For a more complete description of simulcast distortion, it’s affects, and some countermeasures that might (or might not) help, see the Simulcast Distortion page in the Wiki.
Basically, in a simulcast site, you have multiple transmit towers, sub-sites, all transmitting the exact same radio traffic, on the same frequency, at the same time. While radio waves move at the speed of light, these signals, coming from more than one location, at various distances, arrive at your scanner at slightly different times. While that is a very tiny time difference, they are out of sync enough to impact the scanner’s ability to correctly monitor the system. So, you might either hear nothing, or the scanner is so “confused” that all you hear is a garbled mess of audio, nothing that can be understood.
With a “better” antenna, you’re getting signals from most, if not all, of these subsites. With “less” (or a worse, less effective, antenna) you might eliminate some of the more distant, weaker, signals, letting the closest, stronger, signal override the weaker ones, giving the scanner a chance to monitor correctly.
In some cases, using a paper clip as an antenna is so ‘bad’ that the weaker signals are not strong enough for the scanner to use them. That may allow you to successfully monitor the site. Of course, the down side is that while it might improve your reception on the simulcast, you will lose too much signal strength to monitor weaker, more distant, sites that are not simulcast.
