ka3jjz
Wiki Admin Emeritus
The new Cross Country Wireless HF Preselector | The SWLing Post
Let's talk briefly about a preselector. It acts as a tunable 'gate' that rejects out of band signals and other spurs, such as FM stations showing up on HF. These are very useful for older single conversion desktops for this very reason.
Broadbanded SDRs such as the RTL-SDR and even the SDRPlay would benefit from such a device, especially when using antennas of any size near an urban area where MW and FM stations abound, often running with a bunch of power. Their front ends are not designed to work well in such environments.
There are 2 kinds of preselectors - passive and active.
Passive units like the one above, and the 2 sold by MFJ, are preferable in high RF density areas since they don't have an amplifier.
Palomar used to sell the P-305 (I think they also made a P-308) preselector that had a tunable amp with about 20db of gain - this is an example of an active preselector. The P-305 had a gain control that allowed you to drop the gain if needed. These are great in remote or less developed areas, where the additional gain would cause less problems with overloading issues.
There are schematics on the web for these units, but construction can be a bit of a challenge, particularly if you're unaccustomed to winding toroids. Proper bonding and shielding is an absolute must.
If there are other commercially made preselectors that you know about, feel free to post the URL here.
Mike
Let's talk briefly about a preselector. It acts as a tunable 'gate' that rejects out of band signals and other spurs, such as FM stations showing up on HF. These are very useful for older single conversion desktops for this very reason.
Broadbanded SDRs such as the RTL-SDR and even the SDRPlay would benefit from such a device, especially when using antennas of any size near an urban area where MW and FM stations abound, often running with a bunch of power. Their front ends are not designed to work well in such environments.
There are 2 kinds of preselectors - passive and active.
Passive units like the one above, and the 2 sold by MFJ, are preferable in high RF density areas since they don't have an amplifier.
Palomar used to sell the P-305 (I think they also made a P-308) preselector that had a tunable amp with about 20db of gain - this is an example of an active preselector. The P-305 had a gain control that allowed you to drop the gain if needed. These are great in remote or less developed areas, where the additional gain would cause less problems with overloading issues.
There are schematics on the web for these units, but construction can be a bit of a challenge, particularly if you're unaccustomed to winding toroids. Proper bonding and shielding is an absolute must.
If there are other commercially made preselectors that you know about, feel free to post the URL here.
Mike