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For example, if the shortened helical antenna has
7 MHz resonance frequency, then it provides good reception
at this frequency - 7 MHz, and additional at the second
harmonic frequency in 14.5 MHz, and at the third harmonic
frequency in 23 MHz. The antenna
provides satisfactory reception below 7 MHz and in the ranges
lying between the main resonance frequency in 7 MHz and
the second harmonic frequency in 14.5 MHz. In the frequency range between the second harmonic
in 14.5 MHz and the third harmonic in 23 MHz, the reception
is already not satisfactory. Antenna, on the reception above
the third harmonic in 23-MHz, works not satisfactory. At
the fourth harmonic frequency in 32 MHz the helical antenna
works on reception very bad.
It was tested shortened helical antennas on reception
with ATU. It was used ATU described in the Reference
8. Figure 12 shows ATU with vertical helical antenna.
However, I experimented with dipole helical antenna connected
to the ATU as well. It was found that ATU does not dramatically
change the operation of the antenna on the reception. Of
course, the reception was improved especially on the frequency
range between the resonance frequencies of the helical antenna.
But the improving was not so much.
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At testing of the helical antennas on TX it was got
the same results. After the final testing the shortening
helical antennas on RX and TX I did the conclusions:
Shortened helical antenna could provide effective
operation on the fundamental resonance frequency. Shortened
helical antenna could provide satisfactory operation on
the second and sometimes on the third harmonic frequency.
The shortened helical antenna cannot provide satisfactory
operation on the fourth harmonic frequency.
ATU could improve operation of the shortened helical
antenna on the frequencies below the main resonance frequency
of the antenna. ATU could improve operation of the shortened
helical antenna on the frequencies in range between the
main resonance frequency and the second and third harmonic
frequencies.
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