The Log Periodic Antenna:
A log periodic antenna is a multi element directional antenna which operates over a wide range of frequencies. It was invented in 1952 by John Dunlavy. It consists of a series of dipole elements arranged in a specific geometric pattern. A Yagi aerial has a driven element (a dipole) a reflector and several directors. In a log periodic, all the elements are driven. Each one is a dipole.
The antenna works by taking advantage of the properties of the dipole elements. Each element is a half-wave resonator for a particular frequency. The dipole elements are arranged in a geometric pattern where the length of each element decreases as you move toward the end of the antenna. This dipole arrangement creates a series of resonant frequencies that overlap. As the frequency changes, different elements become resonant which allows the antenna to operate over a wide range of frequencies.
As a signal is received or transmitted by the log periodic antenna, it is captured by the dipole elements that are resonant for that frequency. The signal is then combined and sent to the receiver or transmitter through the feedline.
One of the advantages of the log periodic antenna is its directional properties, which allow it to focus the signal in a specific direction. One of the key advantages of the LPA is its wideband performance, which means it can operate over a broad frequency range with a constant gain and impedance.
Further Info
A log periodic antenna is a directional antenna that works over a very wide frequency range, often a ratio of 2:1 or more, with fairly constant gain, pattern and feed impedance across the whole span. The most common form is the log periodic dipole array (LPDA): a row of dipoles of steadily decreasing length, fed from a common transmission line.
How it works
At any given frequency, only the few elements that are close to a half-wavelength long do most of the work. This group is called the active region. As the frequency rises, the active region slides forward towards the short elements; as it falls, it slides back towards the long ones. The rest of the elements stay largely inert, so the antenna behaves like a small Yagi that moves along the boom to suit the frequency.
The feed is the clever part. The elements connect to a two-wire feeder (or a pair of square boom tubes), and the connection is crossed between each adjacent pair, giving a 180° phase reversal. Power is fed in at the short (front) end. The result is that radiation from the active region adds up in the direction of the short elements, so the antenna fires towards its narrow end. Many builds add a short stub or termination behind the longest element to absorb any energy that reaches the back.
Performance
- Bandwidth - set by the longest and shortest elements. The longest should be a little over a half-wave at the lowest frequency, and the shortest a little under a half-wave at the highest, so the active region never runs off either end.
- Gain - modest for the size. Practical amateur LPDAs give roughly 5 to 9 dBi (about 3 to 7 dBd), depending on τ and σ. Gain stays fairly flat across the band rather than peaking at one spot.
- Pattern - a single main lobe towards the short end, with typical front-to-back ratios of 15 to 20 dB. The pattern is broad enough to make rotator pointing forgiving.
- Impedance - the feeder impedance and element diameters set the input impedance, usually somewhere between 50 and 100 Ω. Many designs are tuned to 50 Ω directly; others use a 1:1 or 4:1 balun. SWR typically stays below 2:1 across the whole design range.
The log periodic antenna gives up a few dB of gain compared with a monoband Yagi, but in return it covers a huge frequency range with steady gain, pattern and match on a single feeder. For an amateur who wants one beam for many bands, or a reliable wideband antenna for receiving and testing, it is hard to beat.