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For the resonance frequency of the fourth harmonic
of the shortened helical antenna I cannot find clear rule
for the resonance. I only found roughly rule:
The fourth resonance harmonic
frequency may be either slightly lower or significantly
higher than the frequency of the first resonance multiplied
by four.
For example, if the shortened helical antenna has
the main resonance at a frequency of 7.00 MHz, then the
fourth harmonic resonance may be at frequency of 27 MHz,
that is, slightly lower than the frequency of 28 MHz, or
at frequencies above 32 MHz.
Some types of helical shortened antennas may not
show the third and fourth resonances:
Helical shortened antennas with
shortened factor more the 10.
Helical shortened antennas with small pitch in the
helix.
Helical shortened antennas wound by thin wire with
diameter less the 0.3- mm.
Helical shortened antennas wound on the form with
high dielectric permeability.
Shortened helical antennas wound on the form with
small diameter- less the 16- mm.
The input impedance at the frequencies
of the second and third harmonic of the shortened helical
antenna is usually slightly higher than the input impedance
at the fundamental resonance frequency of the antenna. The
input impedance at the fourth harmonic frequency may differ
in any direction from the input impedance at the main resonance
frequency of the antenna.
For example, if the shortened helical antenna has
input impedance of 40 Ohms at the frequency of the main
resonance of 7.00 MHz, then it is possible assume that at
the frequency of the second (14.500 MHz) and third (23 MHz)
the input impedance of this antenna will be approximately
50 ohms.
However, if this antenna has a fourth resonance,
then we cannot say anything about the input impedance of
the antenna. It may be either higher or lower than 40 ohms.
Only a practical measurement of the input impedance of the
antenna will help us know its true value at the frequency
of the fourth resonance.
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The Q factor of the shortened
helical antenna at the frequencies of the second and third
harmonics of the antenna, as a rule, is slightly higher
than the Q factor at the fundamental resonance frequency
of the antenna. The Q factor of the antenna at the fourth
harmonic frequency, as a rule, does not exceed the quality
factor at the fundamental resonance frequency of the antenna.
For example, if the shortened helical antenna has
Q factor equal to 30 at the fundamental resonance frequency
of 7.00 MHz, then we may suppose that at the second resonance
harmonic frequency (14.500 MHz) and at the third resonance
harmonic frequency (23 MHz) the Q factor of the antenna
would be approximately 40.
If this antenna has the fourth resonance, then we
may suppose that there Q factor of the antenna would not
exceed 30.
Operation
of the Helical Antennas to Receiving and Transmitting
To test the efficiency of the shortened helical antenna
in reception mode I did A-B test of the antennas compare
to 41- meter long wire. At the test the control receiver
was tested with the long wire and then with the shortened
helical antenna. Test was made on HF ranges.
Both antennas connected to the receiver without any
matching devices. I used several different receivers: old
military tube R311 receiver (USSR), old military tube US-9
receiver (USSR), old military tube transceiver R-105 (USSR),
R-108 (USSR), R-109 (USSR), broadcast semiconductor receiver
ISHIM- 003 (USSR), and semiconductor transceiver K- 116
(made in Ukraine).
It was found that the shortened helical antenna provides
good reception at the first resonant frequency and at harmonic
frequencies, if any are present. At these frequencies it
was like bursts in reception of radio stations.
Top loading helical antennas worked very well at
their fundamental resonant frequency, but provide weak reception
at the harmonic frequencies. In the frequency range between
the harmonics frequencies the shortened helical antenna
provided weak reception compare to long wire antenna in
41 meter length.
It was found that the reception is going to bad with
increasing frequency compare to the fundamental resonance
frequency of the helical antenna.
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