Sunday, January 23, 2011
An Unassuming Antenna - The Ferrite Loopstick
Ferrite core antennas, known as loopsticks, are widely used as antennas for radio receivers, particularly for AM broadcast band radios. This is done by winding a coil or loop, generally with tightly-spaced turns over one end of a ferrite rod or bar. The ferrite has the effect of concentrating and intensifying the received magnetic field inside the loop. Like other forms of loop antennas, loopstick antennas are relatively free from RF noise, as they react to the magnetic portion of the RF energy received and are relatively immune to the electrical component, which is prone to noise caused by electrical sources.
Radio waves have both an electrical component (E) and a magnetic component (H). They are 90 degrees in relation to each other and also 90 degrees out of phase. Looking at a signal eminating from a typical vertical AM broadcast tower, the groundwave electrical component (E) is vertical, or perpendicular to the ground like the tower, and the magnetic component (H) is horizontal, or parallel to the ground. Receiving antennas respond mostly to one component or the other: small, closed loop (less than 0.1 wavelength in wire length) antennas to the magnetic component, open-ended wire antennas and large, closed loop antennas to the electrical component.
Antenna polarization is defined by the orientation of the electrical component of the emitted signal. Since AM broadcasters transmit using a vertical antenna causing the electrical component to be vertical, the polarization is called vertical. The near-field wave polarization is vertical and the distant wave polarization generally remains the same out to the limits of the groundwave's propagation, normally 100-150 miles or more.
Since ferrite loopstick antennas respond mostly to the magnetic component of a radio wave, maximum groundwave signal pickup is when the radio is held in the horizontal, or usual position with the loopstick horizontal (parallel) to the ground.
Skywave signals, like those encountered from extremely distant stations during nighttime hours, can have an odd combination of the two polarizations (vertical and horizontal). Multi-path propagation may also be present, where the signal arrives and combines from two slightly different ionospheric directions.
Of course mediumwave DXers know that a ferrite loopstick has roughly a figure-8 pattern, with deep nulls off the ends of the ferrite stick. By rotating the radio with the end pointing to the station, the signal will null. By rotating the radio so the side faces the station, the signal will peak. Different radios, and different ferrite loopsticks, have different nulling and peaking qualities. Some have sharp peaks or nulls, some less defined. The nulls tend to be more pronounced than the peaks.
So, the following question begs an answer: What are we doing when we tilt the radio towards a vertical position, that is, causing the ferrite loopstick to rotate towards vertical - are we nulling the received signal, or just what?
For a groundwave signal, when tilting the radio towards vertical you are lessening the signal pickup of all stations, not just the station tuned to, by reducing the pickup of the magnetic component of all signals. This can be a bonus, sort of a novel way to reduce the RF gain on a receiver. Do you have a strong local that is giving you problems? Tilting the receiver towards vertical may be enough to reduce the overly-strong offending signal so that a weaker signal can be pulled through some distance away in frequency from the local. Just be aware that rotating a ferrite loopstick towards vertical effects all signals across the entire band, not just the signal tuned to.
During nighttime operation, when distant skywave signals may be arriving at slightly elevated angles of arrival or at skewed polarizations, tilting the radio may even increase signal pickup a little, and tend to reduce pickup of local stations.
The modern ferrite loopstick antenna, used in most handheld, portable, and even tabletop radios, is a marvel of antenna science. Measuring from perhaps a little more than an inch in length (as in the Sony SRF-M37V Walkman), to about 200mm (nearly 8 inches) in the large portables, its ferrite rod concentrates the received component of a mediumwave station's broadcast signal to produce such a respectable strength as to rival that of a wire antenna. Additionally, it is highly directional and infinitely steerable in all planes, all in a small enough package to hold in your hand.
Next up in this series: The Twin Coil Antenna Patent