Issue link: http://digital.copcomm.com/i/1351516
C A S Q U A R T E R L Y I S P R I N G 2 0 2 1 19 other sites. The excellent Khan Academy site has an amusing short description: tinyurl.com/4a7unla2 The short story here is that if you're getting mixing from two transmitters close to one of your receivers—and transmitting on frequencies close enough to generate a mixing product in the passband of the receiver—moving one twice the distance (which could be inches!) can make a huge difference to reduce or eliminate local mixing and intermodulation distortion in AM and FM systems. Next, there is fundamental overload. That is, a very strong signal close to the receiver that overwhelms the input stage such that it can't filter out all the energy, resulting in the receiver becoming "deaf" to the intended transmission. This usually happens when the signal is close in frequency as well, but sufficient out-of-band power levels can sometimes also wipe out reception. Sharp mechanical filters might help, but often the signal penetrates the case of the receiver itself and winds up directly in transistor junctions. Time to move something! In crowded urban environments, you can experience both intermod and fundamental overload, but the former is more likely. Modern wireless receivers are very sensitive, and a 10 mW signal can be heard for a few hundred yards in a quiet (RF) environment, sometimes with rubber duckie antennas. Most people have elevated dipole or log-periodic directional array (LPDA) antennas, sometimes amplified. These broadband antennas are great for picking up all your radios tuned from 470 MHz-610 MHz. As the first element in your receive chain, they also pick up everything else that appears in that spectrum; both fundamentals and mixing products. Especially with powered antennas, when someone keys a walkie nearby, the active components will start nonlinear mixing with all kinds of signals. Hold that thought. With a multiple receiver setup, there is some kind of antenna distribution network. Some have tuned circuit filters built in to limit all this amplified crap of non- interest, but most do not. You can buy external bandpass filters to place inline, and they help a lot. You might be using an antenna that's too good. Try eliminating the amplifier circuit, lowering the mast, pointing the antenna away from the suspected interference source, or going to a lower gain antenna. After you've done this, then you can raise transmitter power. The idea is to both lower the noise and raise the signal so the receiver can accurately decode the modulation. There are even "doormat" antennas that have a very limited pickup range, popular with installed systems in (RF) noisy environments. With a little extra knowledge, we can successfully overcome some of the things that may interfere with our day. Some best practices to help maximize the signal-to-noise ratio of your system: • Use good, low-loss coax with your antenna • Consider putting a few turns of your coax through a toroid choke • Orient your antenna to minimize pickup from unwanted sources • Make sure your connectors are clean and tight • Use a bandpass filter in front of your antenna distribution system • Powered antennas introduce nonlinear components that can contribute to IMD, avoid them in RF-dense environments • Physically separate transmitters from receivers and from each other • Separate transmitters by frequency to avoid IMD and use frequency-allocating tools • Remember the inverse square law, move interfering sources away Log-periodic directional array (LPDA) antennas from Lectrosonics and Wisycom.