MF/HF Noise and Interference

Elimination of noise and nulling of interference is often a factor in choosing a MF or HF receiving antenna, where the most important criteria is the signal to noise ratio at the receiver.

Receiving antennas can be subject to considerable noise and interference. Atmospheric noise varies throughout the day and the yearly cycle. It also varies with frequency, being of greatest magnitude statistically at the lowest radio frequencies and decreasing as the frequency increases. Interference can come from strong regular radio communications in the direction of the signal you want to receive or closeby to the receiver.

Of course noise causes the greatest trouble at LF/MF frequencies where antennas are normally physically short and working at lower efficiencies, but HF communications can also be seriously affected.

C: Atmospheric Noise, tropical storm centre (night)

B: Atmospheric Noise, temperate zone (night)

A: Atmospheric Noise, temperate zone (day)

At these frequencies, the range of communications is usually determined by the ambient noise level, and the lower the frequency the higher this becomes. During the daytime, the variation is normally in the region of 20 dB, but thunder storms in the vicinity of the receiving station can increase the level to 80 or 100 dB. At night, thunder storms and AM radio broadcast stations can travel great distances via the sky wave. Noise levels are greatest in the tropics.

Choice of antenna can make all the world of difference in dealing with known interference from competing or overpowering signals in the vicinity of the receiving station.

typical loop radiation pattern antenna effect

Loop antennas are designed to null out unwanted signals. The Moonraker BC Loop antenna for AM reception, for example, has a broad maximum signal pick-up pattern in the plane of the loop and two sharp deep nulls broadside to the loop. Reception can be enhanced by placing the interfering signal to coincide with the nulls, so that the wanted signal can be picked up clearly. With this type of antenna, electrostatic shielding is required to ensure minimum interference from nearby objects and to avoid antenna effect which causes the antenna to become unbalanced and act partially as a small vertical antenna, distorting the radiation pattern and filling in the nulls.

The shielding usually takes the form of a tube about the winding made of a conductive but non magnetic material (eg copper/ aluminium). Its purpose is to maintain loop balance with respect to ground by forcing the capacitance between all portions of the loop and ground to be identical.

Because radio signals are made up of electrical and magnetic waves it is also possible to use the magnetic component of the electro magnetic wave with minimum response to the electric component. As most interference is electrical or electrostatic waves, by only responding to the magnetic wave it is possible to reduce noise. Provided that the antenna is well designed and constructed, electrostatic shielding is not normally required. Magnetic loop antennas use this principle and have the advantage of high and low angle reception and elements of both horizontal and vertical polarisation. When the loop has a high Q tuned circuit, it will be very selective in operation giving a high dgree of attenuation to unwanted signals.

Another way to achieve noise reduction is to match the antenna to a coaxial shielded cable by way of a transformer, such as the MRA RX. Using this method, the antenna can be located away from the noise, such as the bow of the ship or on the roof of building.

Obviously, a primary consideration towards improving the signal to noise radio is to locate receiving antennas as far away as possible from man-made, mains wiring or other conductors by which noise range may be extended.

VHF/UHF Reception and Diffraction

In recent years there has been a trend for radio broadcasting stations to leave MF frequencies for the higher VHF frequencies. The move to the FM bands promised better quality broadcasting than was possible on the AM frequencies, and in stereo. However, many listeners have found difficulty in receiving the new FM stations in stereo quality sound and sometimes in receiving the signal at all.

This is due to the nature of VHF direct waves which get shorter, travel shorter distances and are less able to compensate for hilly terrain as the frequency increases. The MF ground wave, by contrast, is much longer, follows the curvature of the earth and is able to broadcast over a much larger and diverse terrain with the added benefit of long distance broadcasting over the skywave at night.

If you live any distance from your local FM stations and there are hills in between, you will probably have experienced reception difficulties. You may also have experienced deterioration in television reception where stations have been required to move from lower VHF to higher UHF frequencies.

Diffraction at VHF frequencies

At VHF frequencies, the direct wave has the ability to bend and scatter through a process known as diffraction , to fill in around corners to some extent. However, as the frequency increases this ability decreases and the size of the shadow zone where no signals reach gets larger.

The result, whether it be radio or television, is that the promised increased quality reception that the higher frequencies offer is only available to those who are close to the transmitter (or relay) or have an uninterrupted line-of-sight reception path.

For cellular phone performance please see Cellular Systems, analogue v digital