Stanley Briggs, K8SB, instructor for the attenuator construction project, demonstrates how to test the circuit before assembly into the box.

Pictures by Hollis 'Chip' Locke, WB8ALW

The Kalamazoo Amateur Radio Club


Hosts a Successful Saturday
Fox Hunting Attenuator Construction Project

The next best thing to a Saturday fox hunt is an afternoon construction project at your local club. Such a project was held Saturday, June 11th by the Kalamazoo Amateur Radio Club at the Red Cross Chapter House in Oshtemo. Billed as a 'training event', the project for the day was to build a K8SB Offset Attenuator for fox hunting. According to Michael Richey, KD8IHB, KARC Vice-President and project chairman, 16 members signed-up for the project.

Instructor for the event was Stanley Briggs, K8SB, who was a past member of KARC back in his college days and is a current member of The Motor City Radio Club. Stan has developed his attenuator over several years and has supervised many constuction projects for many Michigan clubs. His attenuator is of the offset (or "offset') type, which means that input signals from the radio antenna are combined with another signal in an active circuit to produce an "offset" signal that is much better to use for fox hunting.

How do attenuators work?

How do attenuators work? All fox hunting attenuators serve the same purpose, they keep the hidden transmitter from overloading the receiver when the two become close together. Fox hunters often use their s-meters to tell when their antenna is pointing toward the hidden transmitter. When the hunter becomes quite close to the fox, the s-meter reads full scale in all directions and the hunter must insert some kind of attenuation to reduce the input signal to the point where the s-meter is useful again. There are three or four popular ways to introduce this attenuation.

Body Fade is the attenuation technique of using your body beween the receiver and the hidden transmitter. The operator holds the HT receiver close to his/her chest and turns until the signal is minimum. The transmitter is then behind the operator's body. Many hams have success with this technique.

Fixed attenuators are usually made from modified cable connectors and each contain a single attenuation network. The operator can install several of these devices in series with the antenna to drop the input signal level. This can be cumbersome, but it works in a pinch.

Step or manual attenuators are generally characterized by a row of 5 to 10 switches in a shielded box. Each switch controls a small attenuation network capable of 5 to 20 DBs of attenuation. When all switches are turned on, the box introduces over 100 DB of attenuation to the antenna signal. When the switches are off, the attenuation is very small. This device provides attenuation in many combinations of steps. This type of attenuator device is quite difficult to construct because each switch and it's associated attenuator components need to be isolated in their own shielded chamber within the switch box. Another shortcoming of this device is that after all the attenuation has been applied, the signal from the hidden transmitter can still leak into the cable between the attenuator and the HT receiver as well as through the case if the HT itself. For very close work, some hunters actually shield the HT case with aluminum foil. Having said all that, the step attenuator is a very usefull tool for attenuating the fox signal. Several commercial model are available which are very good.

Active (or Offset) attenuators are like the attenuators built in the KARC construction project. They consist of active transistor circuitry that creates an offset signal exactly 1 MHz above and 1 Mhz below the hidden transmitter's output frequency. The circuitry consists an an oscillator, a variable resistor to adjust the amplitude of the oscillator output and three passive components used to mix the oscillator output with the signal from the hunter's directional antenna. If the hunter installs the offset attenuator between his antenna and HT, and keeps the receiver tuned to the hidden transmitter's basic frequency, he will see little effect. But, if the hunter tunes his HT receiver to the 1 MHz offset signal, he finds a very useful hunting signal. This signal sounds very much like the normal hidden transmitter output, but it's amplitude into the HT is controlled by the oscillator output voltage, i.e. the setting of the variable resistor knob. The operator simply sets the attenuator knob for a mid-range reading on his s-meter and rotates his antenna to locate the fox direction. In this setup, it doesn't matter if the transmitter signal leaks into the cable between the attenuator and the HT, because it is at a frequency other that the one the HT is reading. This operation is not as complicated as it sounds to a beginner. It appears to be like having a volume control on the foxes signal. Here is a simple example.

In this example, the hunter installs the active attenuator between his directional antenna and his HT. (Attenuator power should be off to save battery life, but it doesn't really matter here.) The fox is transmitting on 146.530 Mhz. The hunter sets his HT to the same frequency as the fox signal (145.530 MHz) and can hear the foxes signal and can detect the foxes bearing by looking at his s-meter and rotating his antenna. As the hunter gets closer to the fox, the signal gets a lot stronger and the bearing to the fox can no longer be detected because the s-meter is always at maximum. The attenuator is now needed. The hunter turns the attenuator ON and sets his HT receiver frequency to 146.530 Mhz plus 1.00 MHz or 147.530 MHz. This is the offset signal generated by the attenuator. Our hunter adjusts the attenuation knob until he can hear the fox signal and the s-meter reads near midrange. As the hunter closes-in on the fox he continues to adjust the attenuator knob keeping the foxes signal within the useful range of the s-meter.

There are 16 new fox hunters with super attenuation tools thanks to Stanley Briggs, K8SB, and the kalamazoo amateur radio club. Good job guys.....

Mike, W8DER

inside view
The photo (left) shows the inside view of the K8SB Offset Attenuator.

The photo (right) shows the fully assembled K8SB Offset Attenuator.
inside view