Wave Propagation & Antennas
RF gain can only be obtained through Antenna design, no matter what book you read or what RF course you went for this is an undisputed fact. Antenna is a natural organic part of an insect’s body that we humans have adapted in order to transmit Electromagnetic waves.
Electromagnetic Waves: Now I find Electromagnetic rays very fascinating because these two oscillating fields an electric field and a magnetic field have a relationship that’s always 90 degrees apart, unlike acoustic waves that need a medium to travel, EM (Electromagnetic) rays can even travel in ‘ether’ or we earthlings call vacuum at a propagation velocity of the speed of light. No this is not jargon or rocket science, you would have probably learnt about it in high school/college at an elementary level, not knowing the importance of EM in your everyday professional life.
Wave Equation & Losses of EM: The higher the frequency the shorter the wavelength and a standard RF frequency for entertainment RF is around 600Mhz which equates to around 2 Feet, now here is how the wave propagation can get effected that if an object is 2 feet of bigger in the ‘line of sight’ between the transmitting antenna and receiving antenna there will be surely drop-outs not forgetting that a human body that’s primary composition is of salt-water is a large absorber of the of RF energy and averagely reduces the signal strength by 6db (that’s a lot). Interestingly the more the fat in a humans body the greater the loss and this ends about here before I am accused of ‘body shaming’ but if you are facing RF problems and everyone else isn’t probably its time to check your waistline. Love handles don’t do well with RF.
EM also is subjected to ‘inverse square law as soon as it begins radiating into the environment whether is air or free space and metallic objects in its path too cause havoc in the propagation of the waves this is enough to cause an RF shadow and degradation of the waves.
Polarization Of Waves: Polarization is the electric field orientation of an EM wave. A vertically polarized EM wave is produced when the transmitting antenna is perpendicular to the earth and vice versa. The receiving antenna will get the maximum reception if the transmit antenna has the same polarization and if the transmit and receive antenna are orthogonal/90 degrees apart then the losses can be as low as 20db.
Polarization losses are in addition to the environmental and propagation losses cumulative effect is often referred to as ‘Fade’.
Modulation: Modulation is the process of superimposition the information we wish to transmit (in this case audio) on a high frequency carrier for the purpose of efficient wireless transmission. For example an IEM system, the transmitter takes an audio input signal, modulates this onto a high frequency carrier, then via the antenna, radiates the signal as an Electromagnetic wave. The opposite process occurs at the belt- pack receiver; the high frequency Electromagnetic wave is detected by the receive antenna and is demodulated to recover the original audio signal. There are a multitude of modulation schemes available, both analog and digital.
And Now Antennas: Most audio engineers are aware that it is best practice to maintain a clear line of sight between transmit and receive antennas whenever possible; however, there is sometimes confusion regarding what a clear line of sight actually is. A clear line of sight does not just mean the antennas are visible. The electromagnetic wave should be able to propagate from transmit to receive antenna without interference from obstacles. Transparent materials such as glass, Perspex and water should be considered obstacles, as should seemingly transparent structures such as chain link fences and cages. In entertainment audio, helical antennas have been extremely successful as the orientation of the entertainers bodypack or handheld can keep changing and do not adhere to the transmitting and receiving antennas orientation for maximum reception, so the moment the EM wave enters the helical antenna it gets circular polarized and directional as well thus being able to provide effective transmission and reception.
The standard LPDA (Log Periodic Dipole Antenna) a.k.a ‘fish plate’ antenna too used in most productions effectively do their job well to in indoor applications and in studios where you can mount antennas overhead even omnidirectional do an amazing job. Using the general whip antenna are passé’ and expecting to put on a robust RF environment with them is as good as putting the production on peril.
Diversity: In most wireless microphone applications, even small signal dropouts are unacceptable. Multi-path fading typically causes dropouts, rather than weak signal strength caused by too great an operating distance. One method to reduce the likelihood of dropouts is to use a diversity receiver with two antennas. While it is common practice to connect two physically separated antennas to these inputs, there are other antenna designs that achieve the same goal of reducing dropouts.
The Diversity Fin is my personal favourite and a proprietary and innovative antenna design for entertainment production wireless that statistically eliminates the leading cause of dropouts in low-power wireless applications—multi-path interference. Incorporating orthogonally (perpendicular) opposed elements, a single antenna unit can now provide both cross polarization sensitivity and dual feeds for diversity wireless microphone receivers without the need to deploy two physical antennas at a calculated or estimated distance.
If you look at the Azimuth of the Diversity Fin, one antenna is a Log Periodic Dipole Array giving you a good forward gain with high sensitivity and ignoring unwanted information in the rear. Whilst the dipole has a somewhat omnidirectional unity gain and providing reception to signals that have a polarization above 45 degrees that greatly reduces dropouts due to cross-polarization.
Antenna Location & Distance Between Them: In terms of location, receive antennas should not be positioned closer than one wavelength to any reflecting surface. The RF signal detected by the receive antenna is a complex mix of direct and reflected waves, the nature of which is constantly varying as the transmitter moves. Positioning antennas less than one wavelength from reflective surfaces can increase the risk of a dropout due to reflection induced multi-path phase cancellation. Sometimes environmental circumstances dictate antenna positioning and recommendations such as this cannot be adhered to. In these cases, it usually only takes a minor adjustment to antenna location to completely change the system response, so experiment with positioning to achieve best results.
Another common point of confusion relates to the distance at which antennas should be placed apart. For effective diversity performance, antennas should not be positioned less than 1⁄4 wavelength apart, and ideally, they would be placed 1⁄2 to one full wavelength apart. Diversity systems work best if receive antennas are positioned close enough to exist in the same field of RF reflections, but far enough apart that a multipath drop out condition at one antenna is unlikely to simultaneously exist at the other.
RF Boosters, Filters & Accessories: A lot of people think that the name means adding gain however these electronic boosters are merely used to compensate the cable losses, they can never be thought of boosters for antenna gain, that’s simply a misnomer. RF filters are often used to eliminate spectrum into the receiver front end that’s of no use to it other than adding unwanted noise and intermodulation and such practices are never used in India because merely they aren’t known of or available of the shelves. Trajectory of the antenna comes from good positioning and the use of stands and clamps to achieve line of sight is the key to success, however sometimes you do not want it too high as it unnecessarily increases cable length.
Conclusion
Now this is why such a simple practice is so complex because application engineering is very weak and even if anyone has good knowledge he ends up getting absorbed by the manufacturer, which leads me to tell you why we do not have a single RF tech in India, so why hasn’t anyone wanted to pursue a career in RF Engineering, you in all likelihood will have the busiest job in the audio industry in time to come when the spectrum becomes smaller.