RF 201 – Antennas, Distro, and More!

We covered the basics and theory of RF (radio frequency) mics and in-ear systems in RF 101 – The Basics of Wireless Mics and In-ear Systems. In this article, we’ll dive deeper and talk about products you can use to improve your system. These tools can be critical as we deploy more channels of wireless in less available space. Thanks to the FCC’s sell-off of the 600MHz spectrum, we need all the help we can get.
Antennas
If you’re using wireless already and have all your systems sitting in a rack with the included antennas poking out the back, you may have gotten grief about that. Even if it’s working!
The truth is, sometimes you don’t have a choice! We need to keep in mind that wireless is a pass-fail system. If an RF signal on nearly the same frequency arrives at your antenna at a hotter level than the transmit signal from your gear, your receiver will switch to that signal. That’s when things get ugly and noisy.

Antennas are transducers, which is just a fancy way of saying they convert a signal from one type of energy to another type of energy. Just as speakers (and microphones) are transducers that convert electrical energy to acoustic energy (and vice versa), antennas convert electromagnetic energy (as radio waves) into electrical energy. In fact, it can be helpful to think of antennas as the speaker and microphones of our RF systems. Some of the concepts that apply to audio apply equally to RF!

We covered antenna approaches in the article How to Improve Wireless Performance Using Antennas, which offers additional information.

Antennas that are designed to be remotely mounted fall into two broad categories: omni and directional. The Sennheiser A1031-U is an example of an omni antenna, but it is important to understand that the phrase “omni” has a different meaning when it comes to antennas. They are only omnidirectional in a single plane.
Figure 1: Omni antennas have almost no coverage directly above and below.
This antenna and those built into microphones, bodypacks, and rackmount receivers actually have a doughnut-shaped pattern, which means it matters how you orient them. This lack of coverage from above and below is also true of directional antennas, but it is less expected on the omni, so it is worth keeping in mind.
This is one of the reasons that we recommend orienting dual-diversity antennas at a 90-degree angle to each other. This helps make sure that you don’t get transmit and receive antennas in each other’s dead spots. RF reflections help combat this indoors, but it can be problematic outside.
Directional antennas can be particularly useful when trying to focus on an RF source that is coming from a specific direction or trying to avoid interference from an identified source. LED lights make lots of noise in the UHF range. Knowing that an antenna has locations where it is less sensitive — along the sides and the top and bottom — can help you aim the antennas at the transmitters you want to lock onto and exclude the interfering frequencies that you are trying to avoid.
Special-case Antennas

Circumpolar antennas like the RF Venue CP Beam and the Shure Helical feature spiraling antennas under those covers that accomplish two things: A) tighter pickup patterns and B) antennas that are always in polarity with their complementary antenna. This makes them ideal choices for IEM applications, because bodypack receivers typically have one antenna, making them non-diversity. They are also on the move with the musician, so the antenna is constantly changing its position. The corkscrew shape of the helical antenna helps keep the antennas in polarity with each other for better reception.
Figure 2: A clear helical makes it easier to see the spiral on the antenna.
Another clever antenna is the RF Venue Spotlight. The Spotlight is intended for use in harsh RF environments to make the inverse square law work for you instead of against you. This antenna can go under a carpet or stage so that the performer is literally on top of the antenna — perfect for when the LCD wall is wreaking havoc with your wireless systems.
Figure 3: The RF Venue Spotlight is a clever antenna design that can mount on or under a stage.
Active Versus Passive
Keep in mind that “active antenna” is a bit of a misnomer. This is simply an antenna with an amplifier strapped onto it. This is not for better reception but rather to overcome signal loss in long RF cable runs. An active antenna can actually be worse in short runs (less than 25 feet of antenna cable), because the signal could overload your wireless receiver. Also, a passive antenna like the RF Venue Diversity Fin or the Sennheiser A2003 can be used as a transmit or receive antenna, making those far more flexible.
Distro and Combiners
These terms tend to get thrown around freely when talking wireless, so it is worth defining them. An antenna distribution system (distro) allows you to share one pair of diversity antennas with multiple wireless receivers. An RF combiner takes multiple RF signals and mixes them so you can transmit them on one single antenna or RF output. This could be for multiple IEM units to share a single antenna or to combine the output of two antennas that are in different locations.
With an antenna distribution unit, you are getting two main benefits. First, you can share a better pair of antennas across a bunch of wireless receivers. Moving a pair of antennas closer to the stage is far easier than moving a rack of 16 wireless receivers closer to the stage. Secondly, a distro unit can help you avoid the dreaded antenna farm.
Figure 4: An antenna farm — photo courtesy of RF Venue.
Not only is this ugly, but it can also impact the wireless performance because it is impossible to get the antennas into good orientation, and letting antennas touch each other degrades performance. Also, having this many antennas close to each other causes interactions that can change the directional characteristics and sensitivity of the antennas.
Combiners can be the most important pieces of equipment in an RF system. I often see IEM transmitters and wireless receivers in the same rack. This is terribly bad RF hygiene. Transmit antennas that are close to each other create much more interference than those that are separated, but even if you have only one IEM in the rack, it is lowering the efficiency of all the receivers it is close to by overloading their inputs.
A combiner like the Sennheiser AC 41 is expensive because combining high-level RF signals requires careful engineering. By doing this, you will greatly lower the RF distortion between IEM units; getting the antenna out of the rack and at least 10 feet away from the receivers will cause your whole system to work better and be less prone to dropouts.
Tools for Assessing the Danger
One of the most powerful tools you have for troubleshooting your system is the front panel of the wireless receiver. Any quality piece of wireless should show you two things on its front panel: RF level and which antenna is active in a diversity system.
During operation, the antenna indicators (labeled A and B on the Shure ULX-D) should be switching back and forth. If they are not, you may have a problem with your antennas or their placement. If you have an entire rack of wireless systems that are always on the A antenna, you have a serious problem.
Figure 5: The Shure ULX-D has great front panel feedback.
Those RF meters are incredibly useful as well. You should always turn your transmitters off if you are having RF issues. If you see lots of activity on these meters when its paired transmitter is off, then you likely have something on a near frequency that is interfering. It is time to find a new channel to set this system to.
Free software tools like Shure’s Wireless Workbench and Sennheiser’s Wireless Systems Manager (WSM) allow sophisticated management and coordination of wireless systems. They even allow you to use connected wireless receivers to scan for competition in your facility.
If speed is the critical issue, then a dedicated RF Scanner like the RF Venue RF Explorer is a must-have tool. It can continually scan for interference across the entire legal bandwidth and be connected to software tools to export those scans into Workbench and WSM. It is faster than a scan from a wireless mic and doesn’t require taking a wireless microphone off-line to accomplish the scan.
There are myriad things that can cause wireless to have problems. If you are having issues that you need to address, give your Sweetwater Sales Engineer or our Tech Support a call at (800) 222-4700.
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