Right after Issac Newton's "Mathematical Principles of Natural Philosophy" come "Rik’ART (standing for Terrestrial Radio Antennas in French) Biquad Antenna Principles". Rik’ART system includes four major components:
A) The CPL2X8 is a RF power amplifier and splitter, also named a "coupler". It provides 2 inputs for the antennas, and 2 X 8 outputs, for feeding up to 8 true diversity Radio Mike receivers. Each input is fitted with a 5th order bandpass filter, a high dynamic RF amplifier and an 8 outputs power splitter. The 3rd order intermodulation equivalent input power level is 39 dBm, thus allowing a 80 dB dynamic range for a -1 dBm input power level. Due to this excellent feature, the system offers a great linearity, even with a high level of interference.
B) Two directionnal antennas are connected to the CPL2X8. A passive antenna offering 10 dB gain allows, in theory, to increase the range by a factor of 3 (range is inversely proportionnal to the square of the distance between emitter and receiver). Of course, this antenna can only be used on the receiver side of the Radio Mike system. When connected to the CPL2X8, two antennas therefore allow feeding 8 true diversity RM receivers with the RF signal. The two antennas must be perpendicular to each other, with a 2.3 feet spacing, and 5 feet above the ground. The relative orientation (90°, cross polarized) of the antennas is mandatory to protect the reception of signals within close frequencies or at the same frequency. This technique minimizes mutual interferences, especially when the signals come from the same source but follow a different path ("Fading").
C) A single twin-antenna unit is also provided, when space is at premium, i.e. when the two main biquad antennas are too bulky for the shooting location. This small antenna unit is fitted with a stand and can be set on the CPL2X8.
D) A radio interference analyser (A.I.R). A.I.R is a handy, portable and standalone spectrum analyser which operates in the 470-870 MHz range; it collects measurements of the RF field and displays the frequency bands free from disturbances, according to the frequencies of the Radio Mike systems previously stored. With all RM transmitters turned off, simply connect A.I.R to the CPL2X8 (any output), and get a view at once of the RF interferences on the shooting set. Then accurately rotate the antennas in order to minimize the level of the disturbing frequencies, and therefore to improve RM reception.
Purists can review (in French) their knowledge of electromagnetic waves at http://www.montefiore.ulg.ac.be/~vdh/elen0076-1.html
and get a few reminders about wave propagation.
- Rayleigh Zone: also known as "near field zone"; in this zone, waves propagate without any attenuation, as antenna and medium only exchange reactive power. More at http://master-telecom.unilim.fr/MR2/ouverture.pdf
-Free Field: an area where there is no reflection of any kind, nor any disruptive source. The waves, beyond the Rayleigh Zone, propagate in a straight line. Ground and environment have no influence. Obviously, these conditions are ideal and are never fully satisfied. Generally, an area free of obstacles (trees, poles, buildings...) with a flat ground (ploughed field) is good approximation of a Free Field, as far as there is absolutely no disruptive source in the vicinity.
Let's give an exemple for a disruptive source. Consider this case: A GSM relay antenna radiating 40 W at 900 MHz (+46 dBm) induces a power level of -33 dBm 900 meters away (Free Field condition). An Audio Ltd transmitter (Radio Mike) radiating 40 mW (+16 dBm) with an antenna gain of 0 dB (making abstraction of the attenuation due to the proximity of the human body) induces a power level of -33 dBm at 10 meters. Problems that may arise can easily be figured out when the Radio Mike receiver gets closer to the GSM relay, or when the actor moves further away from the receiver on the sound cart. Luckily enough, in town, the energy radiated from a GSM relay is much less than 40 W, and is futhermore easily absorbed by the surrounding buildings.
- Fading: a more or less fast change of received signal magnitude, usually noted when signals emitted by a single source reach the receiver with a phase shift.
-Range: the distance at which the received signal power equals the noise power (S/N = 0 dB), in the considered frequency band. The term "useful range" is preferred by defining the weakest signal that can still be received (usually 10 dB over the noise).
Note that when it comes to range, the whole system has to be considered : raising any component performance increases the range. RF specialists use a shorcut when they talk of range as a squared distance in a passive system (wave travels one way only), as opposed to range as a quadratic distance in an active system (wave travels and returns) like a radar.
For those reasons, raising the antenna gain by 10 dB increases the range by a factor of 3 because the attenuation of the propagating wave is proportionnal to the square of the distance (20*log(3) = 10 dB).
After several monthes practice and some sound mixers comments, we have adapted the general ergonomics of the system for a mobile use on set that implies quick mounting.
RIK’ART system (to be used without restraint), is provided by DC Audiovisuel with each complete 6-8 radio mike set.
- a CPL2X8 splitter
- two directive antennas with fast plug mounting
- a small monobloc double-antenna
- sixteen push-pull SMA-SMA jacket
- two low dissipation SMA-SMA for big antennas
- a radio frequencies analyzer A.I.R.
The complete set is packed in an easy transportation SACCAR box.
Rik’ART system is also available on sale.