Short wave transmission method and apparatus therefor

In a short wave transmission method, a carrier in a short wave band is circularly polarized and radiated from an antenna, thereby removing an influence of fading. A short wave transmission apparatus has a transmission apparatus including a carrier generation unit, a subcarrier generation unit, a first mixer, a phase shifter, a second mixer, and two orthogonal dipole antennas. The carrier generation unit generates a carrier in a short wave band. The subcarrier generation unit generates a subcarrier. The first mixer receives the carrier and the subcarrier. The phase shifter phase-shifts the subcarrier generated from the subcarrier generation unit by .pi./2. The second mixer receives the carrier and a subcarrier which is phase-shifted by .pi./2 by the phase shifter.

BACKGROUND OF THE INVENTION 
The present invention relates to an improvement of a communication system 
in radio transmission/reception using a short wave and, more particularly, 
to a short wave transmission apparatus which removes the influence of 
fading to allow high-quality signal transmission/reception. 
As is conventionally well known, in radio communication in the short wave 
band, fading occurs to temporarily disable reception. 
To prevent such a reception disabled state caused by fading, conventional 
short wave communication employs a reception system wherein an AGC circuit 
is arranged on the receiver side to minimize the influence of fading. 
In the actual short wave communication, however, fading cannot be 
completely removed only by the AGC circuit. For this reason, the quality 
of the short wave line is largely degraded to result in a deterioration in 
reliability. 
For example, short wave radio broadcasting uses a plurality of frequency 
bands (frequencies in 3-, 6-, and 9-MHz bands) for transmission. Of these 
frequencies, the most satisfactorily receivable frequency is selected and 
received in the reception site or on the receiver side. 
Regardless of this problem, the short wave band is used as an important 
communication medium because of its excellent characteristics representing 
that the short wave is reflected by the ionosphere and propagates a long 
distance. 
For example, the International Maritime Organization and the World 
Administrative Radio Conference have decided to introduce short wave 
narrow-band direct-printing telegraphy using a digital selective calling 
system to replace the conventional Morse communication, and are planning 
to use this system not only for general communication but also as a medium 
for search and rescue communication. 
In international broadcasting as well, needless to say, the short wave band 
is used as an important medium. 
As described above, the short wave band is useful for radio communication 
because of its wide coverage. Without fading, the short wave band can be 
used as the most excellent terrestrial link. 
The present inventor has previously proposed a radio communication system 
which removes fading to allow high-quality signal transmission/reception 
(Japanese Patent Application No. 1-113192). 
In this radio communication system, the plane of polarization (a vibrating 
direction of the electric vector) of a carrier in the short wave band is 
rotated at a high speed of, e.g., 100 kHz, much higher than the rotation 
speed at which fading is caused by the geomagnetic field, i.e., the 
rotation speed of the vector of an electric field passing through the 
ionosphere, which takes about three seconds to one minute for one 
revolution (1/3 to 1/60 Hz). With this processing, the influence of 
rotation of the plane of polarization caused by the magnetization plasma 
due to the geomagnetic field is removed, thereby preventing the reception 
disabled state caused by fading. 
In the radio communication system of the prior art, as described in 
Japanese Patent Application 02291731 of Nippon Denshi orthogonal dipole 
antennas are used, and radio waves whose balanced-modulated signals are 
phase-shifted from each other by 90.degree. are sent from the respective 
antennas (an antenna X and an antenna Y). 
FIG. 8 shows the arrangement of the main part of a transmission apparatus 
for the radio communication system of the prior art. Referring to FIG. 8, 
reference numeral 1 denotes a carrier oscillator; 2, a low-frequency 
oscillator; 3, a 90.degree. phase shifter; 4X and 4Y, balanced modulators 
(double balanced modulators); 5X, a 90.degree. upper sideband filter 
(bandpass filter); 5Y, a 0.degree. upper sideband filter (bandpass 
filter); 6X, a 90.degree. lower sideband filter (bandpass filter); 6Y, a 
0.degree. lower sideband filter (bandpass filter); 7X and 7Y, SSB (upper 
side) transmitters; 8X and 8Y, SSB (lower side) transmitters; 9X and 9Y, 
output synthesizers; 10X and 10Y, baluns (antenna matching devices); 11X, 
an X antenna; and 11Y, a Y antenna. 
In FIG. 8, the carrier oscillator 1 on the left side has a function of 
generating a short wave of a carrier frequency (F.sub.1). 
The low-frequency oscillator 2 shown on the right side of the carrier 
oscillator 1 generates a low-frequency signal of a frequency (F.sub.2) for 
rotating the plane of polarization of a radio wave. 
Therefore, in the balanced modulator 4Y shown on the upper side, a carrier 
frequency A is balanced-modulated with a low frequency B for rotating the 
plane of polarization of the radio wave, thereby obtaining two signals of 
frequencies (F.sub.1 .+-.F.sub.2). 
The two signals output from the balanced modulator 4Y are separated at the 
next stage by the 0.degree. upper sideband filter 5Y and the 0.degree. 
lower sideband filter 6Y and sent to the SSB (upper side) transmitter 7Y 
and the SSB transmitter (lower side) 8Y, respectively, which perform power 
amplification. 
Thereafter, the two signals are synthesized by the output synthesizer 9Y 
and supplied to the Y antenna 11Y through the balun 10Y. 
On the other hand, the low-frequency signal of the frequency B is input 
from the low-frequency oscillator 2 to the balanced modulator 4X shown on 
the lower side of FIG. 8 through the 90.degree. phase shifter 3. 
For this reason, two signals of frequencies (F.sub.1 .+-.F.sub.2) are 
output, which are balanced-modulated signals with a phase shift of 
90.degree. with respect to the balanced-modulated frequencies (F.sub.1 
.+-.F.sub.2) output from the balanced modulator 4Y. 
The two signals output from the balanced modulator 4X are also separated at 
the next stage by the 90.degree. upper sideband filter 5X and the 
90.degree. lower sideband filter 6X and sent to the SSB transmitter (upper 
side) 7X and the SSB transmitter (lower side) 8X, respectively, which 
perform power amplification. 
Thereafter, the two signals are synthesized by the output synthesizer 9X 
and supplied to the X antenna 11X through the balun 10X. 
Therefore, one pair of in-phase balanced-modulated waves which have a phase 
shift of 90.degree. with respect to the other pair of balanced-modulated 
waves and the same carrier frequency A are supplied to the corresponding 
orthogonal dipole antenna (Y or X) shown on the right side of FIG. 8, and 
radiated from the Y antenna 11Y or the X antenna 11X. 
The waveforms of signals radiated from the dipole antennas (Y and X) will 
be described below. 
FIG. 9 shows the waveforms of signals radiated from the dipole antennas in 
the radio communication system of the prior art. 
FIG. 10 shows a view plotted using a computer, which shows a state wherein 
the plane of polarization is rotated when radio waves shown in FIG. 9 are 
radiated from the orthogonal antennas. 
Radio waves having waveforms as shown in FIG. 9 are radiated from the 
orthogonal dipole antennas. In this case, the plane of polarization as a 
synthesized vector is rotated at a speed of, e.g., .theta./2.pi. 
(=1/F.sub.2) per second. 
As is apparent from FIG. 10, these radio waves are rotated as the 
synthesized vector of the radio waves radiated from the orthogonal 
antennas X and Y in accordance with output currents i.sub.x and i.sub.y 
and propagate in a Z direction. 
The plane of polarization is normally rotated once in several seconds to 
one minute. However, in the radio communication system shown in FIG. 8, 
the plane of polarization is rotated at a high speed of, e.g., about 
.theta./2.pi. per second, so that the influence of the geomagnetic field 
can be ignored. 
The frequency of the rotating radio wave is F, and the amplitude A is 
constant. 
A change in frequency (spectrum) and phase of each signal passing through 
the blocks of the transmission apparatus shown in FIG. 8 will be described 
below using equations. 
FIG. 11 explains a change in frequency (spectrum) and phase of each signal 
passing through the blocks of the transmission apparatus shown in FIG. 8 
using equations. The same reference numerals as in FIG. 8 denote the same 
blocks in FIG. 11, and DBM represents a balanced modulator (double 
balanced modulator). 
In FIG. 11, the frequency of a carrier output from the carrier oscillator 1 
is F.sub.1, the amplitude of the carrier is A, the frequency of a 
low-frequency signal output from the low-frequency oscillator 2 to rotate 
the plane of polarization of the radio wave is F.sub.2, and the amplitude 
of the low-frequency signal is B. A voltage of about 100 mV is appropriate 
for the amplitude A of the carrier, and a voltage of about 10 mV is 
appropriate for the amplitude B of the low-frequency signal. 
The angular frequency of the carrier is .omega., the angular frequency of 
the low-frequency signal (corresponding to the angular frequency of 
rotation of the plane of polarization) is .theta., and 90.degree. is 
represented as .pi./2. 
In this case, the carrier (short wave) output from the carrier oscillator 1 
is represented as follows: 
EQU short wave carrier F.sub.1 =A.multidot.sin.omega.t (1) 
The low-frequency signal output from the low-frequency oscillator 2 is 
represented as follows: 
EQU low-frequency signal F.sub.2 =B.multidot.cos.omega.t (2) 
Therefore, the two signals represented by equations (3) and (4) are output 
from the balanced modulator 4Y and input to the 0.degree. upper sideband 
filter 5Y and the 0.degree. lower sideband filter 6Y. 
Frequencies F.sub.11 and F.sub.12 of these signals are represented as 
follows: 
EQU F.sub.11 =C.multidot.sin(.omega.+.theta.)t (3) 
EQU F.sub.12 =C.multidot.sin(.omega.-.theta.)t (4) 
where C is a voltage amplitude attenuated upon modulation and almost equal 
to B. 
The two signals represented by equations (3) and (4) are sent to the SSB 
transmitter (upper side) 7Y and the SSB transmitter (lower side) 8Y. Upon 
power amplification, the amplitude is increased to M, so that the signal 
frequencies are converted as represented by equations (5) and (6): 
EQU F.sub.11 =M.multidot.sin(.omega.+.theta.)t (5) 
EQU F.sub.12 =M.multidot.sin(.omega.-.theta.)t (6) 
Thereafter, the two signals are synthesized (added) by the output 
synthesizer 9Y to generate a signal represented by equation (7): 
EQU F.sub.11 +F.sub.12 
=M.multidot.sin(.omega.+.theta.)t+M.multidot.sin(.omega.-.theta.)t(7) 
On the other hand, a low-frequency signal represented by equation (8) is 
input from the 90.degree. phase shifter 3 to the balanced modulator 4X 
shown on the lower side of FIG. 11: 
EQU F.sub.2 =B.multidot.cos(.omega.-.pi./2) (8) 
Therefore, two signals output from the balanced modulator 4X are 
represented as follows: 
EQU F.sub.21 C.multidot.sin{(.omega.+.theta.)t-.pi./2} (9) 
EQU F.sub.22 C.multidot.sin{(.omega.-.theta.)t-.pi./2} (10) 
The signals represented by equations (9) and (10) are separated by the 
90.degree. upper sideband filter 5X and the 90.degree. lower sideband 
filter 6X and sent to the SSB transmitter (upper side) 7X and SSB 
transmitter (lower side) 8X, respectively. After power amplification, the 
two signals are synthesized (added) by the output synthesizer 9X to 
generate a signal represented by equation (11): 
EQU F.sub.21 +F.sub.22 
=M.multidot.sin{(.omega.+.theta.)t-.pi./2}+M.multidot.sin{(.omega.-.theta. 
)t-.pi./2} (11) 
In this manner, the signal represented by equation (7) is generated by the 
output synthesizer 9Y and output to one (Y antenna 11Y) of the orthogonal 
dipole antennas through the balun 10Y while the signal represented by 
equation (11) is generated by the output synthesizer 9X and output to the 
other dipole antenna (X antenna 11X) through the balun 10X. 
Therefore, a signal whose plane of polarization is rotated by .theta.t is 
radiated from the orthogonal dipole antennas. 
FIG. 12 shows the waveforms of the short wave signals radiated from the 
prior art orthogonal dipole antennas 11X and 11Y shown in FIG. 8. The 10Y 
output is the signal output from the balun 10Y, and the 10X output is the 
signal output from the balun 10X. 
The signals having the waveforms as shown in FIG. 12, i.e., the short wave 
signals whose planes of polarization are rotated by .theta.t are radiated 
from the dipole antennas. 
When these radio waves are received by a reception antenna, and only the 
upper sideband is selected by a receiver, a signal without fading 
represented by equation (12) is obtained: 
EQU F=M.multidot.sin{(.omega.+.theta..+-.p)t} (12) 
In equation (12), the small increase and decrease in angular frequency of 
rotation of the plane of polarization, which are caused in the ionosphere 
are represented by Ip. 
As described above, in the radio communication system of the prior art, two 
balanced-modulated radio waves, i.e., two balanced-modulated radio waves 
of angular frequencies (.omega.+.theta.) and (.omega.-.theta.) are 
generated on the transmission side, and only one of them is received on 
the reception side. 
On the reception antenna, two high-frequency currents having angular 
frequencies (.omega.+.theta.) and (.omega.-.theta.) and the same amplitude 
are induced and interfere each other, thereby forming the 
balanced-modulated waves. 
FIG. 13 shows the interference waveform between currents which are induced 
on the reception antenna by the radio waves whose planes of polarization 
are rotated. Referring to FIG. 13, a, b, and c represent phase shift 
points. 
This interference waveform is the same as a waveform obtained by 
balanced-modulating an arrival wave k.multidot.A.multidot.sin.omega.t! by 
cos.omega.t!. 
At the points a, b, and c, the envelope crosses the X-axis, where the phase 
is shifted by 180.degree.. 
The amplitude of the current at the phase shift point is "0", and the 
amplitude near the phase shift point is almost "0". 
Two phase shift points are generated every .theta./2.pi.. If the plane of 
polarization is rotated at a low speed such as one revolution per second, 
a long dead period of several seconds is generated at the phase shift 
point. 
On the other hand, only polarization fading is considered as a possible 
cause for deep fading which periodically disables reception by a receiver 
having an AGC function. 
The reason for this is as follows. A receiver with a high sensitivity for 
radio communication requires an input voltage of about 0 dB.mu.V to obtain 
a reception output of 50 mW at an S/N ratio of 20 dB in the short wave 
band. Therefore, it can be said that the reception output cannot be 
perceived due to fading when the input level to the receiver decreases to 
-14 dB.mu.V because the S/N ratio decreases to, e.g., about 6 dB at this 
time. 
If dipole antennas are used as reception antennas, reception is disabled 
when the field strength decreases to -26 dB.mu.V/m at an average gain of 
about 12 dB, though it depends on the frequency. 
In the radio communication system of the prior art, to remove the influence 
of polarization fading, the vector of the electric field passing through 
the ionosphere is rotated at a high speed of, e.g., 100 kHz, much higher 
than the speed of Faraday rotation which takes three seconds to one minute 
for one revolution (1/3 to 1/60 Hz). 
The two frequencies forming the balanced-modulated wave are detuned from 
the carrier frequency by .+-.(.theta./2.pi.) Hz on the upper and lower 
sides. Therefore, the interval between the two frequencies is 
(.theta./.pi.) Hz. 
When the two frequencies are detuned by, e.g., 12 kHz, .theta. must be 
about 37.7 krad. In this case, the plane of polarization is rotated at 
about 6 kHz. 
However, if the wave with a rotating plane of polarization has an occupied 
bandwidth of 6 kHz in accordance with modulation, the entire occupied 
bandwidth becomes 18 kHz. 
The present radio regulation provides that the permissible occupied 
bandwidth in the short wave band is 6 kHz or less for radio stations 
except broadcasting stations. 
Therefore, the radio communication system of the prior art, i.e., the 
communication system in which the plane of polarization of a radio wave is 
rotated at a high speed to remove fading is inconsistent with the 
regulations. 
The radio communication system of the prior art, i.e., the communication 
system in which the plane of polarization of a radio wave is rotated at a 
high speed to remove fading is inconsistent with the existing regulations. 
This problem becomes a large obstacle in putting the system into practice 
for normal radio stations. 
SUMMARY OF THE INVENTION 
The present invention has been made to solve the above problem, and has as 
its object to provide transmission method and apparatus which remove 
fading to allow high-quality transmission/reception in radio communication 
in the short wave band which is an important medium used for short wave 
narrow-band direct-printing telegraphy in maritime communication, general 
communication, search and rescue communication, and international 
broadcasting. 
In order to achieve the above object according to an aspect of the present 
invention, there is provided a short wave transmission method comprising 
the steps of circularly polarizing a carrier in a short wave band and 
radiating the carrier from an antenna, thereby removing an influence of 
fading. 
According to another aspect of the present invention, there is provided a 
short wave transmission apparatus having a transmission apparatus 
comprising carrier generation means for generating a carrier in a short 
wave band, subcarrier generation means for generating a subcarrier, a 
first mixer for receiving the carrier and the subcarrier, a phase shifter 
for phase-shifting the subcarrier generated from the subcarrier generation 
means by .pi./2, a second mixer for receiving the carrier and a subcarrier 
which is phase-shifted by .pi./2 by the phase shifter, and two orthogonal 
dipole antennas, wherein an output from the first mixer is input to one of 
the two dipole antennas, and an output from the second mixer is input to 
the other of the two dipole antennas, thereby radiating a circularly 
polarized wave. 
The basic principle of the present invention will be described below. 
The present inventor was convinced from theoretical calculations and a lot 
of experimental results that, when the radio communication system of the 
prior art was implemented, the influence of fading could be technically 
removed to allow high-quality signal transmission/reception. 
To put this radio communication system into practice, a system usable 
within the limitation of the permissible occupied bandwidth in the short 
band, which is consistent with the existing radio regulations, i.e., the 
permissible occupied bandwidth of 6 kHz or less (for radio stations except 
broadcasting stations) must be realized. 
In FIG. 11, the change in frequency (spectrum) and phase of each signal 
passing through the blocks of the transmission apparatus shown in FIG. 8 
has been described using equations. 
In FIG. 11, the angular frequency of a carrier is .omega., and the angular 
frequency of the low-frequency signal (corresponding to the angular 
frequency of rotation of the plane of polarization) is .theta.. 
More specifically, theoretical calculations and experiments were conducted 
by setting .theta. (corresponding to the angular frequency of rotation of 
the plane of polarization)=the value of the angular frequency .omega. of 
the carrier. 
In this case, the relationship between the change in frequency (spectrum) 
and the change in phase of each signal passing through the blocks will be 
described below in correspondence with FIG. 11. 
FIG. 2 explains a change in frequency (spectrum) and phase of each signal 
passing through the blocks when the angular frequency .theta. of rotation 
of the plane of polarization of a radio wave is set to be equal to the 
angular frequency .omega. of the carrier in the present invention. The 
same reference numerals as in FIG. 8 or 11 denote the same parts in FIG. 
2. 
FIG. 2 corresponds to FIG. 11 and shows a case wherein .theta.=.omega.. 
For this reason, the low-frequency signal output from the low-frequency 
oscillator 2 is represented by equation (13) instead of equation (2): 
EQU low-frequency signal F.sub.2 =B.multidot.cos.omega.t (13) 
Therefore, two signals represented by equations (14) and (15) are output 
from the balanced modulator 4Y shown on the upper side of FIG. 2 and input 
to the 0.degree. upper sideband filter 5Y and the 0.degree. lower sideband 
filter 6Y, respectively: 
EQU F.sub.11 =C.multidot.sin(.omega.+.omega.)t (14) 
EQU F.sub.12 =C.multidot.sin(.omega.-.omega.)t (15) 
Equation (14) corresponds to equation (3) where .theta.=.omega.. Similarly, 
equation (15) corresponds to equation (4) where .theta.=.omega.. 
Since the latter equation (15) is "0", the signal input to the 0.degree. 
lower sideband filter 6Y becomes "0". Therefore, the signal component 
power-amplified by the SSB (lower side) transmitter BY also becomes "0". 
For this reason, only the signal represented by equation (14), which is 
output from the balanced modulator 4Y, i.e., only the signal 
power-amplified by the SSB (upper side) transmitter 7Y is synthesized 
(added) by the output synthesizer 9Y to generate a signal represented by 
equation (16): 
EQU F.sub.11 =M.multidot.sin(.omega.+.omega.)t=M.multidot.sin.omega.t(16) 
A low-frequency signal represented by equation (17) is input from the 
90.degree. phase shifter 3 to the balanced modulator 4X shown on the lower 
side: 
EQU F.sub.2 =B.multidot.cos(.omega.t-.pi./2) (17) 
Therefore, the two signals output from the balanced modulator 4X are 
represented as follows: 
EQU F.sub.21 =C.multidot.sin{(.omega.+.omega.)t-.pi./2} (18) 
EQU F.sub.22 =C.multidot.sin{(.omega.-.omega.)t-.pi./2} (19) 
In this case, the latter equation (19) is converted to equation (20): 
##EQU1## 
As a result, the signal input to the 90.degree. lower sideband filter 6X 
becomes a constant "-C", which is independent of the angular frequency 
.omega. of the carrier or rotation of the plane of polarization. 
Therefore, the signals represented by equations (18) and (20), i.e., the 
signals sent to the SSB (upper side) transmitter 7X and the SSB (lower 
side) transmitter 8X, power-amplified, and synthesized (added) by the 
output synthesizer 9X at the next stage, thereby generating a signal 
represented by equation (21): 
##EQU2## 
The signal represented by equation (21) is a substantially circularly 
polarized wave which is decentered because the value of the second term is 
independent of t. 
In this manner, according to the transmission apparatus shown in FIG. 2, 
the signal represented by equation (16) is generated by the output 
synthesizer 9Y and output to one (Y antenna 11Y) of the orthogonal dipole 
antennas through the balun 10Y while the signal represented by equation 
(21) is generated by the output synthesizer 9X and output to the other 
dipole antenna (X antenna 11X) through the balun 10X. 
The signals output from the two baluns 10Y and 10X are equalized with the 
carrier which is phase-shifted by 90.degree.. 
Therefore, a signal whose plane of polarization is rotated by the angular 
frequency .omega. is radiated from the orthogonal dipole antennas. 
From the above analysis result, it was proved that, when the angular 
frequency .omega. of the carrier was equalized with the angular frequency 
.theta.(=.omega.) of the low-frequency signal for rotating the plane of 
polarization of a radio wave, the carrier of the angular frequency .omega. 
was rotated at a high speed and eventually radiated as a circularly 
polarized wave of an angular frequency 2.omega.. 
In this case, the signals supplied to the elements of the orthogonal dipole 
antennas, i.e., the Y antenna 11Y and the X antenna 11X are not 
balanced-modulated waves. Therefore, no balanced-modulated wave is 
generated on the reception antenna in the electric field radiated from 
these antennas 11Y and 11X, so no fading occurs. 
In the radio communication system of the above-described prior art, two 
radio waves are present. In this case, however, one circularly polarized 
wave is used. For this reason, the spectrum of one of the radio waves is 
erased, so that the occupied bandwidth can be set within the range of 
regulation values. 
On the basis of this recognition, of the blocks in FIG. 2, as for the Y 
wave on the upper side, the 0.degree. lower sideband filter 6Y and the SSB 
(lower side) transmitter 8Y to which a signal of "0" is input are not 
needed. As for the X wave on the lower side, the 90.degree. lower sideband 
filter 6X and the SSB (lower side) transmitter 8X, which have only a 
function of slightly shifting the center of rotation to the left side 
because the constant -M is independent of t, are not functionally needed. 
For this reason, when the substantially insignificant blocks are removed, a 
transmitter apparatus shown in FIG. 1 (to be described later) can be 
constituted. 
The short wave transmission apparatus of the present invention 
substantially removes fading which is conventionally regarded as 
inevitable, thereby allowing high-quality radio communication. At the same 
time, the short wave transmission apparatus is highly practical because 
the permissible occupied frequency in the short wave band is consistent 
with the existing radio regulations. 
In the present invention, to solve the problem of the radio communication 
system of the prior art, i.e., the problem that two radio waves are 
necessary for one-line communication, and the permissible occupied 
bandwidth inconsistent with the existing radio regulations impedes 
practical use, the plane of polarization of a radio wave is rotated with 
the same frequency as that of the carrier, thereby allowing 
transmission/reception using only one radio wave. 
In addition, since only the apparatus on the transmission side is improved, 
the conventional short wave reception function suffices on the reception 
side. Without fading, power saving can be achieved with a small power 
consumption.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment! 
A short wave transmission apparatus according to the first embodiment of 
the present invention will be described below in detail with reference to 
the accompanying drawings. 
The first embodiment corresponds to claim 2. 
FIG. 1 shows the arrangement of the main part of the short wave 
transmission apparatus according to the first embodiment of the present 
invention. Referring to FIG. 1, reference numeral 21 denotes a carrier 
oscillator; 22, a multiplier; 23, a subcarrier oscillator; 24, a .pi./2 
phase shifter; 25X and 25Y, mixers; 26X and 26Y, tuned amplifiers; 27X and 
27Y, power amplifiers; 28X and 28Y, baluns; 29X, an X antenna; and 29Y, a 
Y antenna. Reference symbol f.sub.o denotes a carrier frequency; f.sub.s, 
a subcarrier frequency; f.sub.s, an output frequency of a subcarrier which 
is phase-shifted by .pi./2 by the .pi./2 phase shifter 24; f.sub.i, an 
output frequency from the mixer 25Y, which corresponds to an output 
frequency from the power amplifier 27Y; f.sub.i, an output frequency from 
the mixer 25X, which corresponds to an output frequency from the power 
amplifier 27X; and n, a multiplication factor of the frequency multiplier 
22. 
A carrier of the frequency f.sub.o is output from the carrier oscillator 1 
shown on the left side of FIG. 1 and multiplied by n by the frequency 
multiplier 22 at the next stage to generate a carrier in the short wave 
band. 
The carrier multiplied by n is input to the mixers 25Y and 25X. 
A subcarrier of the subcarrier frequency f.sub.s is output from the 
subcarrier oscillator 23 and input to the mixer 25Y and the .pi./2 phase 
shifter 24. 
On the upper side associated with a Y wave, a signal of the output 
frequency f.sub.i =(f.sub.o .times.n)-f.sub.s is generated from the mixer 
25Y. The signal is amplified by the tuned amplifier 26Y and the power 
amplifier 27Y and supplied to the Y antenna 29Y through the balun 28Y. 
On the lower side associated with an X wave, the carrier multiplied by n 
and the subcarrier of the output frequency f.sub.s' which is 
phase-shifted by .pi./2 are input to the mixer 25X to generate a signal of 
the output frequency f.sub.i' =(f.sub.o .times.n)-f.sub.s'. 
The signal of the output frequency f.sub.i' is amplified at the next stage 
by the tuned amplifier 26X and the power amplifier 27X and supplied to the 
X antenna 29X through the balun 28X. 
Therefore, a circularly polarized wave of a frequency f.sub.i is radiated 
from the Y antenna 29Y and the, X antenna 29X of the orthogonal dipole 
antennas. 
Since the circularly polarized radio wave has a single spectrum, the 
occupied bandwidth is smaller than that in the radio communication system 
of the prior art and sufficiently consistent with the existing radio 
regulations. 
Therefore, when the circularly polarized wave radiated from the orthogonal 
dipole antennas is received, a high-quality short wave without fading can 
be received. 
In FIG. 1, to generate the carrier (frequency f.sub.i) as a reference, the 
carrier oscillator 21 and the frequency multiplier 22 are arranged. 
However, a synthesizer oscillator or the like can also be used, as a 
matter of course. 
As described above, the radio communication system and transmission 
apparatus of the present invention have a very simple arrangement. In 
addition, this system is not influenced by fading, unlike the prior art. 
Furthermore, this system is sufficiently consistent with the existing radio 
regulations, and particularly, the regulations of the permissible occupied 
bandwidths. 
Therefore, this system can be optimally used for radio communication in the 
short wave band which is important for maritime communication or 
international broadcasting, and its practical effect is conspicuous. 
Experimental Example 
In an experiment, the Y antenna 29Y and the X antenna 29X, which are 
orthogonal to each other, were excited by carrier currents (in the short 
wave band) accurately phase-shifted from each other by .pi./2. 
For a facsimile apparatus, frequency shift signals of .+-.400 Hz having a 
center frequency corresponding to the carrier frequency were used. A shift 
signal of -400 Hz represented "black", and a shift signal of +400 Hz 
represented "white". 
One horizontal scanning period was set at one second, and the index of 
cooperation was set at 576, which is a constant defined as the WMO 
standard in a short wave facsimile apparatus. 
A pulse of 500 ms corresponding to 1/2 the horizontal scanning period on 
the image transmission side was generated by dividing another accurate 
oscillation frequency, and the transmission output on the X antenna side 
was intermittently switched. 
According to this method, a linear polarized wave and a circularly 
polarized wave are sequentially switched every 500 ms and transmitted. 
Therefore, when these radio waves are received by the facsimile apparatus, 
the linear polarized wave reception portion and the circularly polarized 
wave reception portion are separated to the left and right sides on the 
received image. 
At the same time, when the reception antenna output voltages are 
continuously recorded, the influences of fading in the two waves can be 
compared and observed. 
The transmission frequency was set within the range of 10.100 MHz to 10.150 
MHz. Powers radiated from the two dipole antennas were accurately adjusted 
at an equal level of about 75 W. 
The reception site was separated from the transmission site by about 350 
km, where it was expected that, in this frequency band, the direct wave 
had no coverage although the main path of an ionospheric wave had 
satisfactorily coverage, and the multipath of a multiple reflection wave 
could also be received (Tokyo and Yokkaichi-shi in Mie Prefecture) 
At the reception site, a .lambda./2 dipole antenna was extended, drawn into 
the room through a coaxial cable, halved by a divider, and connected to a 
facsimile receiver and a short wave receiver. 
The facsimile receiver was used to receive a transmitted image, and the 
short wave receiver was used to simultaneously measure the antenna output 
voltage. 
An image was received while sequentially switching between the linear 
polarized wave and the circularly polarized wave. The image of the 
circular polarized wave had very high quality. However, the image of the 
linearly polarized wave had almost unreadable image portions due to 
fading. 
Experiments were conducted while changing the form of the transmission 
radio wave. 
FIG. 3 shows an experimental result, which shows a change in reception 
antenna output voltage caused by the circularly polarized/unmodulated 
radio wave. Time is plotted along the abscissa, and the antenna output 
voltage is plotted along the ordinate. 
FIG. 4 shows an experimental result, which shows a change in reception 
antenna output voltage caused by the linearly polarized/unmodulated radio 
wave. Plotting along the abscissa and ordinate is the same as in FIG. 3. 
As is apparent from comparison between FIGS. 3 and 4, the variation in 
reception antenna output voltage in FIG. 3 showing transmission/reception 
using the circularly polarized wave is much smaller than that in FIG. 4. 
Therefore, it was confirmed that the influence of fading was largely 
decreased to allow practical high-quality transmission/reception. 
FIG. 5 shows an experimental result, which shows reception antenna output 
voltages at the fading points of a circularly polarized wave and a 
linearly polarized wave. Time is plotted along the abscissa, and the 
reception antenna output voltage is plotted along the ordinate. A broken 
line a indicates an estimated output level in transmission using only the 
circularly polarized wave; a chain line b, an estimated output level in 
transmission using only the linearly polarized wave; c, a transmission 
portion without fading; d, the circularly polarized wave transmission 
portion; and e, a linearly polarized wave transmission portion. 
In FIG. 5, the circularly polarized wave and the linearly polarized wave 
shown in FIGS. 3 and 4 were alternately transmitted for 500 ms. 
The transmission portion c shown on the left side has the same level as 
that of the circularly polarized wave because no fading occurs in the 
linearly polarized wave. 
The low level portion e shown on the right side is the linearly polarized 
wave transmission portion with fading. 
The portion d with a minimum level change on the right side is the 
circularly polarized wave transmission portion. 
As is apparent from FIG. 5, fading occurs in the linearly polarized wave 
transmission portion, although no fading occurs in the circularly 
polarized wave transmission portion. 
Each tooth of the comb-like shape (transmission time of the portion d or e) 
corresponds to 500 ms along the time axis. 
The reason why the level decreases at the linearly polarized wave portion 
(portion e) is that the plane of polarization of the radio wave arriving 
at the reception antenna is rotated due to rotation of the plane of 
polarization caused in the ionosphere. 
The level changes by 26 to 30 dB from the maximum level although the level 
of the circularly polarized wave portion changes only about 6 dB. 
The data in FIG. 5 was obtained by calibrating the relationship between the 
input voltage to the receiver for measurement and the AGC voltage with a 
measurement result obtained using a standard signal generator and an 
experimental frequency. 
From the above experimental result, it was proved that circularly polarized 
wave transmission was more stable against the influence of fading than 
linearly polarized wave transmission. 
Therefore, it was confirmed that the short wave transmission apparatus 
according to the present invention was stable against the influence of 
fading, and the occupied bandwidth was consistent with the existing radio 
regulations. 
As the modulation method for a facsimile apparatus, the frequency 
modulation method (e.g., signals for discriminating "white" and "black" 
are transmitted within the range of .+-.400 Hz) is optimally used. 
However, the present invention can also be applied to the normal amplitude 
modulation method or another known modulation method. 
As described above, in the conventional short wave broadcasting, a 
plurality of frequency bands are simultaneously used for transmission, and 
one frequency with minimum fading is selected and received, thereby 
avoiding the influence of fading. However, according to the short wave 
transmission apparatus of the present invention, high-quality 
transmission/reception can be performed using only one frequency with 
sufficient coverage. 
In short wave broadcasting, no radio wave can be received within a range 
of, e.g., about 100-km radius. However, when a total of only two stations 
are set, i.e., when one station is placed in Kanto area, and the other is 
placed separated therefrom by 100 km or more, all areas in Japan can be 
covered. 
In this case, a transmission output of 10 kW or more is conventionally 
required. However, for the short wave transmission apparatus of the 
present invention, a transmission output of 1/10 that of the prior art, 
i.e., a transmission output of about 1 kW suffices. Therefore, power 
saving or size reduction of equipment can be achieved with a small power 
consumption. 
Second Embodiment! 
The second embodiment will be described below. 
This embodiment corresponds to claim 3. 
FIG. 6 shows the arrangement of the main part of a short wave transmission 
apparatus according to the second embodiment of the present invention. 
Referring to FIG. 6, reference numeral 31 denotes an oscillator; 32, a 
frequency multiplier; 33, a buffer amplifier; 34, a power amplifier; 35, a 
power divider; 36X and 36Y, coaxial cables; 37X and 37Y, baluns; 38X, an X 
antenna; and 38Y, a Y antenna. Reference symbol f.sub.o denotes an 
oscillation frequency; L, a length of the coaxial cable 36X; L', a length 
of the coaxial cable 36Y; and p and q, contacts. 
In the short wave transmission apparatus shown in FIG. 6, a circularly 
polarized wave is transmitted from a single transmission apparatus through 
the X antenna 38X and the Y antenna 38Y, which are orthogonal to each 
other. 
When a signal generated from the oscillator 31 has the oscillation 
frequency =f.sub.o, i.e., .omega..sub.o =2.pi.f.sub.o, and the amplitude 
is A, an oscillation current i.sub.o is represented as follows: 
EQU i.sub.o =A.multidot.sin(.omega..sub.o .multidot.t) (22) 
This signal is output to the frequency multiplier 32 at the next stage. 
When the multiplication factor is n, a multiplied frequency f.sub.c 
=n.multidot.f.sub.o, and .omega..sub.c =2.pi.f.sub.c. When the amplitude 
is B, a current i.sub.1 is represented as follows: 
EQU i.sub.1 =B.multidot.sin(.omega..sub.c .multidot.t) (23) 
Thereafter, the signal of the frequency f.sub.c is amplified by the buffer 
amplifier 33 and the power amplifier 34. When the amplitude at the last 
stage is C, a transmission carrier current i.sub.c is represented as 
follows: 
EQU i.sub.c =C.multidot.sin(.omega..sub.c .multidot.t) (24) 
The power divider 35 at the next stage halves the input power, so that the 
amplitude of each current becomes C/.sqroot.2. 
When currents supplied to the coaxial cables 36X and 36Y are i.sub.c1 and 
i.sub.c2, respectively, 
##EQU3## 
When the length of one coaxial cable, e.g., the coaxial cable 36X is L, the 
length L' of the other coaxial cable 36Y is represented as follows: 
EQU L'=L+.lambda./4 (26) 
where .lambda. is the wavelength. 
In this manner, the length of one of the pair of coaxial cables 36X and 36Y 
is delayed by 1/.lambda.. When the current i.sub.c1 at the contact p is 
represented as follows: 
##EQU4## 
the current i.sub.c2 at the other contact q is represented as follows: 
##EQU5## 
so that a circularly polarized wave is generated. Third Embodiment! 
The third embodiment will be described below. 
This embodiment corresponds to claim 5. 
FIG. 7 shows the arrangement of the main part of a short wave apparatus 
according to the third embodiment of the present invention. The same 
reference numerals as in FIG. 6 denote the same parts in FIG. 7, and 
reference numeral 41 denotes an impedance matching device; 42, a 
reflecting plate; and 43, a helical antenna. 
The short wave transmission apparatus shown in FIG. 7 has the same basic 
arrangement as in FIG. 6 and is different only in that the helical antenna 
43 is used in place of the dipole antennas. 
More specifically, a transmission carrier current i.sub.c amplified by an 
power amplifier 34 is supplied to the helical antenna 43 having the 
reflecting plate 42 through the impedance matching device 41. 
A circularly polarized wave can be directly obtained because of the antenna 
characteristics of the helical antenna 43. 
In this case, since the circumference of the helical antenna 43 must be 
equalized with a wavelength .lambda., the diameter of the helical antenna 
43 becomes about .lambda./3. 
The present invention is not limited to the above embodiments, and various 
changes and modifications can also be made without departing from the 
scope or spirit of the invention as defined in claim 1. 
According to the short wave transmission apparatuses of claims 1 to 2, in 
radio communication in the short wave band which is not conventionally so 
useful, the reception disabled state caused by fading is prevented, so 
that high-quality transmission/reception is allowed. 
This apparatus can be optimally used for radio communication in the short 
wave band which is important for maritime communication or international 
broadcasting and is also very useful for practical use of short wave radio 
transmission of digital data or facsimile data. Therefore, the application 
range can be largely increased. 
In addition, the apparatus on the transmission or reception side can be 
constituted with a simple arrangement. Therefore, economical advantages 
and effective utilization of precious radio waves can also be achieved.