An offset attenuator, sometimes called an active attenuator, is used in transmitter hunting to shift the receiver frequency 2 MHz or 4 MHz up or down from the transmitted frequency. This solves the problem of the main frequency leakage into the receiver that essentially renders a standard attenuator useless when getting close to the transmitter. It is good for getting close (within a foot) to perhaps a watt or two. If you will be hunting higher power transmitters, then a shielded box might be necessary and possibly a switched 20dB attenuator to keep the module from getting saturated with the unwanted signals. The offset is available in either 2 Mhz or 4 MHz, and both seem to work just fine. The 4 MHz offset is probably preferable to reduce the possibility of inband interference. Generally, the 2 MHz is used for receivers that cannot tune out of band.
My experience is only using it on 2M but others have used it quite successfully on the 440 MHz band.
The offset attenuator modules are on a PC board 3/4" x 1 1/4". The kit consists of the circuit board with all the surface mount components soldered in place, and the other parts necessary to complete the unit (diode, potentiometer, crystal, and 9V battery leads.) The surface mount components include all capacitors, resistors, and the voltage regulator. An enclosure is not included. When ordering, please specify either a 2 MHz or 4 MHz offset. Included with each kit is a one page schematic and parts list.
||PCB w/ SMT components soldered in place|
||Oscillator (2 MHz or 4 MHz)|
||1N4148 or 1N914 Diode|
||9V Battery Lead|
|Assembled/Tested - 12 or more||
|Kit - 12 or more||
Note: Due to increased PC board and component pricing, I've had to raise the price by $2.00.
To order, payment can be made by check, money order, or PayPal (my email address is my PayPal ID.) The total due will be the kit(s) prices plus $5.50 USPS Priority mail including Delivery Confirmation. International buyers, please check for mailing cost.
Please email me first to confirm what kits you are purchasing and if you are ordering the 2 MHz or 4 MHz offset (2 MHz is normally used for receivers that can't tune out of band.)
My email address is: .
This kit is provided with permission from Joe Moell, K0OV. It was originally featured in the 73 Magazine Homing In column for May 1998, and more information is available on the HomingIn website.
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This page updated on 1/17/2014