
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 212 MHz.antenna was developed for the required efficiency and a
minimum 3.5 kHz bandwidth independently of the variations in the
environment.
With a 2.2m^{2} radiating surface the halfloop 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 Ci1 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
