Modern communications radios usually have a 50-ohm input to the receiver.  A short whip or rubber duck will present an extremely high impedance, resulting in very little energy transfer to the radio.  The Ducker overcomes this problem using autotransformer matching to bring these two extremes together over a wide range of operating frequencies.  The result is outstanding - improvements of 30-40 dB in received signal strength are common on the HF/SW bands.

The tuned response of the Ducker-antenna combination provides substantial preselection and gain relative to an untuned duck.  This results in a reduction of interference and a strong peaking behaviour while tuning the unit - signals that were inaudible with the untuned antenna will pop up loud and clear, and interfering signals such as strong nearby AM stations can be reduced or eliminated completely.

Because it's a passive device, the Ducker is immune to effects such as intermod, mixing and overload that plague most active antenna designs.  And it contributes no noise whatsoever to the received signal.  No active antenna works so well.

For radios like the 817 and other tranceivers, you can in fact tune the rubber duck to transmit on HF as well.  While we can't promise DX using a rubber duck antenna, there are many ways to have fun exploiting this capability.  And one day soon, when the sunspots kick back in, DX contacts will be a reality - stay tuned!