How many counterweights do you need? Scheme, description

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How many counterweights do you need? Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / HF antennas

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Vertical antennas, which require a small installation area, are very popular with shortwavers. However, their effectiveness essentially depends on the artificial "earth" - counterweights. The general recommendation "the more, the better" is not always acceptable. It should be noted that in relation to counterweights, there are two fundamentally different situations. One of them arises when it is possible to raise the antenna high enough and the counterweights are removed from the "ground" (the roof of a building, etc.). This is typical for VHF bands. Here the number of counterweights, in essence, is not important: from one to three or four. In the first case (with one vertical counterweight), a simple vertical dipole is obtained. In the second, the counterweights are installed at some angle to the emitter, placing them evenly around the circumference to provide a circular radiation pattern. But both options are united by the fact that the influence of the "ground" on the operation of counterweights is insignificant.

Another thing is if the counterweights have to be placed very close to the "ground". This is a common situation on the HF bands, when the balances are from the "ground" at a distance significantly less than the wavelength. An insufficient number of them will lead to significant losses in poorly conductive "ground" (soil, building roofs, etc.) and, accordingly, to a noticeable decrease in the antenna efficiency.

An analysis of the number of counterweights required in this case was carried out at the time by W2FMI (Jerry Sevick. Short Ground-Radial System for Short Verticals. - QST, 1978, April, p. 18). Two conclusions follow from the results of the analysis. First, for a given number of counterweights, there is a limit to their length, exceeding which no longer leads to an increase in the effectiveness of the counterweights.

Secondly, with a given length of counterweights, there is a limit for their number, the excess of which also does not contribute to an increase in efficiency. The concept of "limit", which appears in these conclusions, is rather vague - the efficiency varies smoothly depending on the number of counterweights and their length.

Expanding on the above points derived from the work of W2FMI, G3SEK proposed (John White. In Practice. - RadCom. 1999, February, p. 45) a simple practical criterion for determining the length and number of counterweights, which combines both conclusions of W2FMI in one ratio. According to his estimates, the length of the counterweights and their number should be such that the distance between the ends of the counterweights (see figure) is kl, and the value of k can lie within 0,02 ... 0,05. If k is greater than 0,05, the loss will be significant. Reducing k to 0,02 really improves the efficiency of the antenna. However, a further decrease in k no longer gives a noticeable effect.

How many counterweights do you need?

In the descriptions of many antennas, "resonant" counterweights with a length of l / 4 appear, located at a small height above the roof. Based on the G3SEK criterion, it can be argued that such counterweights will become effective if their number is at least 30. All the above ratios are valid for counterweights having the same length.

The ratios discussed in this material define reasonable limits for the combination "number of counterweights - length of counterweights". Obviously, under the condition that these ratios are met, the best of the two systems for a given antenna will still be the one that has a large length of counterweights.

This publication ("Radio", 1999, No. 6) aroused the interest of the readers of the magazine, since the GP antenna is always popular with shortwaves. Here is more detailed information about the results obtained by W2FMI (Jerry Sevick. Short Ground-Radial System for Short Verticals. - QST, 1978, April, p. 30-33) in the study of the influence of the number of counterweights and their length on the efficiency of the antenna.

We are talking about a HF antenna installed close to the ground (practically without a mast). In these experiments, the soil under the antenna according to the W2FMI measurements was "average", i.e. it had a conductivity of 15...30 mS/m (higher values refer to soil after rain, lower values to dry soil). "Bad" for antennas after rain, smaller - to dry). "Bad" for antennas is soil that has a conductivity of less than 5 mS / m (stony, sand), and "very good" - about 100. The reinforced concrete roof of a modern building, unfortunately, most likely refers to "bad soil".

Figure 1 shows the dependence of the antenna input impedance at the resonant frequency on the number of counterweights obtained by W2FMI. It includes the radiation resistance (useful part) and the loss resistance. The calculated value of the input impedance for the used W2FMI driver diameter and ideal (lossless) ground was 35 ohms.

How many counterweights do you need?

As can be seen from fig. 1, a close to this value of the input resistance is achieved only when the number of balances is more than 50. In other words, with a small number of balances, a significant part of the transmitter power is not emitted by the antenna, but literally goes into the "ground". For the most common GP version with three or four counterbalances, the input impedance will be approximately 70 ohms and, accordingly, the antenna efficiency will be about 50%.

From the data shown in fig. 1, it also follows that the length of the counterweights does not greatly affect the efficiency of the antenna.

This issue has been investigated in detail by W2FMI. The measurement results are shown in fig. 2, which shows the dependence of the antenna efficiency on the number of counterweights for three options for their length - l/4, l/8 and l/16. An analysis of these curves allows us to draw several conclusions.

How many counterweights do you need?

First, the longer the counterweights, the more effective they are, generally speaking.

Secondly, with a small number of counterweights, their length has little effect on the efficiency, so the efforts and funds spent on the manufacture of long counterweights may not give a noticeable result in this case.

Thirdly, under certain conditions, short (less than l/4) counterweights can provide the same antenna efficiency as long ones.

Let us explain the latter in more detail. From fig. 2 it can be seen that the same efficiency is provided by four counterweights with a length of l/4, five or six counterweights with a length of l/8 and seven counterweights with a length of l/16. What's more, twenty l/16-length counterweights provide the same efficiency as eight l/4-length counterweights. And the design advantages provided by the use of short counterweights (especially in the low-frequency ranges) are obvious.

Author: Jerry Sevick

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