exist which occasionally cause what are apparent 
holes or areas of absorption in the ionosphere 
where radio waves are not reflected. Ordinarily, 
during daylight hours, the ionosphere consists of 
three layers of ionization capable of reflecting 
radio waves. This is best illustrated in Figure l. 
The signal from point A to point B traverses three 
different paths simultaneously but, due to the dif- 
ferences in path lengths, do not arrive at point B 
simultaneously. This is another characteristic 
of the ionosphere called multipath propagation. 
When reliable signals are requiredthat 
are not distortedinamplitude, frequency, or time, 
HF systems cannot be tolerated. It was stated 
earlier that the pulse modulation techniques used 
in VHF and UHF systems were not usable in HF 
systems and only to a degree in scatter systems. 
This limitation results from the effects of multi- 
path. 
HF TECHNIQUES 
Perhaps the oldest use to which the HF 
spectrum has been applied for pulse propagation 
is radio-telegraph. Here data are transmitted in 
the form of short or long pulses called dots and 
dashes. The alternate signal paths caused by 
multipath ordinarily do not cause differential 
delays of more than 2 to 3 ms although, in some 
cases, delays of up to 14 ms have been measured. 
Three-ms delay is negligible at normal telegraph 
speeds andtherefore causes no trouble. However, 
attempts to speed up a conventional radio-tele- 
graph transmission beyond, say, 300 wpm, 
become unsuccessful because delayed, or multi- 
path, components of one signal element start to 
overlap the first arriving components of the suc- 
ceeding signal element. Preceding the arrival of 
those pulse components over longer transmission 
paths, the first arrived pulse will.have an am- 
plitude determined solely by ionospheric con- 
ditions, transmitted power, antennas, frequency, 
and like factors. The succeeding train of received 
pulses resulting from the one transmitted pulse 
and arriving via different propagation paths, will 
be of such phases andamplitudes as to increase or 
decrease the amplitude of the first arrived pulse 
and to distort its waveform. Inthe simple case 
of the two paths providing equal signal strengths, 
the resultant received signal may vary from zero 
totwicethe amplitude of either in the overlapping 
area, depending upon the relative r-f phase. This 
point is best shown by using an illustration. 
Figure 2 showsa series of transmitted pulses with 
the assumed position of a single multipath com- 
ponent. If the r-f of the multipath pulse is in- 
phase with the first arrival or primary pulse, 
both amplitude and width distortion will occur. 
If out-of-phase distortion occurs for a nonsyn- 
chronous system, the number of pulses have been 
multiplied. Figure 3 shows actual photographs of 
atransmitted pulse andthe corresponding received 
pulse train with the many multipath pulses. 
Another 
radio-teletype. 
old HF pulse system is FSK 
This system uses two carrier 
62 
frequencies separated conventionally by about 
800 cps. When one carrier is on it is calleda 
teletype mark, the other carrier being calleda 
teletype space. The two channels are never keyed 
simultaneously. The pulses, at normal speeds of 
60 or 100 wpm, are somewhat distorted by multi- 
path components but usually do not cause garbling; 
however, greatly increased speeds are not reliably 
attainable. 
Several other pulse systems exist which 
are used in the HF spectrum. For these the same 
argument holds thatthe pulse repetition frequency 
must be limited to retain intelligence. Any of 
these systems may be used quite successfully for 
slow speed data systems. They should be con- 
sidered seriously if they will handle the data rate 
desired. 
STATE-OF-THE-ART 
The communications industry has been 
faced with this multipath problem for years in 
their digital systems for data, printed-message 
communications, and speech. The solution to the 
problem lies inkeeping the pulse width sufficiently 
narrow to allow the primary pulse to completely 
arrive before its multipath components and to 
space these primary pulses out in time so as to 
allow all the multipath components of one pulse 
to arrive before the second pulse. If it is 
assumed that a3-ms gap between pulses will 
allow for the arrival of all important strength 
multipath components and a 1-ms pulse is trans- 
mitted, the data rate becomes 250 pps. 
The problems of multipath have been 
overcome successfully in several systems built 
for the U. S. Government. These systems utilize 
a modulationtechnique called Quantized Frequency 
Modulation. Many tests have proved that these 
systems will continue to perform satisfactorily 
when the more conventional systems fail. This 
technique enables the data rate to be increased 
and, at the same time, provides multipath protec- 
tion. Quantized Frequency Modulation (QFM) is a 
frequency shifting technique applicable to digital 
transmission systems in which the transmitter 
carrier frequency is caused to change cyclically 
with time in quantized increments. By means of 
discrete frequency changes in the receiving sys- 
tem, in synchronism with those in the transmitter, 
the receiving system is made responsive to the 
QFM channels in use at any instant intime, and 
ignores the other channels. Alternatively, the 
receiver may have a bank of filters, one for each 
QFM channel and, by means of sampling tech- 
niques, achieve the same results. In both cases, 
signals propagatedto the receiver over paths that 
have transmission time delays that differ from 
that of the prime path, arrive at the receiver at 
a time such that they are not normally effective 
in the receiver's digital decision process. In 
this manner, the deleterious effects of multipath 
are greatly diminished. A similar technique, 
termed Quantized Phase Modulation (QPM), 
contains the intelligence in the r-f phase as 
