.however these duplicate frequencies have different pl tones.can someone what this means and what i should do.
The best way to describe PL (aka "private line") is that a very low frequency audio tone is sent along with the transmission, and when you have that specific PL tone enabled on the channel you want to monitor, only transmissions with that tone will unsquelch the radio.
A good example are those FRS or "family radio" handhelds. Take one of those to a very busy park, beach or public event, and you will hear everyone that is on the same channel. This could get annoying really fast. If the FRS radio has PL capability, and you assign your group a PL of say "100 hz", then ONLY transmissions with that pl will unsquelch the radio. Helps to cut down on the annoyance factor of hearing unwanted traffic.
The use of PL can also help by keeping your speaker squelched when there is interference, as nothing in nature generates pl tones naturally. It won't reduce the interference, but just keep your receiver squelched until there is a transmission with the pl you programmed.
Note that pl is anything but "private" - that is a misnomer as all anyone has to do to monitor any frequency with pl is to just turn off the pl, or use a scanner that doesn't have it in the first place.
The overall affect with the use of PL is to enable groups of users to share the same frequency without annoying each other too much, but this pales in comparison to sharing a group of frequencies with trunking.
So if you have pl enabled on a channel, make sure it is the right one as listed in the database. If you don't know it, either turn it off completely, or perhaps use the scanner's pl-search feature to find it.
The big problem with self-assigned PL operations is that while it will keep the receiver squelched until a specific pl tone is heard, groups sharing the same channel on a different pl won't hear each other, and may unintentionally jam each other by trying to transmit at the same time. So the polite thing to do, especially with FRS radios, is to use an "unsquelch" button or "pl-disable" or something similar temporarily, to see if the channel is clear before transmitting.
For hilltop repeater systems where neighboring cities or counties might use the same channel, the use of different pl's can help keep the repeaters or receivers from activating on those weaker signals that are not really intended to use anything but the local repeater.
