Adjacent Channel Interference is usually caused by one of 4 factors:
1) The receiver's IF bandwidth being higher than what you need to pass your video, plus your highest audio subcarrier. The result is that your receiver actually sees AND AMPLIFIES part of the channel ABOVE and BELOW your assigned channel. This can be cured with our High Selectivity PMI-70 Series SAW (Surface Acoustical Wave) Filter Module, which gives you an additional 60 db of Adjacent Channel Rejection.
2) The interference comes from 2 channels BELOW you in 2 Ghz systems (35 Mhz away). The signal 35 Mhz BELOW you appears as a 35 MHz carrier in the mixer of your receiver, and its 2nd harmonic is 70 MHz, which just happens to be the EXACT CENTER of the IF (Intermediate Frequency) which virtually all receivers use for their amplifier, limiter, and discriminator / detector stages. At 7 GHz and above, the normal channel spacing is 25 MHz. A signal 3 channels UNDER your assigned frequency can produce a 75 MHz signal in the mixer stage of your receiver. This signal falls within the 70 MHz IF passband of your receiver, and can raise the noise floor of your channel, reduce fade margin in STLs, and sometimes can be seen as video or sync running through your video. This can be cured by adding an EXTERNAL (switchable if desired) RF Filter, or by having us re-align or replace your present single channel filter.
3) BRUTE FORCE Signals that overload your receiver. This is most noticeable when you replace your old receiver with a new one. "State of the Art" receivers might handle a -20 dbm signal if it is on your channel, but they cannot handle multiple signals higher than typically -36 dbm without producing intermod (signal mixing products and spurs generated within you receiver that mask weak signals, and can sometimes interfere with audio subcarriers and video). One of the most common occurrences is when an old receiver in a Duplexed STL / TSL (Transmitter and Receiver share the same antenna), is replaced with a new receiver. The new receiver will typically have an LNA stage with 20 to 30 db of gain. If you have -30 dbm of transmitter power (Reflected Power from your antenna, or direct ingress or leakage through the duplexer filters) getting through to your new receiver, you are going to overload your new receiver, reduce your fade margin drastically, and possibly see your own TSL floating through your "ON AIR" or return video. During a minor fade that you would barely notice with your old receiver, you could lose not only your video and audio, but your data / command channel(s) also. This can virtually always be cured by having us retune your duplexer filters for a tighter bandwidth. For ENG Reception, give us a call.
4) If none of the above 3 items apply, OVER DEVIATION or Excessive Transmitter Sidebands caused by the Transmitter on the adjacent channel may be the problem. Wasted power spilling over to the next channel is usually caused by high frequency video components above 4.2 MHz. This not only takes precious power away from YOUR channel, it causes Digital Stream corruption, distortion, sync buzz, and noise in your own AUDIO SUBCARRIERS, and possible DATA CORRUPTION in command / control channels. It also causes annoying phone calls and complaints from users of adjacent channels. Video Modulation should not exceed the standard +/- 4 MHz. This can easily be verified using Bessel Functions. We recommend installing a MATTHEY TBW-446B "Brick Wall Video Filter" (available from TV Equipment Associates, phone: 845-278-0960, ask for JOE) on all ANALOG ENG and STL transmitters, at the video input to the transmitter, after all amplifier and switching stages . This will solve many formerly unexplained ghosts and overshoots in your video, normally caused by the high slew rates of today's modern equipment and video enhancers. This also prevents you from re-broadcasting a 26 MHz IFB carrier from your ENG news van from yourself or someone parked near you, and, along with a proc amp with a good "clipper", reduces "white noise" splatter when incoming carriers are lost in repeater/relay systems. The splatter usually kills subcarrier control channels.
PHILLIPS MICROTECHNOLOGY, INC