Sunday, January 30, 2011

The Twin Coil Antenna Patent

Let's continue our discussion of the ferrite loopstick, a most interesting device.

A curious animal appeared on the radio scene in March of 2003. It was called the Twin Coil Antenna. Its inventor was one Christopher Justice of the C. Crane Company.

A July 2001 application for United States Patent number 6529169, "Twin Coil Antenna", shows an "antenna for a radio receiver comprising a ferrite rod; spaced coils wound about the ferrite rod; a variable capacitor element connecting each coil together; and a combiner connecting each coil together." It was a souped-up ferrite loopstick.

In analyzing the standard tuned ferrite rod utilizing a single coil, Justice determined there is a "need for improved efficiency and a higher signal-to-noise ratio antenna that does not saturate at high frequencies", and it was to these ends that his experiments were directed.

Ferrite core antennas 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.

Unfortunately, the ferrite core tends to absorb some of the signal power. Additionally, the combined resistance or impedance of the antenna and ferrite core is typically just a few ohms or less. In operation, the antenna must be coupled to the larger input impedance of the receiver. This is generally accomplished by adding a capacitor to tune the ferrite loop to a certain frequency, creating a resonant circuit.

When a signal enters a ferrite rod antenna that is tuned to resonance with a coil and capacitor, the magnetic lines of flux begin to saturate. When this saturation occurs, the ferrite rod antenna takes on polarity, much like a magnet does. Ferrite antennas having only one pick-up coil result in loss of efficiency and limit the signal-to-noise ratio available to the receiver.

In the words of the patent application, "The invention provides an antenna for a radio receiver that has high efficiency, high signal-to-noise ratio and that does not saturate at high frequencies. This is accomplished by an antenna structure which employs a ferrite core having two (or more) coils coupled together and located on the ferrite core such that the magnetic fields coupled to the coils induce signals which combine to produce a resulting signal level equivalent to the combined signals in the coils. The coils may be coupled through a transformer where the combined signals of the coils are received in the primary of the transformer. The transformer coupling the antenna coils can dramatically narrow the bandwidth of the received signal. In addition, the antenna coils and the transformer windings may be connected to a capacitor to form a resonant circuit. The capacitor may be variable so the resonant frequency of the antenna may be set by tuning the capacitor. This results in an increased signal level and reduced interference and noise."

Read more about this fascinating antenna device on Google Documents' Twin Coil Ferrite AM Antenna. Justice's patent was accepted and issued on March 4, 2003.

CCrane sells their version of the Twin Coil Antenna on their website.

Next up in this series: The Vertically-Challenged Ferrite Loopstick

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

Sunday, January 2, 2011

New Years 2011 Mediumwave Daytime DX

Haven't gotten the Sony 2010 out in awhile, so charged a fresh set of batteries and went on a little expedition out in the desert about 1300L yesterday. The sun was high, the temperatures cold for southwestern Arizona - 40 degrees during mid-day. I picked a nice hilltop site and commenced scanning some of the weaker channels. Along with me was a modified, spiral-wound 24-inch passive loop to aid in the DXing.

1315L KMZQ-670 Las Vegas, NV (30KW) 197.9 miles

KMZQ's three tower array has a pattern to the northwest, so pumps most of its signal away from me. Only about 12KW comes this way.

1327L KKOH-780 Reno, NV (50KW) 517.0 miles

Nice signal for this distance, strongly competing with KAZM-780 out of Sedona, AZ (5KW, 160.4 miles). KKOH runs a single tower with omni-directional pattern.

1334L KNWZ-970 Coachella, CA (5KW) 110.8 miles

KNWZ's three tower array has a pattern to the southwest, and I am in its null. Weak. Only about 1600 watts comes this way.

1347L KHQN-1480 Spanish Fork, UT (1KW) 464.7 miles

Weak, but a great catch for 1KW at that distance, the best of the day. KHQN runs a single tower with omni-directional pattern. Well under Phoenix's KPHX-1480 (5KW, 123.7 miles)