There are several items to look at, starting first with the SIFM program:
1) Are the parameters correct? Specifically, how far away from tuned
frequencies are 3rd and 5th order products being restricted - A minimum of
100KHz for 3rds and 75KHz for 5ths is recommended to start. Have *all*
wireless devices in the same general spectrum present in the
venue been
entered into SIFM (other mics, coms, IEMs and IFBs)? Are the chosen
bandsplits correct?
2) Calculating in analog TV is one thing; did you consider the pilot tone of
any strong nearby digital TV stations (.310MHz above the
channel's lower frequency
edge)?
3) Did you check the FCC website to confirm all analog and digital TV
stations that might be problematic for your location? Remember that several
DTV stations surrounding your bandsplits raises the overall RF noise floor.
As to the equipment itself:
4) In order to simulate as close as possible to 'real world' intermodulation
interference, spread out all your transmitters within the performance area,
keeping them separated by at least two feet, and turn them all on. Now turn
one off and look at it's receiver; does the RF signal indicator show a
complete loss of RF or at least an energy
level below the
squelch threshold?
If yes, good; turn the transmitter back on, go to the next one and repeat
the process for all transmitter/receiver combinations. If no, there is then
an intermod issue, a faulty/misaligned receiver front end or one of the
transmitters still on is faulty.
5) You indicate the "The mics are right in front of the
stage, which is
elevated about 3 feet, so I'd say they have a pretty
clear line of sight,
and the distance from the receivers to the transmitters is never more than
maybe 30 or 35 feet": Are you using individual antennas on each receiver or
an antenna multicoupler(s) with remote antennas? If the former, ensure both
antennas of each receiver
face the performance area and the receivers are
separated enough so that each pair of receiver antennas are at least one
wave length (of the lowest frequency used) apart. If the latter, What kind
of antennas are used? In this case, directional *UN-amplified* log
periodic, band limited 'paddles' would be the best option.
6) If using directional antennas, where are those antennas facing in
relation to TV broadcast towers?
7) Do the "
drop outs" consistently occur at the same
point in the production
or is it random?
Drop outs occurring at the same
point in the production may
indicate that performers in front are physically
blocking upstage
transmitters.
8) Where are the transmitters located on the performers; are the performers
wearing any metal (or have any implanted)? Are antennas straight and
vertical? Are the antennas coming in direct contact with skin?
Lastly, a simple check that all TX and RX electronics, antennas,
coax and
connections is in proper working order is an often overlooked part wireless
woes.
Henry Cohen
Production Radio Rentals