log-periodic antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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Modern requirements for a home-made amateur radio antenna can be reflected in several points:
- Provide operation on several bands, including WARC.
- Have a gain (relative to the dipole) of at least 5-8 dB and a back attenuation of at least 25 - 30 dB.
- The constructive implementation should be quite simple and not require large material costs. (If a radio amateur has the means, he can purchase an industrially manufactured antenna).
- Must have minimum SWR on all operating bands. This requirement is especially relevant for working on “branded” transceivers that do not have an automatic antenna tuner.
- It must be an electrically closed system, as it is least susceptible to electrostatic atmospheric noise.
- It should work without additional adjustment after lifting, which in practice is labor-intensive or impossible.
A log-periodic antenna can be proposed as the most suitable for these requirements, which is confirmed by the ever-increasing number of antennas of this type produced by well-known companies CushCrft, KLM, etc.
Structurally, the antenna is made on a conventional tubular boom.
The elements are made of lightweight duralumin tubes. My design consists of four-section antennas. The sections are inserted into one another and latched with spring locks. Such antennas are in short supply and radio amateurs can approach the manufacture of elements based on available materials. The diameter of the pipes does not matter much (which is also an advantage of Logoperiodic antennas) and can be made from antenna cable. Similar structures with a cord are described in the literature.
The insulation of the elements “broken” in the middle is carried out as follows: a square-section nylon profile is used (a water conduit used in agriculture), into which duralumin tubes (ski poles) with an outer diameter of 17 mm are pressed. The tubes of the elements themselves are inserted into them and secured with self-tapping screws. Two holes with a diameter of 8 mm are drilled in the middle of the nylon profile. for attaching to the boom with a clamp. The clamp is used from television antennas and has spikes that prevent the element from turning. Fastening of elements can be carried out in the traditional way on insulators on textolite or fiberglass platforms.
The antenna is powered by a standard symmetrical wire (collection) line made of copper wire with a cross-section of at least 1.5 mm in high-quality insulation and has two matching lines:
- C3adi (relative to the direction of radiation) is short-circuited, determining backward attenuation. This line additionally turns the antenna into a short-circuited system, which corresponds to paragraph 5 of the general requirements.
- In front there is a matching one and connected to a supply and balun HF transformer (balloon) having a ratio of 1:4. The balloon is wound on a ferrite ring having a magnetic permeability of 600 with two twisted wires in polyethylene insulation. Number of turns - 6. Ferrite - from the TV deflection system. The ring is disassembled into two halves and glued with glue with a ferrite filler, which sharply reduces power losses in it. A measurement was made of losses with equivalent load.
Accepted designations:
L1 is the length of the element.
S1- Distance between elements.
LS- Matching short-circuited line.
LC- Antenna power matching line.
B - Boom length.
K - Estimated coefficient.
t- Progression coefficient.
l- wavelength.
N is the number of elements.
Antenna size table from 14.000 to 29.000 mHz:
| LS=112 |
LC=64 |
t = 0.82259 |
K = 3.514 |
B = 600 |
| Element length L cm. |
Dist. between el. S see |
| L1 = 1070 |
S1 = 171 |
| L2 = 880 |
S2 = 141 |
| L3 = 724 |
S3 = 115 |
| L4 = 595 |
S4 = 95 |
| L5 = 490 |
S5 = 72 |
| L6 = 403 |
|
The antenna was assembled and raised to a height of 12 meters near a wooden country house using a lightweight telescopic mast. A cheap Chinese-made rotating mechanism was used, designed for rotating television antennas, purchased from the Radiospectr company.
The obtained technical parameters can be assessed as exceeding those specified at the beginning (except for 14 mHz, since the rise height is low for this range). Parameter measurements were carried out on all bands, including WARC with a radio amateur located at a distance of 15 km line of sight. The resulting characteristics are on average F / B - 54/56; F/S - 52; F/D - 59 +25db.
The accuracy of measurements is relative, adjusted for amateur radio equipment. Unfortunately, measurements with the GOS laboratory did not take place because The antenna is installed outside the city and there is no direct visibility.
The reflected wave coefficient throughout the entire range from 14 to 29 mHz turned out to be unchanged 1.3 at any measured point in the specified range. At frequencies of 13.2 and 30 mHz it tended to infinity. The measurements were carried out using an Icom-746 transceiver SWR meter.
It should be noted that the antenna is very “picky” about nearby metal objects and the lifting height. The antenna was first tested at a height of 7.5 meters and the test result at a frequency of 18 mHz showed that at this height the front lobe is blurred and the F/B ratio is no more than 5-8 decibels.
Some shortwave operators have a negative attitude towards Log Yagi antennas, considering them “noise collectors”. I agree with them on one condition - this occurs if a receiver is used with a preselector that has a wide range and a weak dynamic range. Conventional Yagi itself works as a preselector with good quality factor and reduces the requirements for receiver characteristics. Therefore, I advise shortwave operators to use an additional preselector or high-performance receiver when using Log Yagi; this will completely eliminate the problems caused by using Log Yagi.
For shortwave operators who wish to calculate the antenna to suit their capabilities, the calculation formulas used to calculate the described design are given. The number of elements and the lower frequency are chosen arbitrarily. Not recommended, with an antenna range of 14 -28 mHz, the number of elements is less than 5/6 - the antenna efficiency drops sharply.
L1 = l/2; based on the lower operating frequency (14.0 mHz).
Ln = 405 cm for the upper frequency 29 mHz, based on a review of industrial and amateur radio designs (which contradicts the theory, but practical measurements confirm the correctness of the choice based on the measured SWR).
Author: R. Tarshish (RU3UJ), ru3uj@mail.ru; Publication: qsl.net/ru3uj
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