300 KHz-30 MHz MF_HF(Goldberg1966)

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  • 8/17/2019 300 KHz-30 MHz MF_HF(Goldberg1966)

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    IEEE TRANSACTIONS

    O N

    COMMUNICATION TECHNOLOGY VOL. COM-14,

    N O .

    6 DECEMBER 1966

    In addition to the device reliability considerations, an

    environmental study was made to verify that the system

    could withstand shipping, storage, andoperating condi-

    tions on the customer's premises. Laboratory tests which

    included temperature shock, high relative humidity, and

    vibrationwereused tostimulate he expectedenviron-

    mental extremes to which the equipment would be sub-

    jected in actual use. As a result of these tests minor design

    modifications were incorporatedprior to initial production.

    The majorityof equipment malfunctions a,re expected to

    be repairable by merelynterchangingprintedwiring

    boards. Trouble shooting

    of

    common circuitry consists of

    the analysis of symptoms to narrow the troubledown to a

    small number of possible circuit packs, and then the re-

    placement of theseone at a imeuntil he defective

    board is located. Troubles on traffic circuits, such as reg-

    isters and centraloffice trunks, can be isolated by a feature

    which enables a repairman to route test calls to specific

    circuits. To facilitate testing, t,he built-in test equipment,

    fuses, and alarms are located at the front of the cabinet ,

    at eye level.

    CONCLUSION

    The 800A

    PBX

    was introduced into commercial service

    in August 1966. Operational experience with he system

    has been very good.

    300

    kHz-30

    MHz MF/HF

    Abstract-A

    tutorial presentation is made in broad and general

    terms regarding the properties of the

    MF

    and HF portions of the

    radio spectrum as they pertain to and affect communication systems.

    The fine grain behavior in terms of amplitude and phase variations

    are presented in conjunction with the effects of fading periods, t ime

    and frequency spread, and atmospheric noise.

    A

    discussion of both

    theoretical and experimental bounds in error rate levels of digital

    systemsas a function

    of

    the basic attributes of the ionospheric

    channel is undertaken in connection with the adaptive approach to

    communication system design. Two adaptive systems are described

    briefly in terms of their ability to cope with the time variant dis-

    persive ionospheric channel.

    T

    E PURPOSE of this paper will be to serve as

    a

    broad tutorial coverage of the elements and factors

    employed for characterizinghe arious channels, as

    segmented assignments, in he frequency spectrum extend-

    ing from 300 kHz to 30 MHz. In th is connection, some of

    the propert ies f significance such as the temporal behavior

    in termsof signal levels and noise, channel transfer proper-

    ties, interference, fine grain behavior, and system perform-

    ance as exemplified by both theory and experimental data,

    will be covered.

    Liberal usewill be made of materia l already in the open

    literature, material available to USAEL through their var-

    ious contracts with industry andniversities, and data and

    informationgeneratedasa result of USAEL'sown in-

    houseprograms.

    Although a good portion of the mater ial to be covered

    will not be new to workers in this field, it is considered

    Paper 19CP65-482 presented

    at

    the 1965

    IEEE

    Communications

    Manuscript received March 28, 1966; revised August

    I,

    1966.

    Convention, Boulder, Colo.

    Thenthor is withhe Communications/ADP Laboratory,

    U.S.

    Army Electronics Command, Fort

    R'lonmouth, N. J.

    appropriate that it stillbe presented for the edification of

    those people desiring to understand this area of activity

    and for the sake of completeness.

    In order to do justice to the broad spectrumovered by

    this paper, it will be necessary to break apart the 0.3-30

    nilHz frequency slot into four categories and then discuss

    three of these categories (the medium frequencies) in

    a

    limited way while reserving the bulk of the discussion to

    the 4th category 3.0-30 MH z (the high frequencies).

    The so-called medium requency (MF) spectrum ex-

    tending from 0.3 to 3

    Hz

    for the purpose of this presenta-

    tion,as uststated, will bedivided into hreedistinct

    regions approximated by region A00-550, kHz, region

    B,

    550-1650 kHz and region C, 1650 to 3000 kHz. Region A,

    employing

    CW

    transmission almost exclusively, generally

    is utilized for navigational purposes, for mobile, aero-

    nautical and ship communications, for emergency survival

    communications, and for time and frequency synchroniza-

    tion. Region

    B

    is employed for standard broadcast service

    and region C

    is

    and may be utilized for fixed and mobile,

    land, maritime and aeronautical navigation, and commu-

    nication purposes.

    In

    these three regions most of the useful distant field

    energy is propagatedby heground

    or

    surfacewave.

    The sky wavegenerally presents a sourceof trouble, how-

    ever, it is occasionally utilized as the primarymode, espe-

    cially for region C.

    As far as the surface wave support is concerned, which

    can be viewed as due toan earth-atmosphere wave guide,

    the signal strength is reasonably well behaved. Generally t

    follows an inverse distance law with the value of signal

    strength being

    a

    function of the polarization, operating

    frequency,and hegroundconductivityand dielectric

    767

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    765 I E E E TRANSACTIONS O N C O M M U N I C A I lO NE C H N O L O G YE C E M B E R

    constant along the pathof propagation. In this regard, sea

    water (conductivity4 X lo- EMU, ielectric constant80

    ESU , rovides the pathwith the least attenuation. Aoor

    earth , hat is, earthwith low conductivi.ty

    EMU

    and low dielectric constant

    3-5 ESU

    ields a path with

    relativelyhigh ttenuation.There is littlediurnal or

    annual variation in the ground wave characteristic.

    In region A, under good conditions, ground wave propa-

    gation can reach 000miles with only40 dB more loss than

    that due to the inverse distance loss. Theoretical work of

    significance in this area has been performed by Sommerfeld,

    Morton,vanderPolandBrenner,Watson,andWait.

    Sky wave propagation for this groupf frequencies exhibits

    properties which are dependent upon he stateof the iono-

    sphere with signals experiencing, change in level by a fac-

    to r of from 2 to 5 as a function of sun spot activity. The

    existence of the sky wave gives rise to fading and inter-

    ference effects t locations where110th the groundwave and

    sky wave are received. This interference effect tends to

    tak e place with maximum severity at distances of a few

    hundred miles from the transmitterwhere both theground

    wave and the skywave are of equal strength.

    I n general, sky wave signals experience diurnaland sea-

    sonal variations superimposed upon the variations due to

    the sun spot cycle. Fortunately, during the daylight hours

    there is high absorption in the :D region, hence, the sky

    wave tends to be aroblem only during nighttime whenhe

    D layer disappears. The impact

    o:i

    ionospheric propagation

    on medium frequencies and high frequencies (HF) will be

    covered in the detailed discussion of the frequency region

    from 3-30 MHz.

    Some pertinent properties of sky wave transmission at

    MF, however, are best cited at this time. The envelope of

    the received signal n the majority.f cases tends to ollow

    a

    Rician distribution which could be viewed as the combina-

    tion of a Rayleigh distribution and a specular component.

    The fade rate is roughly 0.01 per second mplying ong

    fades. Equally rough estimates of the correlation distance

    forspaced antennas ndicates thatabout 20 km is re-

    quired for decorrelation t o a value 1 / ~ .

    As already tated for the lvIF region, theground

    waves enerally the mostlnportant primarily be-

    cause the energy is reasonably constant (nonfading) and

    appears compacted as a specular ray. It is interesting to

    note thatbecause of this specular nonfading characteristic,

    diversity reception would not enhance system reliability

    unless it could opera te on theresence of uncorrelated noise

    or interfence. In the ower frequelncy portions of this spec-

    trum limitationsdevelop n erms of antenna efficiency

    with values of

    10

    percent being considered good and with

    a communication bandwidth capa,bility n the ange

    100

    to

    500 cycles being typical. Unfortunately, for this portion f

    the radio spectrum (region A) atmospheric noise is quite

    high being roughly about two ord ers of magnitude greater

    tha n he level n the high requency 3-30 MHzband.

    I n general, communications reliability in this region tends

    to

    be

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