RF Antenna Placement
By Andrew Papastephanou
Part 1 - Spacing, Antenna Size and Diversity:
At an average RF value of 550 MHz in the UHF band, the wavelength is around 22 inches. Shure recommends spacing their rf antennas about 1/4 to one wavelength apart with no perceived additional benefit over one wavelength regarding diversity reception. For example, that means at 550 MHz you can have your antennas as close as 5.5 inches together without any issues. Officially they suggest somewhere between 6-24 inches which also covers lower range frequencies.
I should point out that the most important aspect of spacing to me is diversity. I will note however that when antennas are too close to each other, they will start to interfere and warp their receptions patterns. This is a big problem when getting your antennas too close. Once you get past that 1/4 wave you’re out of that danger zone. A lot of people make the mistake of stacking RF receivers on top of each other with all the whips being near each other. You’ll actually get better results spacing them horizontally from each other or using an antenna combiner if they’re in a rack.
Here is an excerpt from Shure’s RF manual where they actually say you can go as low as4 inches but they are also referring to RF in the 700 MHz band in that case:
"Diversity refers to the general principle of using multiple (usually two) antennas to take advantage of the very low probability of simultaneous dropouts at two different antenna locations. "Different" means that the signals are statistically independent at each location. This is also sometimes called "space diversity," referring to the space between the antennas.
For radio waves, this "de-correlation" is a function of wavelength: a separation of one wavelength results in nearly complete de-correlation. In most cases, at least one-quarter wavelength separation between antennas is necessary for significant diversity effect: about 40 cm for VHF systems and about 10 cm for UHF systems. Some increased benefit may be had by greater separation, up to one wavelength. Spacing beyond one wavelength does not significantly improve diversity performance, but large or unusually shaped areas may be covered with greater antenna separation.”
With a 6” wide stereo bar + a pair of M2’s actually extending the spacing about another two inches you are well within the recommended “significant diversity effect” sweet spot. If you wanted to, another option would be to set up a full wave spacing by placing your paddles on an O2/O2X with two M2’s. You’d want to space the paddles about two feet apart in that case.
Choosing between different spacingsand techniques will vary based on your location and intended application. For the application I’m using the T-Bar + M2’s on (TV Studio interview on a couch), I’m getting 95-100% reception at all times from both paddles.
Part 2 - RF & Axis:
Unlike audio, radio waves travel on two different axis. One for the electric field and one for the magnetic field. One factor in RF dropouts can be when a microphone’s transmitter is in a position where the transmitting antenna is perfectly misaligned with the receive antenna. People found that if you offset two receive antennas from each other at 90 degrees, it could help reduce this potential area of RF dropout since at least one antenna would be on axis even if the transmitter was held perfectly out of phase of the other. This is why the paddles are 90 degrees offset of each other. I have confirmed this in person with Shure and they agree that it is a good technique. I’m not saying that it is the ONLY technique. Other manufacturers also urge their users to do this practice.
There is another antenna design called helical where they run the antenna wire in a spiral pattern down a tube. This is used to reduce the chances of a mic being off axis of the receiver but also tends to create a very focused field (which is very good in most applications but not all). The downside is that helicals are larger antennas and from what I understand you can actually get wider coverage with a cardiod style log-periodic antenna.
Part 3 - Amplitude:
One other factor that usually kills RF before you get anywhere near 1/4- full wave spacing is amplitude. People tend to overpower their paddles. This can potentially amplify the noise as much as it does the RF signal. I actually have a pair of active paddles with 0db boost in the other location specifically because I know that I’m very close on one paddle and my other one is so high up that I’m constantly in line of sight of the artist.
So what am I getting with the picture that I sent over?
- I’m getting excellent range and reception due to the nature of the log-periodic paddles (even though they are passive) because of their height and line of site to the interview couch which is 25 feet away. That is helped with the small but heavy T1 base and OA extensions.
- I’m getting excellent reception of my transmitters regardless of what axis they are on due to the 90 degree offset from the M2’s.
- I’m getting excellent diversity due to my larger than 1/4 wave spacing provided by the T-Bar.
- I’m getting a system that performs excellently and is both portable and compact. I could make it even smaller if I installed it in the ceiling with an IO-C!
Would I use it every time? Honestly no. In fact I’m using a totally different design I worked out with the guys at Shure that throws the whole wave spacing out the window at the other location. Instead it leans more on having each paddle be superior in coverage of different areas. When the artist is on stage there is a paddle downstage right that shoots across the stage. Meanwhile I’ve installed the second paddle about 20 feet up in the lighting and that one is aimed downstage and a little into the audience. The benefit is that while the stage level one covers the entire stage and hallway, the second one covers downstage and all the way to the front of house position. Diversity chooses which one is the best and because they’re are at different angles, the off axis situation takes care of itself.
If you have to set up a tight paddle system, the picture I showed you is one of the best ways to do it. It is not the ONLY way. For this application I would pick this almost every time.