Introduction

I needed a 6db attenuator to attenuate the transmit path before feeding the signal to the PA input to not cause an input overload. While attenuators of various types are available in the market, it is easy to homebrew one.

Considerations

Even a simple project like an attenuator needs a bit of considerations before you start melting solder. The main consideration that dictates further design decisions is:

Where in the transmit chain are we going to insert the attenuator?

The answer to this question will tell us a bit about the amount of attenuation we need and the power that is being dissipated in the attenuator.

In my case, I have a 25mW (~14dbm) signal that is fed into a pre-amplifier that produces 1W. The input to the pre-AMP only needs about 10dbm to produce full 1W. So I need to reduce at least 4db. I chose to go with a standard 6db attenuator. The power levels are not too much here. Some back of the envelope math is useful here:

3 db corresponds to doubling (or halving the power, depending on the sign)

10 db corresponds to multiplying (or dividing) the power by 10.

In this case our input is 25mW and our desired output is only around 6.5mW. So, we need to reduce 25mW by 4. So, 3+3 = 6db reduction. Input power is only 1/4 of a watt, so 1/4W resistors is enough. I chose 1W because those are the ones I had in my junk box.

connections

The other consideration is, how are we going to insert these attenuators. I decided to use an SMA-M on one side and SMA-F on the other side, so that all the existing cables/connectors can be reused, since the interface as far as the rest of the chain is concerned, remains the same.

calculations

I used an online calculator to create a Pi-network. The equations are not complicated and can even be derived with some basic math. We can just use one of the calculators available.

              R1
  o------.-\/\/\/\/\.-------o
         |          |
         /          \
         \ R2       / R2
         /          \
         .          .
         |          |
          ----------
              |
              ⏚

Plugging in 6db into the above calculator link, gave me an R1 of 37Ω and R2 of 150Ω. Since I had 300 Ω resistors (2 parallel 300 Ω gives 150 Ω and 39 Ω resistors with me, I decided to use that. The 39 Ω instead of 37Ω does not change the attenuator much.

Build

I cut a small piece of copper clad board and filed the edges to make it more or less straight and smooth, used a piece of sandpaper to clean the surface and then soldered one of the SMA connectors to it and soldered the two 300Ω resistors from the center pin of the SMA connectors to the PCBs. Same way with the other SMA as well and then soldered the 39Ω from center pin of one SMA to the center pin of the other.

Conclusions

This is a simple homebrew project and can be done by almost every radio amateur. Why buy an expensive attenuator when one can build it in an hour or even less? (Bezos sells it for over 2500 Indian Rupees when I can homebrew one for almost the cost of the connectors alone..)