Adept Instruments - Active Offset Attenuator
Model AOA1 Instructions and Data Sheet
"Fox Hunting" / Radio Transmitter Tracking Accessory
Model: AOA1, Price: $49.00 - Fully Assembled and Tested.

Ready to use with CR2032 Lithium Battery Installed.


The Adept Active Offset Attenuator is used during Amateur Radio "Fox Hunting" or to help in the tracking and location of animals, model airplanes, model rockets, high altitude balloons, lost hobby and recreational drones, and so on. Generally as the distance to the transmitter is reduced, attenuation of the signal becomes necessary. Some receivers have a one or two-level built-in attenuator, and this can help, but additional attenuation of the signal will be required. The incoming signal must be kept at a low level, the lower the better. Then as the directional antenna is turned away from the signal, the strength level of the signal drops off dramatically.

If the signal coming in to the receiver is too strong, the receiver's front end circuitry will be overloaded. Also, at very close range a significant amount of signal gets into the receiver through its unshielded plastic case. This will cause the signal strength to appear to be the same from all directions. We need a way to attenuate the signal before it reaches the antenna input.

Why use an Offset Attenuator, and what are the benefits of an Offset Attenuator?

An Offset Attenuator, also known as an Active Offset Attenuator is an electronic device that typically runs on battery power. The Adept Offset Attenuator includes a 4 MHz transistor oscillator that is crystal controlled. It uses a 225 mAh, 3 volt lithium battery and draws very low current, about 350 microamps. Battery life is more than 600 hours. The 4 MHz local oscillator generates a sine wave voltage, and the load resistor is an audio-taper potentiometer that allows adjustment of the oscillator output from near zero to maximum.

This local oscillator signal is mixed with the antenna input signal using a simple diode mixer. Two new frequencies are created, the input signal plus 4 MHz, and the input signal minus 4 MHz. Adjusting the level of the local oscillator will also adjust the levels of the two new frequencies that are passed on to the circuit output, and to the receiver's antenna input. The receiver is tuned to one of these two new frequencies, and this allows the receiver's input signal strength to be adjustable from near full strength to near zero. This emulates an attenuation function. The original input signal of interest stays at the same high level, but it is so far away from the receiver's input frequency (4 MHz away), that virtually none of the signal that penetrates the unshielded plastic case of the receiver can have any effect.

Due to the whole nature of, and characteristics of, an Offset Attenuator, the Adept Active Offset Attenuator comes in a non-shielded plastic enclosure which typically sits atop a non-shielded HT or hand held receiver that also comes in a plastic enclosure. Shielding is generally not necessary. However, if a customer wants a shielded metal case, we can also supply that option.

Another thing, actually a good thing, about an Offset Attenuator is that when the frequency is offset higher or lower by 4 MHz, the actual incoming frequency being received may be outside of the Amateur Radio band. This is true of the 2 meter band. This means that your handy talkie probably won't allow you to transmit. This is a good thing. Transmitting directly into the Attenuator will probably damage the internal circuitry. Because of this, the operator should choose the lower frequency or the higher frequency, which ever one will be outside the Amateur band. NEVER TRANSMIT WHEN THE ATTENUATOR IS BETWEEN THE ANTENNA AND THE HANDY TALKIE.

This product does not come in kit form. However, a few versions can be supplied in partial-kit form.

Using the Adept Active Offset Attenuator

The AOA1 is designed to be mounted directly atop a hand-held receiver or Handy Talkie. The male BNC connector connects directly to the antenna connector. An adapter connector (SMA male to BNC female) may be required if your receiver or HT has an SMA antenna connector. Connect the coax from the directional antenna to the BNC female connector at the top of the Offset Attenuator.

Turn the control clockwise to turn on the power to the Attenuator. Turn further clockwise to increase the injected 4 MHz signal. All the way clockwise the signal is the strongest (minimum attenuation). All the way counter-clockwise toward the off position offers the maximum attenuation. This apparent reversal of the control direction was chosen to take advantage of the audio-taper potentiometer. With the audio-taper control there is more range control of higher attenuation values in the counter-clockwise direction.

This is an Offset Attenuator. The receiver's input frequency must be set to the input frequency plus 4 MHz or to the input frequency minus 4 MHz. For example, if the primary frequency you wish to receive is 145.55 MHz, then set your receiver input to 145.55 plus 4 = 149.55, or to 145.55 - 4 = 141.55.

Note: if one of the offset frequencies is occupied, then try the other. You may also try an offset of 8 MHz, the second harmonic of 4 MHz. The attenuation at an offset of 8 MHz will be even higher.

Here we show a spectrum display (2 MHz per horizontal division, 10 dB per vertical division) of the AOA1 outputs with a strong input signal in the center, the signal that we wish to attenuate. On either side of the input signal are the diode mixer outputs. The closest two signals are the input plus 4 MHz and the input minus 4 MHz. These signals are each about 50 dB below the input level. The two additional signals are the input plus 8 MHz (the second harmonic of 4 MHz), and the input minus 8 MHz. These signals are down by an additional 12 to 15 dB.

This display shows only partial attenuation of the input signal. The AOA1 adjusted to full range would completely remove the mixer output signals from this display example.


Effective Attenuation variable from 12 dB to 95 dB typical.

Frequency Range from less than 10 MHz to more than 500 MHz.

Case Dimensions: 2.13 by 1.39 by 0.59 inches. Overall Dimensions with Connectors and Knob: 2.13 wide by 2.9 high by 1.22 inches.

Weight: 1.7 oz.

Operational Current Consumption: 0.35 mA nominal. Operational Battery Life: 600 hours.



Adept Instruments, Inc. warrants to the original purchaser that this product is free of defective parts and workmanship and that it will remain in good working order for a period of one year from the date of original purchase. This product will be repaired or replaced within one year of purchase if it fails to operate as specified, if returned by the original purchaser and if it has not been damaged or modified, or serviced by anyone other than the manufacturer.


Additional CR2032 3V Lithium Battery, Price: $1.29       2/16/16

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