Loading of ADR Antennas:
David J Jefferies and Athanasios Koulouris
here has been some discussion in antenneX magazine on the Antenna Theory Forum pages, about whether the size of an antenna can be reduced significantly by loading it with dielectric, or even encasing it in dielectric. In the year 2001 we did some inconclusive laboratory experiments on dipoles, which only served to show that if there is an effect, it is rather small. Alan Boswell had this to say on the subject via the above-mentioned Antenna Theory Forum:
". . .it can be shown that coating a wire with dielectric has the same effect as increasing its diameter - the formula is: . . .
Effective diameter = outer diameter * (wire diameter / outer diameter ) ** (1/epsilon) . . .
The formula is valid as long as the diameter is a fraction of a wavelength, and it shows that using a very high dielectric constant coating has the same effect as using wire of the same outer diameter. The effect reduces the resonant frequency of a dipole but not by a lot. . . . Increasing the diameter of the wire in antennas like dipoles does not increase the radiation resistance, but it helps performance by reducing the reactance of the antenna at off-resonance frequencies. This increases the bandwidth so that frequent re-tuning is less necessary as you roam across a band. It also has the advantage that, with antennas used well below resonance (i.e. small antennas), lower-loss tuning components can be used to match them."
Our feeling about this comment was that it was fine for situations where the electric field within the dielectric is substantially in a direction radial to the axis of the wire. Square or circular loop antennas, with a diameter comparable to a quarter wavelength or so, have regions where the generated electric field close to the antenna has a significant component parallel to the wires. Perhaps the shortening of the wavelength inside the dielectric, by an amount 1/(square-root of dielectric constant) will result in a lowering of the resonant frequency. As far as we are aware, there has been no reported work on encasing loop antennas in dielectric.
So this paper now discusses dielectric loading of the ADR antenna design of Dan Handelsman. He discusses this in a loop antenna article in antenneX in May 2000, Archive IV, Article No. 13 http://www.antennex.com/archive4/May00/May6/qloops1.htm
Subsequently, David Jefferies and Mat Ariff presented experimental results on a microwave version of the ADR in an antenneX article. In this short article, the available data supports the hypothesis that there is a slight size reduction of loop antennas to be had from dielectric loading them. The effect is only a few tens of percent (e.g., at most between about 10 to 30 percent) and probably will not greatly aid those people who seek substantial progress towards the goal of truly compact antenna structures. It is certainly not as much as the factor 1/(square-root of dielectric constant) might lead us to expect. But it is probably significantly more than the "Boswell effect" of the fattening of the antenna wire; clearly Alan Boswell's remarks apply more readily to rod antennas where the wire is straight.
Figure 1 shows the configuration of the ADR antennas used, and Figure 2 shows a schematic indication of the dielectric loading employed, where the small antenna structure (of the order of 10 cm square) is sandwiched between two or more sheets of dielectric. Antennas were constructed from various diameters of wire. A collection of different dielectrics was assembled; the dielectric constants were determined at low frequencies (10kHz) by measuring the capacitance of parallel-plate dielectric sandwiches. A table of the deduced dielectric constants, with associated experimental uncertainties, is shown in Table 1.
|Perspex||28.55||70 x 70 x 5.2||(3.42)
3.4 ± 0.3
|Acetal||64.56||102 x 92 x 5.2||(4.04)
4 ± 0.2
|Nylon 66||73.45||102 x 92 x 5.3||(4.68)
4.7 ± 0.2
|TIVAR 1000||43.40||102 x 92 x 5.1||(2.66)
2.7 ± 0.2
|Gril. L209||116.22||98 x 98 x 3.2||4.37)
4.4 ± 0.3
|99.67||99 x 99 x 3.2||(3.67)
3.7 ± 0.2
|71.12||99 x 99 x 4.2||(3.44)
3.4 ± 0.2
|144.22||99 x 98 x 3.0||(5.03)
5 ± 0.4
|PTFE||17.16||50 x 50 x 3.4||(2.64)
2.6 ± 0.4
Table 1 Calculation of and the respective uncertainties
First we examine the effect of increasing the thickness of dielectric on either side of the ADR wires. Table 2 shows a number of measurements of the effects of 2, 4, 6, and 8 thicknesses of Perspex sheet.
Resonant Frequency (MHz)
|2 Plates of
|4 Plates of
|6 Plates of
|8 Plates of
|Average Values (MHz)|
|Effect on Resonant Frequency with Respect to 2-Plate Case (%)|
|Additional Changes in Resonant Frequency (%)|
Table 2 Resonant Frequency for Different Dielectric Thickness
The Perspex sheet used has measured dielectric constant of 3.4 +/- 0.3, so the factor to which the wavelength is reduced is 1/(sqrt(3.4)) = 0.54 of the unloaded case. The ADR used was made of 2-mm diameter wire and the individual plate thickness was 5 mm or lambda/32. Thus, for the 8 thicknesses of Perspex (4 on each side of the antenna), the total dielectric thickness encasing the antenna was lambda/8. The greatest effect of dielectric loading was produced by the first two sheets (lambda/32), which reduced the resonant frequency by 17%. At a thickness of lambda/8, the total reduction in resonant frequency was 26%, compared to the full amount we would expect from the dielectric constant measurements, which would be 46%. Thus there must be a significant amount of stored near-field energy outside the region of the plates. A curve showing the trend is presented in Figure 3.
The results were checked using two other ADR antennas, one made from 2mm wire again and the other from 6.2mm tube. In the latter case, the reduction for the addition of two plates (5mm thick each) was 5%, and adding extra plates had significantly less effect. This is evidence to support Alan Boswell's hypothesis that thickening the wires has a similar effect to encasing them in dielectric.
The effect of mounting the ADR antennas on a plastic base was also investigated. For various materials (Tivar, Perspex, Nylon66), there was a reduction of about 15% attributed to the 10cm square base alone. It should be remarked that the base plane was orthogonal to the plane of the antenna, and extended beyond the near field region. When plastic sheets were added to the antenna, already mounted on a plastic base, the reduction in resonant frequency was much less, as the antenna was already loaded.
Figure 4 shows the effect of loading an ADR antenna with plastic sheets having differing dielectric constants (relative permittivities).
Figure 5 shows the effect on the bandwidth of the antennas, and Figure 6 shows the effect of the dielectric constant of the plate loading on input resistance of the resonant antennas. It should be remarked that none of these antennas had been optimised for either input impedance or bandwidth, and so we present these figures here for interest, without commenting further.
Other measurements made included varying the position of the dielectric loading sheets within and around the antenna structure, investigating the effect of encasing the antenna in a graded dielectric constant by assembling sheets of different dielectric into a matrix, and investigating the effect on the azimuth radiation pattern (essentially there was no detectable effect).
The effect of doubling the plate area for 3mm thick plates, and using a range of dielectric material, showed also that there was a measurable and consistent reduction in the resonant frequency of the ADR antennas for the larger area plates.
In conclusion, it is clear that significant reductions in the size of an ADR antenna may be made by encasing it in dielectric up to a thickness of about lambda/8. Beyond that we have no data, but it appears that about half the theoretical shortening which would occur in an infinite body of dielectric may be realised.
There are significant perturbations of bandwidth and driving point resistance at resonance.
We didn't extend the investigations to try dielectrics of the ferroelectric class, which have significantly larger permittivities. This would be an interesting follow-up experiment to do. It also appears that placing a sheet of plastic in the near field of the antenna orientated in the azimuth plane may help almost as much as encasing the structure of the antenna in dielectric sheet. –30-
~ antenneX ~ January 2003 Online Issue #69 ~
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