ORAL - International Conference of Agricultural Engineering

open, short and 50 Ω termination. During the experiment a loop antenna and a helix antennawere used. The homemade 433 MHz loop antenna was made from ø1 mm copper wire witha loop diameter of 35 mm. The tuning capacitor was 1.2 pF and Q of the antenna wasapproximately 336 in air. The helix antenna was a Linx Technologies ANT-433-HETH. SMAconnectors were used for both antennas and each antenna was enclosed in a small plasticbox to avoid direct contact with the silage.The experiment was carried out on the 6th of January 2011 and the silage stack used wasmade in July 2010 by and stored at the Faculty of Agricultural Sciences, Foulum, Denmark.The stack was stored in a barn. The resonance frequency and quality, Q, of the antenna wasmeasured in two different circumstances. First it was measured while in air using the VNA.Then a 40 cm hole with a 4 cm diameter was drilled in the grass silage stack, the antennaplaced at the bottom and the resonance frequency and Q was measured again. This wasdone for both types of antenna. Each recorded measurement was averaged on the VNA over100 measurements. Dry-matter analysis was done on all samples.4 Results and Discussion4.1 Properties of the silage samplesTable 1 Dry-matter analysis of the silage samples.Silage baleSilage stackMean Standard div. Mean Standard dev.Moisture content 66.70 % 0.12 66.46 % 1.06Density - - 0.56 g/cm 3 0.10Table 1 contains the dry matter analysis as well as density for the samples from the silagestack and the samples from the silage bale. The moisture content of the silage stack samplesis less stable than that of the silage bale but this was expected as the bale has been treatedmore uniformly and the samples were taken from a smaller volume than those of the silagestack. The density also varies but is well within the variations found in other works (Muck &Holmes 2000).4.2 Complex permittivity measurementsFigure 2 shows the averaged complex relative permittivity for all samples calculated from thereflection coefficient measurements and the standard deviation on both the real an imaginarypart.Figure 2 The real (solid line) and imaginary (dotted line) part of the permittivity and standarddeviation.The figure show a permittivity that drops as frequency increases, which corresponds wellwith observations on biological materials in general (Ponne & Bartels 1995). It can be