Dielectric
Loading of ADR Antennas:
Experimental Results
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 abovementioned 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 offresonance frequencies. This increases the bandwidth so that frequent retuning 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), lowerloss 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/(squareroot 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
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/(squareroot 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 parallelplate dielectric sandwiches. A table of the deduced
dielectric constants, with associated experimental uncertainties, is shown in Table 1.
Material  C_{mean} (PF)  Dimensions
(mm) 
Fractional Uncertainty 

Perspex  28.55  70 x 70 x 5.2  (3.42) 3.4 ± 0.3 
8% 
Acetal  64.56  102 x 92 x 5.2  (4.04) 4 ± 0.2 
5% 
Nylon 66  73.45  102 x 92 x 5.3  (4.68) 4.7 ± 0.2 
5% 
TIVAR 1000  43.40  102 x 92 x 5.1  (2.66) 2.7 ± 0.2 
6% 
Gril. L209  116.22  98 x 98 x 3.2  4.37) 4.4 ± 0.3 
7% 
Gril. TR55 Natural 
99.67  99 x 99 x 3.2  (3.67) 3.7 ± 0.2 
7% 
Gril. TR55 LZ 
71.12  99 x 99 x 4.2  (3.44) 3.4 ± 0.2 
6% 
HTV4H1 Natrual 
144.22  99 x 98 x 3.0  (5.03) 5 ± 0.4 
7% 
PTFE  17.16  50 x 50 x 3.4  (2.64) 2.6 ± 0.4 
13% 
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 Perspex 
4 Plates of Perspex 
6 Plates of Perspex 
8 Plates of Perspex 
1996.333  1904.667  1836.000  1797.000 
1973.167  1892.167  1828.333  1804.000 
1983.500  1893.167  1837.667  1807.500 
1983.000  1889.833  1833.333  1806.667 
1983.000  1895.833  1845.333  1809.500 
1976.167  1877.333  1826.667  1791.333 
1991.167  1895.500  1846.667  1814.833 
1980.667  1880.000  1831.167  1792.667 
1988.333  1882.667  1824.333  1786.000 
1994.667  1876.000  1816.500  1790.167 
Average Values (MHz)  
1985  1889  1833  1800 
Effect on Resonant Frequency with Respect to 2Plate Case (%)  
0  4.8  7.7  9.3 
Additional Changes in Resonant Frequency (%)  
  4.8  2.9  1.6 
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 2mm 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 nearfield 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 followup 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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