QRM Elimination by Antenna Phasing and Noise Cancelling

QRM Elimination by Antenna Phasing and Noise Cancelling

Though the original meaning of QRM was “Are you being interfered with” and QRN was “Are the atmospherics strong”, in current day use in Amateur Radio, QRM indicates man-made noise and QRN indicates noise from natural sources. So noises from nearby solar inverters and other devices which can emit radio frequency will be called as QRM. In modern cities with so many electronic and electrical devices operating in nearby buildings, QRM is part and parcel of amateur radio operation. Another situation when you need QRM elmination is when you wish to work a DX station operating near a much stronger nearby station. There are several commercial devices which allow us to do this by using two antennas and phasing the signals from these two to cancel out the QRM, whether it be the strong station or the noise from devices.

The two antennas need not be of the same type and even the feeder cable need not be of the same type. As any radio amateur operator is aware, it is easy to get a 75 Ohms television cable from the local market while you have to order a 50 Ohms cable needed for your amateur radio, and it is usually more expensive. Hence the option of using 75 Ohms television cable for the auxilary antenna is quite comfortable. The QRM elimination devices have matching modules to match the 75 Ohms cable. The signals from the two antennas are used to cancel out the signals from the strong station or the noise from the interfering device by adjusting the controls on the QRM elmination device. Usual upper range of operation is 30 MHz and hence suitable for HF operations and not VHF or UHF operations.

The QRM elimination device does this by altering the phase of the signal from the two received antennas so that it cancels out. It can be imagined as if you are electronically rotating one antenna in the opposite direction of the stronger signal and the other antenna in the direction of the weaker signal so that the weaker signal is boosted and stonger signal is reduced. Such a system can be a boon in city dwellings with a lot of QRM around. But the down side is that the expensive ones may cost you as much as your radio itself! Another advantage is while working DX contests. Picking up and working rare and weak DX stations can act as a multiplier in the contest.

There is an important precaution while using antenna phasing device for QRM elmination or noise cancellation. A signal from the PTT or push to talk switch of the radio, has to reach the QRM elimination device to bypass it to the transmit antenna, also called main antenna, during transmission. Otherwise the very strong input can damage the very sensitive receive circuits of the QRM elimination device. Remember that there are pre-amplifiers there to pick up weak signals to enhance DX reception. Some protection may be offered by inbuilt input protection circuitry, but the safe way is to bypass the QRM elimination device during transmit cycle.