Shure Apps Tech Tip: Correct use of the UA874US Directional Antenna
Recently, a local theatre group was staging a Broadway musical. During technical rehearsals, there were ongoing problems with the wireless microphone system and I was asked to consult with the production crew. As the theatre was near my home, I stopped by one afternoon.
There were ten channels of Shure UHF-R in L3 band, and twelve channels of UHF-R in the H4 band. All receivers were UR4D+ units, and were grouped in their respective frequency bands, i.e., all L3 units in one group and all H4 units in the other group. I was pleased to find that the operating frequencies had been properly coordinated. But then I saw the antenna set-up and immediately knew the source of the nagging problems.
Here is what I found:
Four UA874US antennas located 60 feet from the stage with direct line-of-sight to the stage;
Two antennas for the L3 receivers; two antennas for the H4 receivers;
All antennas mounted vertically;
All mounted side by side with six inches from antenna to antenna;
All four antenna BNC cables were less than 10 feet in length;
All antennas had gain settings of +12 dB;
One antenna had no LED illuminated.
Before I continue, test yourself. There are three major errors in the antenna setup. (Insert "Jeopardy Final Answer" theme music here.)
Time is up. Here are the errors:
Four UA874US antennas located 60 feet from the stage with direct line-of-sight to the stage
OK.
Two antennas for the L3 receivers; two antennas for the H4 receivers
OK.
All mounted vertically
Minor error. The paired antennas (L3 or H4) should form an angle of 90 degrees. For example, one can be vertical and the other horizontal.
All mounted side by side with six inches from antenna to antenna
Major error. The spacing between the paired antennas (L3 or H4) should be at least 24 inches.
All four antenna BNC cables were less than 10 feet in length
OK.
All antennas had gain settings of +12 dB
Major error. The RF amplifier on the UA874US has one purpose: to offset significant signal loss that occurs over a very long cable run. The antenna cables were short; a cable of ten feet might have an insignificant loss of 2 dB. Setting the gain to 12 dB overloaded the antenna inputs of the UR4D+ receivers. Too much gain is worse than not enough gain! Because of the excessive gain, the receivers became "desensitized" and could not operate reliably.
One antenna had no LED illuminated.
Major error. This indicated that the antenna cable was defective as DC power was not being delivered to the UA874US antenna, and a defective cable also meant that the RF signal from this antenna was not being delivered to the UR4D+ receivers.
What was done?
1) The two antennas feeding the L3 receivers were repositioned with 24 inches of separation; the same was done for the two antennas feeding the H4 receivers.
2) The gain settings on all antennas were set to - 6dB.
3) The defective antenna cable was replaced.
And then the wireless systems worked as expected. When setting the amplifier gain on the UA874US, start with the lowest setting of - 6 dB. If the wireless system works and the receivers show an adequate level of received RF signal, there is no need for a higher gain setting. As Ludwig Mies van der Rohe was fond of saying, "Less is more."