In the last article, I covered the basics of gain structure and why proper setup is important with wireless microphones. Also, part 1 described proper transmitter placement, including the fact that many pastors and CEOs alike should have to spend 6 weeks in microphone basic training before being allowed to stand in front of a crowd. In this article, I want to cover the basics of antenna choice, along with the issue of RF cable loss.
Antennas – Big and Small
First off, one thing I want to get out of the way is that proper receiver antenna choice is a matter of application. “Less is more” generally applies here – in other words the high-gain antennas you often see are not always the best choice for the job at hand. Instead, it is usually best to think about the shape and size of the space where the wireless system will be used, and how far the transmitters are likely to be from the antennas & receivers. The general rule of thumb is that for a broader area but shorter distances – say, a church for instance – omnidirectional antennas are probably best. For longer distances and narrower area of pickup, or when an antenna null might be useful for blocking unwanted RF signals, passive directional antennas will usually work best.
The most basic type of antenna is the “whip”, which is usually a short piece of wire or telescoping tube. These antennas are omnidirectional in the lateral plane and can work very well. Indeed, they are not only supplied as standard with most wireless microphone systems, but are used for everything from stage productions to TV and film work to touring sound. Whip antennas are also “tuned” by being a specific length, usually 1/4 wavelength for the specified center frequency of the operating range.
For instance, if your wireless receiver covers 563 – 589 MHz (Lectrosonics block 22), the center frequency would be about 577 MHz, so the most efficient 1/4 wave length would be 4.48 inches long. Another thing to keep in mind with 1/4 wave whip antennas is that they need a ground plane to work properly. In other words, these antennas are fine if connected directly to the BNC jacks on the receiver, but will not work right if they are attached at the end of coax cables where there is no effective ground plane.
The next step up in many ways is the dipole antenna. This design is also omnidirectional in the lateral plane, but provides its own ground plane. Because of this, it can be placed just about anywhere you can reach with a coax cable. Since they are small, dipole antennas can be easily hidden or at least placed in an inconspicuous location. However, care here must be taken not to place them too close to where the transmitters will be otherwise performance will suffer. Like whip antennas, dipoles should be tuned to the center frequency of the range in which you are working. Also like whips, dipoles have a fairly gentle filtering effect, so precise tuning is not usually required. I’ve seen plenty of installations where there are wireless mics covering everything between 470 and 698 MHz, and the dipole antennas are tuned to the center of the band at about 600 MHz and the system works fine.
Whips and dipole antennas should ideally be oriented parallel to the transmitter antennas – this is the correct “polarization” and will give best performance. Also, note that although dipole antennas are omnidirectional in the lateral plane, they do have a null directly above and below the antenna. Certainly, reflections from the room may overcome this problem. However, I’ve seen installations where a dipole is placed in the ceiling directly over the center of the room – and this is not ideal. In such cases, I usually suggest moving the antenna to one of the walls, so that everyone in the room is within the direct pickup pattern of the antenna.
The more “sexy” type of antenna that is readily visible at many shows is the LPDA (Log Periodic Dipole Array), otherwise known as the “paddle”, “shark fin” or “bat wing”. These antennas have roughly the same directional characteristics as a cardioid microphone, although they look different in the vertical vs. horizontal planes. They are also wide band in their range of frequency sensitivity – most cover the useable band between 470-800 MHz. Specialized models may cover other frequency bands as well.
With these, the goal is to provide some passive gain to the front of the antenna, about 4-6 dB depending on your operating frequency in relation to the antenna’s covered band. The null to the rear is fairly pronounced, and there is also a strong null directly above and below the antenna. As mentioned above, these are best used when you have a relatively narrow (100 degrees or less) angle of coverage needed, and perhaps a longer distance between antennas and transmitters. Proper vertical orientation with LPDAs is important, as it is for whips and dipoles.
More recently, we have seen the proliferation of circularly polarized antennas from several manufacturers. These are directional antennas but also with the added benefit that they are sensitive to signals at any polarization angle.
It is important to calculate your coax cable losses when considering the design of any antenna system for wireless microphones. In general, the thicker the cable, the less loss. RG-174, which is often used for short jumpers, exhibits a loss of 26 dB/100 ft. at 700 Mhz. Obviously this is not cable you would want to use for anything longer than a foot or two. On the other hand, Belden’s 9913F7 exhibits extremely low loss (3.5 dB/100 ft. at 700 Mhz). Such cable may not be needed for moderate runs (say, 15 – 25 ft), where a moderate cable such as RG-8X is a good compromise, exhibiting loss of 11 dB/100 ft. at 700 Mhz. A better RG-8X cable with a braid over foil double-shielded construction will have even less loss – more like 6 dB/100 ft. at 700 Mhz.
Don’t forget that loss through the air is calculated using the inverse square law, while loss through cable is linear. Thus, what may seem like a good idea (hey – let’s do a 250′ run of 9913 to get the antennas closer to the transmitters) may actually be worse than some other method such as going from dipoles to LPDAs. Not to mention it will cost more, too. See the attached PDF for some examples of air loss vs. cable loss.
So, you’ve got a situation where you need to add an RF amp to the equation. By the way, the general rule of thumb is that you want your antenna gain + RF amp gain – cable loss to have approximately unity gain. The reason for this is that more is not better – wireless mic receivers can be overloaded. Another reason to avoid amplifiers is that every gain stage is a place for intermodulation to occur from signal mixing, resulting in unintended signals and an increased noise floor.
Most often – the only reasons for adding an RF amp into your antenna system is either that you have a long cable run, or that you have loss through passive splitters. In most situations, I’ve found that RF amps aren’t needed. For instance, a church I visited had a pair of our ALP650 amplified antennas feeding 50 ft. of 9917 cable. So the cable only had about 2dB of loss, but they were boosting the signal by 12 dB (!) into the cable! So the receivers had 10 dB more signal than they needed. Had I designed the antenna system for this church, I would have specified dipole antennas and the same low-loss cable, thus saving the customer several hundred dollars on the installation.
Thanks to Henry Cohen for his valuable input on this article. Antenna placement is yet another subject and I’ll cover it in Part 3. Until then, keep up the good work!