It’s no secret that today’s environment is RF-heavy. Between cell phone traffic and various signals from the surrounding area, it’s a wonder that there is any space at all left for wireless audio to work in. Or, we might find free space, but the strength of our signal might be inconsistent or noisy.
It is tempting to amplify our RF on the receiving antenna side when we are not getting the desired signal. Instead, we need to look at balancing our signal to noise (S/N) ratio. We want our desired signal to be significantly stronger than background noise or interference but not so strong that it overloads the receiver. One way to control gain in congested RF environments is through an LPDA antenna (Log Periodic Dipole Array), also known as a shark fin, batwing or paddle antenna, such as our active/passive ALP690. LPDA antennas usually cover a wide range of frequencies yet have a directional pattern enabling us to reduce the impact of off-axis RF sources. LPDAs passively boost on-axis signals as well, which may also overload receiver inputs and/or bring up the RF noise floor. The ALP690 has a built-in amplifier with selectable gain and filtering so it can be set to boost or reduce the signal strength according to the gain structure needs of the system, largely dependent on the length and type of coax cable being used for the antenna run.
The ALP690 antenna is powered by DC bias inserted on the coaxial cable connected to the 50 ohm BNC jack. This power can be supplied by a Venue Series or DSQD receiver, an active multicoupler or an inline BIAST. When no bias is applied, the ALP690 automatically switches via relays to function as a passive antenna, so that the antenna can be used for receivers as well as IFB or IEM transmission applications. Please note that when powered, this antenna can only be used for receivers.
System Design
The best approach in pre-planning our environment is to look at our antenna, such as our ALP690, as part of a system. As we mentioned in Wire List #52, appropriately designed antenna systems utilize the following three things:
- A high current, high power amplifier in the RF amp stages to handle high RF signals without overload.
- A band pass filter in front of the RF amplifier to remove intrusive out-of-band signals.
- Low gain amplifiers in the RF amp stages to keep from overloading the output of that same amplifier, and only enough gain to overcome splitter and cable losses.
Another way to improve S/N ratio is by putting the antennas closer to our talent. We will have to use longer cables and potentially will lose signal strength, but the ALP690 can be set to boost the signal strength to compensate without reducing the S/N ratio.
Equipment Considerations
Putting cost (which is an issue for most of us) aside - what should we look for in equipment to use in high RF-environments along with the ALP690? Five things to consider:
- Buy an antenna or antenna system with the minimum frequency range that will satisfy our needs. A system with a response from DC to light brings in certain compromises.
- The addition of a passive filter (like our PF25) just after the antenna can make a big difference in receiver performance. Although this can restrict our frequencies of operation, we can compensate by substituting filters for different locations. We discuss using filters to improve signal to noise ratios with the PF25 in Wire-List #31.
- Avoid low-current/low-power systems, as they will not be adequate for today's crowded spectrum. Similarly, beware of systems that don't quote a third order intercept (IP3) figure. A lack of this specification is a sign that the designer did not take digital and interfering signals into consideration during the design process. Our UMC16B diversity active antenna distributor and our ALP690 both have an extremely high input IP3 of +27dBm.
- If it sounds too good to be true, it probably is. Systems that quote incredible noise figure specs like 0.8 dB or 1 dB are probably built around a high gain, low current (and low cost) part. The adage “we get what we pay for” also applies here.
- Antenna gain and amplifier gain are not the same. High gain amplifiers are not the right choice, except to overcome cable and splitter losses. These days, there are usually better solutions with proper antenna placement, and either shorter cable runs or expensive, low-loss coax. High gain antennas are always directional and therefore reject background noise and interfering signals, and they can be good, provided that they are pointed in the right direction (with the nulls aimed at the RF noise sources). We can help the situation by only using enough amplifier gain to overcome cable or splitter losses. Don't turn up the gain of an amplified antenna to greater than that necessary to overcome cable and splitter losses. Excess gain will also upset the automatic gain leveling stages that exist in most digital receivers. Gain in an antenna amplifier increases not only the desired signal but also noise and any interfering signals. Often times, attenuating our amplifier signal is an excellent way to reduce overloading in subsequent stages of the antenna system or receiver inputs. An overall antenna system “throughput” gain of anywhere from 0dB (unity gain) to - 6dB should work very well. More gain than this, or more loss than this will likely be problematic.
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