltage drop.  Thus for example 1 watt = 10vdc and 5 watts = 22vdc. I have included the formula in my RF spreadsheet here.