Antenna Positioning Online Help
for a MOST COMPACT FREQUENCY SET on a 100kHz grid
1.)The ERP of wireless microphones should not exceed 100 mW in the US
and 50 mW in Europe. These limits prevent the interference with local
TV receivers and reduce the intermodulation of a multi channel WMS.
2.)An amplifying cable would indicate that the dimensioning of the antenna system
is not optimum, because the boosters should only compensate the cable loss. Therefore
do not amplify the received signal more than necessary, because this will only
enhance the non linear distortion; respectively reduce the system Input Intercept Point (IIP3).
3.)The range of the squelch threshold lies usually beween -100 and -70 dBm.
If the WMS operates in a noisy environment (e.g. caused by a local TV transmitter,
adjust the squelch threshold to a minimum of -80 dBm).
4.)Use an antenna with less directivity respectively less passive gain.
5.)Transmitter intermodulation occurs between two transmitters TX1
and TX2 if the RF signal which is emitted by TX1 is received and
retransmitted on a new carrier frequency by TX2.
The attenuation which the incoming RF signal suffers before it is
retransmitted defines the transmitter intermodulation and is called
TRANSMITTER INTERMODULATION ATTENUATION because the intermodulating
transmitter behaves like a linear attenuator. Unlike receiver intermodulation
its power level versus range follows only a square law. Therefore the mutual
separation of intermodulating transmitters is not as effective as separating
a transmitter pair from an intermodulating receiver; because in this case the
receiver intermodulation decays with the 6th power of the distance between
the transmitter and the receiver antenna.
So the receiver intermodulation has the tendency to be most disturbing in the front
of the stage, while the tranmitter intermodulation prevails in the rear of the stage.
5. Remedies for transmitter intermodulation
a. Use a transmitter with less ERP.
b. Increase the minimum spacing between intermodulating transmitters. (e.g. 50 cm)
c. Try to increase the distance between the antennas and the stage.
(This measure is of course a compromise, because it will reduce the
immunity of the system to external interference.
NOTE: One needs a Faraday cage to avoid external interference from a near
TV transmitter.
But their is no remedy against badly shielded computers or strong signals
from near mobile phones operating with full power..
6. Remedies for a weak signal:
a. Increase the ERP
b. Increase the antenna gain.
c. Lower the squelch threshold, if the background noise ( e.g. caused by
a local TV transmitter) allows it.
d. Reduce the used stage size.
Note: One needs a Faraday cage to be free of external interference
like TV signals or the RF emission of unshielded digital devices not to
speak of mobile phones and walky talkies.
7. Remedies to reduce the third order receiver intermodulation :
a. Reduce the ERP
b. Insert attenuators between the antennas and the RX (E.g. 6 dB attenuators).
14. The antennas can also be installed above the stage.
Vertically polarized Antennas which should be mounted at least 0.5 m
below the ceiling have an additional advantage:
Due to a notch in the radiation pattern in the vertical direction,
the necessary height of the antenna above the stage can be reduced
to 75% of its equivalent minimum horizontal antenna/stage distance
while maintaining a satisfactory coverage for the rest of the stage.
Principally one can position the antennas anywhere between the minimum
and maximum distance.
If there exists the threat of external interference , the optimum antenna/stage
distance is the minimum distance.
18. The range is the maximum allowable spacing between the transmitters and
the receiving antennas.
To increase the range respectively coverage :
a. increase the ERP
b. use an antenna with more gain (e,g. a yagi antenna)
c. reduce the cable loss.(Actually the gain in a welldesigned antenna system
should add up to zero for each individual cable section.)
d. reduce the squelch threshold if the background is free of interference
(E.g. adjust the squelch threshold to -100 dBm).
LIST OF THE HIDDEN (FIXED) PARAMETERS :
to cover all situations, some fixed parameters are worst case values.
frequency range: 700-900 MHz (max. error 2 dB)
Audio S/N: 50 dBa (100 dBa companded).
Outage: 0.01 % This is the percentage of the total time the S/N < 100 dBa.
IP3: 0 dBm Third order input intercept point.
Transmitter Intermodulation Attenuation: 67 dB (for 50mW ERP).
CAUTION: ETSI adds a fixed valueof 30 db. So the actual attenuation from the
transmission antenna through the transmitter back to the transmission antenna is
is for the PT/HT4000 27 dB only.
The minimum carrier/intermodulation frequency distance: 100 KHz.
Protection ratio: 7 dB for 100 kHz intermod/carrier spacing
and an intermodulation deviation of 100 kHz caused by a 1kHz sine wave modulation.
The protection ratio is a weighting function for an adjacent channel or the
intermodulation and depends on the frequency spacing between the offender and
the victim carrier; whereby the victim is the wanted RF signal and the
offender is the adjacent channel or the intermodulation. The adjacent
channel signal and the intermodulation differ in their spectral width.
The spectral width of the intermodulation is higher than the spectral
width of an adjacent channel because if a signal is processed by
a nonlinearity its spectral width is multiplied by the order of the
nonlinearity. Respectively the FM deviation is multiplied by the order of the
nonlinearity. The assumed protection ratio of 30 dB (100 kHz from
the carrier) is rather high and lowers the immunity of the
receiver against intermodulation considerably. Therefore the minimum
carrier/intermodulation distance depends on the frequency resource and
should be made as high as possible. But even the worst receiver does not
need more spacing than 300 kHz.
____________________________________
This program has been written to demonstrate, that the optimum antenna
position depends on the stage dimensions and the WMS specifications.
The worst case assumption of 10 dB shadowing allows us to neglect the room geometry and
the effects of reflecting or absorbing objects.
Further this program demonstrates that the transmitter intermodulation is a
decisive mechanism.
SF 021104