ANTENTOP- 03- 2003, # 004

Figure 11

modification concluded in a +10 to +15% extended bandwidths and in +0.5dB to +1dB extra efficiencies all over the frequency range.

II.7. Realisation of a fast tune design

The 2-12 MHz.antenna was developed for the required efficiency and a minimum 3.5 kHz bandwidth independently of the variations in the environment.

With a 2.2m² radiating surface the half-loop reactance is 2mH at 2 MHz and 3.5mH at 12 MHz.

The tuning principle consists in switching capacitors in parallel to create a series of bandwidths with mutual covering at a VSWR < 2.5:1.(FIG 12)

Figure 12

Principle of the capacitor switching

The capacitors which are necessary to tune the antenna reactance are scaled from 3300pF to 60 pF at 2 MHz and 12 MHz respectfully, with a 1,5 pF accuracy at the highest frequencies.

A logarithmic series of n switchable capacitors in parallel defined by Ci=2 Ci-1 with C1=1.5 pF give all discrete value multiple of 1,5pF:

C = S ki Ci from i = 1 to n , with ki= 0 ou 1

C1, which is the smallest used capacitor, defines the accuracy of the C capacitor

The highest individual capacitor value is in theory 3300/2=1650 pF in order to get 3300pF by the addition of all capacitors, and n must be higher than 10.

The total number of capacitors is choosen equal to 12 to takto into consideration the dispersion of the components whose values are guaranteed with a ñ 5% precision, and to recover the possible missing frequency bands.

A special software was created to define and memorize the kiCi arrangements which are necessary to get all discrete capacitor values and recover the possible missing frequency bands. It memorizes the calculated values and the measured values. A calibration at the first installation or in operation in case of a major environment change can be done in less than 6 seconds.

Measured results

The prototype of the antenna achieved a VSWR 2.5:1.

Typical figures are given FIG13.

Figure 13

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