I can hear my tag in two places on my receiver: Sideband Frequencies.

All tracking receivers develop two frequencies from the single frequency of the transmitter. These are called the upper and lower sidebands. The IF (intermediate frequency) filter inside the receiver should all but remove one of them. If the filter was not present you would hear each tag at equal strength at two places on the receiver band. Unfortunately, no IF filter can completely remove the second sideband, so you can still hear the tag at two tuning positions, but one of these should be much louder than the other. Usually the difference is so marked that you will never notice the 'wrong' sideband. If the filter is not as good as it should be, you may accidentally tune in to the wrong sideband. At close range - e.g. as you are putting the tag on the animal - the signal may sound fine. In the field, however, tracking the wrong sideband will give much worse range.

Apart from the difference in signal strength between the sidebands, you can also tell the difference by the way the tone of the bleep changes as you tune through the band. The two sidebands are mirror images of each other. If you start tuning from below the lower sideband, as you tune slowly into it you will hear the pitch of the bleep decreasing. As you continue to tune through, the bleep becomes a thud and may disappear completely (the 'null' point). Keep tuning up through the band and you will start to hear the upper sideband. For this sideband the pitch will start low and increase; the opposite (mirrored) way to the lower sideband.

Once you know which is the stronger sideband on your receiver, you can check the other tags to see if you are tuned to the correct sidebands on these. You can tell by the way the pitch of the bleep changes as you tune up the band.