Homebrew 150 Watt 20 dB Attenuator |
Overview
The is a simple piece of test equipment which should be on every RF experimenter's test bench. The attenuator is based around a common Bird BCP-150 20 dB, 150 watt attenuator chip which can often be found on the RF input to some surplus high-powered RF amplifiers.
For this project, we'll just be using the attenuator chip itself along with the addition of some input and output RF connectors. This will make for a nice stand-alone, high-power 20 dB RF attenuator. This project will not require any precise construction methods, so it's also a good project for beginners.
This is a useful piece of test gear for connecting the output of a high-power amplifier to a spectrum analyzer or frequency counter. It is also useful for testing polarization losses when performing real-world radio path loss experiments. Horizontal/vertical polarization mismatches are often around 20 dB, so if your radio link still works with a 20 dB attenuator on one end, you know you have a solid connection. Attenuators are also helpful in reducing intermod interference in receivers located in RF saturated environments. They keep the incoming RF signals from forcing the preamplifier into its compression region, though using 20 dB is a little too much for that task.
Bird BCP-35 (35 watt) attenuator chips can be ordered from Radio Dan, www.radiodan.com.
Block Diagram

Construction Notes & Pictures

Parts overview.
Off to the left is a large piece of scrap aluminum heatsink. In the middle are two satellite "Videopath Multi-Dish Switches." These are normally used to switch between satellite feeds, but the die-cast metal case they come in is perfect for those little homebrew RF experiments. The threads for the stock F connectors are also the same for BNC and N connectors.
You can often get these switches for free from CATV or satellite installers. Pick up a bunch if you can.
Along the bottom are a few female N connectors or an optional BNC connector. N connectors are preferred as they are designed to operate at higher frequencies and RF power.
On the lower-right, is the heart of this project. A Bird BCP 20 dB, 150 watt resistive attenuator chip. The attenuator is designed to have a standard 50 ohm input/output impedance.

Cut or mill out the bottom of the satellite switch so it can act as a cover for the Bird attenuator chip. Also drill a few holes for mounting the case to the heatsink.

Internal view.
You may have to scrap some weatherproofing compound off the switch case. Also note the mounting tabs on the side of the switch were cut off and cleaned using a Dremel tool. This is optional, but makes the final project look nice.

Drill and tap the aluminum heatsink as required to match the mounting holes in the case.
#4 (red dots) hardware will be used to secure the switch case to the heatsink.
#8 (blue dots) hardware will be used to secure the Bird attenuator chip itself to the heatsink.
The size of the heatsink really isn't that much of a concern. Remember, larger heatsinks will allow you to dissipate more heat, which allows for longer operating times. If you use a smaller heatsink, add an external muffin fan or just limit the RF input time.

Mount the switch case and Bird attenuator chip to the heatsink as shown.
Be sure to use a thin layer of heatsink grease between the attenuator chip and the heatsink surface. Also be sure the heatsink surface is smooth and clean.
You'll note that the tabs on the attenuator chip don't line up with the RF connectors. This mismatch will limit the usable upper frequency of this attenuator. It should be fine up to UHF or so. Microwave use would require better mechanical construction.

Solder the RF connectors to the attenuator's tabs. You may have to "tweak" the RF connector's center conductor a bit.
A small piece of brass tube was used on the top connector.

Finished attenuator overview.
Secure the lid and tape over the other unused RF connector hole. Add a good coat of spray paint to prevent oxidation and it's ready to go.
Attenuators are reciprocal, so either connection can be used for the input or output.