If you need to choose between the two, VHF is a more crowded with more devices being used in that particular frequency range. UHF has a bit more leeway in terms of getting a clean signal that's more practical to work with.
Both kinds work essentially in the same way: The mic transmits an audio signal, which then modulates the signal into a radio frequency and sends it across to a receiver, demodulates the signal, turning it back into an audio signal.
Now let's get into the equipment used. We'll use a body-pack transmitter and a lavalier mic as an example; probably the most used of all the types of systems. The lavalier microphone connects to the transmitter via a mini plug and again, transmits to the chosen frequency on the receiver.
Most receivers will have an XLR output in order to plug it into an amplifying system. Some receivers will provide both an XLR and 1/4" jack. It's your choice, depending on the compatibility of the amplifier or mixer.
A couple of things that you'll want to look for before you buy are a low battery indicator on the body-pack transmitter so you can closely monitor the level of the battery and a display that tells you that tells you what frequency you're operating on.
Let's take look at our tips for operating a wireless system:
1. Allow a clear line of sight between the transmitter and receiver. This will increase the chances that there will be a good connection between devices without much interference. It's still important to know that wireless mics can transmit around walls or other obstacles, even if the connection is not as good. The best advice is to give it a try. If it's satisfactory use it. If not try another frequency. (You won't have that liberty, lest you have a USB system that has the ability to scan frequencies).
2. This piece of advice has to do with how high the level of the battery is. It's wise to check the level of the battery power, but most importantly to carry a backup or two. The last thing you need is to ruin a recording or broadcast by not having such a basic thing.
3. When choosing a wireless mic receiver try to get a "diversity" receiver. RF (radio frequencies) can rebound off of the surfaces in the space resulting in the waves reaching the antennae at different times possibly causing a glitch. A diversity receiver has 2 antennas and 2 demodulators. With two antennas the signal will still be received at different timings but with the two antennas it will switch back and forth and choose the best one.
We hope that these few tips will make your understanding and experience with a wireless system a bit more profitable and trouble free.