Active return amps allow information to return back to the cable company. I don't know why using a passive return amp for a scanner application would pose a problem. Some of these amps are pretty powerful. You may want to make sure you have attenuation capabilities in place.
Yep, a cable TV amp with an active return can actually send out unwanted RF when hooked to an antenna and cause interference to licensed users. In this case, anyone from 5 to 42 MHz could experience interference from an amp with an active return channel. So a passive return would actually be better for monitoring so you don't cause interference to anyone. Most cable TV amps use the same return frequencies for sending signals back to the head end so most active return models will produce unwanted RF at the antenna in the same frequency range.
You must remember that cable TV systems are closed systems so anything amplified should stay in the coax and not radiate out causing interference like can happen when hooked to an antenna which has now become a radiator of the active return frequencies. Luckily, most scanner antennas today are not very resonant at low band and lower frequencies so that minimizes the potential that you may be emitting illegal signals but the possibility still exists.
I'd be cautious using an amp with an active return.
In reality, an amp with active return is not going to help your low band reception anyway as it does not amplify anything coming in, only out.
The active return channel is not a very powerful signal but the potential for interference coming off your antenna still exists, especially with sensitive receivers like low band users would have.
If a user is into low band monitoring and really needs an amp, an preamp made for low band would be a better solution as they don't produce any RF back into the antenna.
It's also possible that a cable tv drop amp may seem to help low band reception by lowering any powerful low band signals hitting the scanner. Those signals could be causing unknown desense which the scanner cannot handle. Adding a drop amp actually lowers low band signals and can allow the AGC circuit to operate giving the owner the impression that the amp is pulling in weak signals not heard before.
Adding a cable drop amp with a passive return can help as it gives you port to port isolation plus it may allow the radio's AGC circuit to work properly by reducing any local low band signals that would otherwise drive the front end into desense.
Over the air drop amps do not have a return path as you are not needing to send a better signal back to a cable companies head end. But... those amps generally have a high noise factor which will usually kill any benefit for the scanner listener. In this case, a preamp made for the scanner bands of interest would be your better choice. Quality preamps for scanners usually have band filtering designed into them so you are not also amplifying unwanted out of band signals such as FM Broadcast radio which can also kill typical scanner reception from desense or overload.
Using a Cable TV drop amp with an active return could land you in hot water if it does wipe out a licensed users radio system!
