Maybe forget everything I asked previosly. Put it this way, What type of indoor device can I buy that will let me listen to, monitor or decode anything I want to at random on my SDRplay? Remember I cannot put up an outdoor antenna.
Sheldon.
Well, there is no such thing as a "SDR device xyz -specific" antenna. Everything depends on what frequency bands you wish to monitor, the distance between your location and the location of each specific system/site you wish to monitor, the elevation of your location vs. the elevation of the site/system of interest, and various other factors.
If you want to monitor a vhf public safety repeater 5 or 10 miles away, a portable whip antenna may work fine. However, if you want to monitor, say, your local regional airport 25 or 30 miles away it's a whole different story. And as was mentioned previously, HF is a whole different discussion.
There are numerous antenna designs/makes/models etc, and some are designed for "wideband" monitoring across several spectrums. Do you have any clue what type of system(s) you want to monitor right now, today? Do you know approximately where the transmitter(s) is located?