In part one of this series, We covered the basics of gain structure as it pertains to wireless mic systems, and transmitter placement. In part two, antenna types and cable loss were discussed. Now, we'll touch on antenna placement.
As already briefly mentioned in part one, transmitters should be placed in such a way that their antennas are not either too close to or contacting something that will absorb RF energy, such as a person's body. For belt pack or "bodypack" transmitters, this can be tricky because the whole point of using a unit like this is to hide it on someone. Nevertheless, as pointed out in that article, mindful placement can make a difference in your success. As mentioned in Part 1 of this series, keeping the antenna away from skin or other boundaries is important, and can be done via several means.
Handheld transmitters can also suffer from diminished range if the antenna is covered or blocked in some way. An example would be if the user cups his or her hands around the base of the transmitter (assuming that's where the antenna is). This is the opposite of when rappers cup the grill basket of the mic to obtain that "rap look". It is fairly easy to see the effect of blocking the antenna in this manner by monitoring the RF level on your receiver as you cup the antenna and then remove your hands. Of course we all know that this advice may not be heeded by talent, CEOs, or pastors, but we can try.
This is where antenna use becomes more under our control (usually) and equally if not more critical for success. In part 2 of this series, we covered the different types of antennas. Once we have chosen the appropriate type for the application at hand, we concern ourselves with placement. The number one rule of thumb is that ideally, you should have line of sight between your transmitter and receiver antennas. In other words, the fewer obstructions the better. The first and most widely used way to achieve this is to have your receiver antennas fairly high up (above head height) in order to minimize how many bodies the RF signal must travel through, be absorbed by, or be reflected by, before arriving at the receiver. At heights above 10 ft - you increase the Fresnel zone clearance at these frequencies and distances, a function of maximizing receiver strength by minimizing the effect of the out-of-phase signals by removing obstacles from the radio frequency line of sight. For more info on the Fresnel zone, click here
Since almost all receivers today have diversity reception, we need to pay attention to where the two antennas are placed in relation to each other. The rule of thumb often mentioned is to have the two antennas at least one wavelength apart, but this proves to be only a bare minimum figure. 10 times the wavelength has proven to be close to the maximum effectiveness because the signals picked up by the two antennas are effectively de-correlated. So you may wonder how well diversity works when both antennas are only inches apart on a portable receiver. Even this distance helps a bit, but a greater distance certainly works much better.
In order to estimate this distance for your setup, you can get close simply by looking at a typical 1/4 whip antenna, about 4.25 inches at 575 MHz (near the middle of the UHF TV band) and multiply this by 40 to get 170 inches, or 14 ft. I have often recommended to people to set up their receiving antennas with one on either side of the monitor console. When consoles were big and ruled the earth, this is not too far off. But with today's tiny varmit-like digital consoles, it is better to use something else as a standard for setting the distance.
Which brings me to the next point: in my (newly revised) opinion a diversity pair of antennas do not have to be on the same side of the stage. One advantage of putting them on opposite sides is that whichever way a performer turns, their body will not be blocking both antennas from the transmitter.
This brings up another question that comes up fairly often: "if my run lengths are significantly different, as in systems where there is an antenna on either side of the stage, do I need to make the cable lengths the same?" Some people swear by this, and from the viewpoint of cable loss balancing, it would make sense. But I've found that the run lengths do not have to be the same. For diversity to work properly, each side of the receiver should receive the maximum, un-amplified signal level possible, with each antenna providing a different perspective on the performance area. When using low-loss cable like Belden 9913F7, an additional loss of a few dB on the long-run side of the diversity system will be a very small difference compared to typically encountered multi-path nulls of 40 dB or more.
Aiming the Antennas
The first thing to do is to follow the manufacturer's recommendation for orientation of the antennas. If they say "vertical" or "up", for instance, please follow this advice. Circularly polarized antennas do not have this issue. But nevertheless, follow the manufacturer's suggestions. Even directional antennas have a fairly broad pickup pattern: LPDAs in the horizontal axis can be compared to cardioid microphones (see Part 2 of this series). Thus, really precise aiming is not terribly critical. Make sure your nulls are used to block unwanted signals (like TV transmissions, for instance) if possible, and point the "front" towards the general direction where you want to pick up your desired signals. Since the antennas are often up in the air, you may want to point directional units downward slightly towards where the talent is most likely to be. Finally, if you are using an IEM system on the same stage or in the same church, etc. as with wireless microphones, you'll want to keep your IEM antennas as far away from your receiving antennas as possible. You can also use antenna nulls to your advantage here as well.
After all That...
Following the steps outlined in this series of articles should apply whether you are setting up one wireless mic or 24 channels, and whether they are analog or digital. In other words, these are the basics that you should be able to use in just about any situation involving RF mics. From there, each brand may have additional suggestions for the optimum setup of their specific equipment, and each technology has its features, benefits and drawbacks. Lean on the manufacturers to help you - we are all here to make sure the equipment is working the best that it can. Now go out there and impress some clients with your new-found knowledge! Who am I kidding - we all know that if everything goes perfectly, no one notices...
Thank you to Henry Cohen for his invaluable assistance on this article.