Monitor transmitter quality using a RF tap
The uBitX is a popular multi-band QRP transceiver. Preparing to assemble my own 40m uBitX, I wanted a means to monitor the RF output to confirm that any harmonics and spurs are at levels that meets FCC requirements. I have a MFJ-260C 300 watt dummy load and adding a RF tap that my RF Explorer, SDR dongle, or RSP2 could use seemed like a great way to extend the usefulness of the dummy load.
What is an RF tap and why do I need one? The purpose of a RF tap is to provide attenuation of the transmitted signal so that the resulting level is within the maximum input levels for any devices connected it it. Connecting a device like a RF Explorer or receiver directly to the output of a transmitter, even with a dummy load, will certainly damage, if not destroy it. The circuit described here will reduce the power at the tap output 40 dB. When 100 watts is applied to the input, the output will see a level of 10 mW (1/100 of a watt) . Additional attenuators can be attached to the output to reduce the level even more.
Full credit for this idea goes to Didier Juges, K04BB, who modified the higher power version of the MFJ dummy load and has provided additional information on the performance of the RF tap at http://www.ko4bb.com/ham_radio/MFJ-264/
I built my tap using 0.6w metal film resistors from an assortment that I had laying around the shack. I used four 2.7k resistors in a series and parallel configuration (to arrive at 2700 ohms) and terminated the RF tap with a 56 ohm resistor. I’m lazy and didn’t do the math regarding power dissipation, but the resistors are cool running 100 watts and following the operating curve as specified by MFJ (90 seconds continuous transmitting at 100 watts). As most of my testing involves power levels well below 100 watts, I am satisfied with the resistor choices.
Additional attenuators can be attached to the RF tap to achieve any level of signal reduction. The image shows a 20 dB attenuator that can be purchased from eBay or Amazon and a RF Explorer protection device that provides 6 dB of attenuation, a DC block (important if there is DC component on the RF output), and clamping diodes to further protect the front end of the device connected to the tap. The combination provides a total of 66 dB of attenuation: -40 dB from the tap, -20 dB from the external attenuator, and -6 dB from the RF Explorer protection device. When all the devices are used, 100 watts fed into the dummy load, results in an output of approximately 0.025 milliwatts.