Our Repair department occasionally gets reports of performance issues with digital transmitters that are being used in an RF-heavy environment. In almost every case, the complaints were traced back to poorly-designed amplified antennas or antenna distribution systems.
To pinpoint the problem and confirm that the transmitter and receiver are working properly before they are sent to us, we ask the customer to first eliminate their amplified antenna or distribution amplifier and try a whip antenna instead. Full digital systems are particularly vulnerable to overload in the RF signal chain, since the full digital modulation has both FM and AM components. If that RF signal passes through an overloaded amplifier, the AM part of the signal is compressed (clipped) and the AM information is distorted or lost. Antenna systems that may work satisfactorily with FM systems can fall apart when full digital signals are used in the presence of other strong RF signals.
Appropriately designed antenna systems utilize the following three things:
- A band pass filter in front of the RF amplifier to remove intrusive out-of-band signals.
- A high current, high power amplifier in the RF amp stages to handle high RF signals without overload.
- 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 and cable losses.
Knowing these things, why aren’t all antennas created equal? In this instance, it comes down to three cost scenarios that manufacturers have to consider:
- Band pass filters add cost and reduce sensitivity due to signal losses.
- Higher cost of the amplifier plus associated power supply parts.
- A “picking the lesser of two evils” corundum: Low gain amplifiers typically have higher noise figures, but higher noise figures reduce the receiver sensitivity. It is difficult to satisfy both low gain and low noise and still be low cost.
What can you do to best work within a high-RF environment, and – putting cost aside - what should you look for in equipment to use? Five things to consider:
- Buy an antenna or antenna system with the minimum frequency range that will satisfy your needs. A system with a response from DC to light is not going to produce satisfactory performance. Our ALP690 active/passive antennas offer bandpass filtering and attenuation: ALP690
- The addition of a passive filter, such as the PF 25, just after the antenna can make a big difference in your output. This can restrict your frequency of operation, but you 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. These features are a sign that the designer did not take digital and interfering signals into consideration during the design process. Our UMC16B diversity active antenna distributors have extremely high input IP3 of +27dBm: UMC16B
- 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, low cost part. The adage “you get what you pay for” also applies here.
- Antenna gain and amplifier gain are not the same. High gain antennas are always directional and therefore reject background noise and interfering signals. High gain amplifiers are bad except to overcome cable and splitter losses. High gain antennas can be good, provided that they are pointed in the right direction. You 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 today, attenuating your amplifier signal is an excellent way to reduce overloading in subsequent stages of the antenna system or receiver inputs.