First Look: Shure UA874US Antenna

When designing receiver antenna systems for radio mics, there are times when too much RF entering the system can be just as problematic as too little.

That's why the Shure UA874US is such a useful new antenna. It's an active sharkfin antenna with switchable gain and attenuation, plus an overload LED to let you know if it's receiving too much RF energy.

There's one primary reason to add gain to the RF signal at the antenna - to overcome the loss of cable and splitter/combiners that are downstream, but what are the reasons for attenuation at the antenna?

The answer to that is a little more complicated:

With radio mics, the desired signal is often relatively weak compared to the strength of the undesired RF signals - I'm thinking specifically of 100 mW wireless mic signals compared to the 1,000,000W TV transmitters found in cities.

A traditional active sharkfin antenna by design has about 8 dB of forward gain - meaning it's almost 3 times as sensitive to RF signals on axis to the front of the antenna than its rear. Because the sharkfin has a wide pass band, all signals between 470-698 MHz are passed to the amplifier - including the very strong RF signals from broadcasters that may be present. which can cause an overload condition in the amplifier of the antenna.

An audio engineer will likely hear this resulting in drop outs. Thinking their receivers are experiencing drop outs from too little RF signal, the audio engineer might increase the gain of the amplifier within the antenna, thus exacerbating rather than solving the problem.

By combining an overload LED with switchable gain or attenuation, the UA874US solves this problem in the following ways:

• RF overload LED. Now an engineer will know definitively if this first amplifier is being overloaded.
  
  
• Switchable attenuation prevents RF overload at the amplifier. Combined with monitoring the RF level at the receiver, the engineer can dial in exactly the correct amount of attenuation.
  
  
• Optimized gain and polar pattern. By using the -6 dB attenuation setting, the engineer can negate any forward gain (thus turning it into the same sensitivity as an omni antenna) BUT with 6 dB of rejection from the rear of the antenna. An engineer can now orient the front of the antenna toward the desired signal and the rear of the antenna toward undesired high powered signals.

The antenna requires bias power of 12V, supplied from many receivers, active splitters, or a Bias-T, and is in stock at a price of $299.95.

- Peter Schneider