September 2023 Lecture and Lab: build an active fox hunt attenuator

If you’ve ever tried to find a hidden radio transmitter, or if you’ve ever wanted to…or if you’ve ever wondered how close you could get to finding a “jammer,” this month’s Lecture and Lab is for you!

We’ll be building the KC9NON active attenuator kit, an easy-to-build device that lets you get up close and personal with the radio transmitter that you’re hunting. The kit cost is only $20.00, and the tools needed are simple: a soldering iron, solder, wire cutters, wire strippers, and a small screwdriver are all that you really need. A hot glue gun and a needle-nose pliers were enough for me to finish my build. You’ll also need a jumper cable with BNC connectors (it can be very short, 2 feet is plenty), and a BNC-equipped antenna and radio, or else a set of adapters.

We’ll be meeting at 10:00 AM on Saturday, September 23rd, in the 3rd-floor community room at the Dallas Medical Center, 7 Medical Parkway Dallas, TX 75234. There are 15 kits available, first-come-first-served.

What does an active attenuator do? Why would you need one?

Surprisingly, the hardest part of hunting for a hidden radio transmitter is not the beginning—it’s the end. It’s easy to tell that signal is coming from the north, or the south, or from somewhere over near the lake. But as you get closer, directions get harder to find. No matter which way you face, or where you point your antenna, the signal you’re looking for seems equally strong.

That’s where an active attenuator comes in. It combines the signal you’re looking for with a 4 MHz local oscillator to create a new signal that’s under you’re control. If the signal you’re hunting is on 146.88 MHz, you can turn the active attenuator on and tune your radio to some multiple of 4 MHz away: 142.88 MHz, or even 138.88. You then adjust the attenuation knob to make the signal louder or stronger, as desired. With an active attenuator, you can walk right up to a transmitter, and still be able to tell what direction the signal is coming from.

Assembly Notes

Follow the manufacturer’s instructions to assemble your circuit board. You can test your board by listening for the 4 MHz oscillator when it’s turned on.

The L-shaped piece of black plastic in the kit holds the LED in place in the case. It should be installed the “tall” way, so that the LED is in line with the center of the adjustment pot. Don’t install it the “long” way so that the LED sticks way out in front of the board. The LED goes through some of the holes in the plastic L, so make sure to install the LED in the plastic L before you solder the LED to the circuit board. I recommend that you put a blob of hot glue on the base of the plastic L after you get it in the right place and solder the LED.

You will need two 3 to 4 inch pieces of speaker wire to connect your BNC connectors. Tin the ends of the speaker wire that you intend to attach to the connectors. It does not matter which connector is used for the antenna and which connector is used for the radio. However, you will want to be careful that no small stray wire strands from one wire touch one of the other wires. Also, be consistent in how you wire up the “+” and “-” terminals to the connectors. I recommend that you use the silver conductor in the speaker wire as the “-,” and connect that wire to the shield of the BNC.

I have had trouble with the knob falling off of the adjustment pot. I recommend that you attach the knob to the shaft with a small amount of glue once you’re sure things are working and you’ve added the nut and washer to secure the pot.

Testing Your Kit

You can test your kit in two ways: first, listen for the 4 MHz local oscillator using any radio that can receive 4 MHz. An HT with a wide-coverage receiver will work fine. Second, use HTs to transmit and receive. Connect an antenna and an HT to your kit and turn the kit on. Listen on 142.520 while another ham transmits on 146.520. You should hear the transmission! Turn the kit back off and your ability to hear the other ham should disappear.