RF Tap Experiments

Here's a quick look at some RF signal taps for some fun.  An RF tap is just a device that attenuates the incident signal  to a convenient port and at a much smaller level so that external monitoring equipment, such as a spectrum analyzer, frequency counter or oscilloscope can view it without it's own front end being overloaded (blown up!).

The first is my own little QRP wattmeter. The wattmeter works well to 70Mhz and I can measure down a milliwatt with accuracy. I modified at a later date to include a simple resistive tap in order to feed things like an external frequency counter. A 1K resistor should provde 27db of attenuation. The result is not bad, within 1db 0-200Mhz but above this frequency range the attenuation becomes lower, about -15db, so beware that above this frequency you could inject a higher signal into external test equipment than you might think. Using better constructed tap of resistors, like SMD parts should extend the higher frequency range accuracy. Depending upon the transmitter levels, you will of course need to choose a resistor value to suit your requirements. Note - S21 plots are just a way of expressing how much loss versus frequency is present, and confusingly called "gain". If the QRP wattmeter is being used as an an RF Tap then the output to the voltmeter terminals must be unloaded (no voltmeter attached).

Next up is an idea that I came across from a well known RF Youtuber. He describes the devices as an RF monitor rather than a wideband device of any kind.  It's made from one of those 100w RF resistors available on Ebay. They are VERY compact and cheap. A small airgapped coupling stub is used as the sensor. Frequency response was very poor. But as the author of the video said, it's an RF monitor.

I was also interested in how good those special 100w 50 Ohm resistors were.  To be honest, not much different to my homebrew PL259 calibration pieces  for my Nano VNA using metal film quarter watt resistors.

S11

500KHz = 49+j211 milliohms

100MHz = 49+j5.17 Ohms

200MHz = 50+j9.7 Ohms

All resistors at DC will perform as "perfect model" type, but as we move up to RF then we start to deviate from that model. The higher we go the more inductive that resistor becomes. Choose your parts carefully. Carbon composites, metal film vs small surface mount types. For example if you used a 100K tap off, that might give the required attenuation level but only valid to a lower maximum frequency.

Conclusion is that for basic harmonic measurements for 0-30Mhz transmitters, like small QRSS rigs then the simple resistive potential divider type shown at the start can be considered quite useful. It's easy to make. The 50 Ohm load power rating needs to be suitable for the TX output, and the tap off resistor simply needs to be of a suitable value in order to provide the required attenuation to feed things like a spectrum analyzer etc. That tap off resistor isn't even passing a milliwatt or so, and can this be a small wattage value. WIth careful construction  techniques and high quality components you can extend the accuracy and higher frequency performance. This stuff is good enough for amateur radio use, but not really ISO9000 quality.

However, in a more professional world, we need things to be a lot better, so using properly constructed in line type RF attenuators is the best choice. They offer a much more constant frequency versus attenuation performance into the Ghz range. I hope this quick set of tests provides some food for thought.