Amateur HF operation, whether for two-way contacts or for listening
to amateur transmissions, imposes stringent requirements
on the receiver. The need is for a receiver that enables an
experienced operator to find and hold extremely weak signals on
frequency bands often crowded with much stronger signals from
local stations or from the high-power broadcast stations using
adjacent bands. The wanted signals may be fading repeatedly to
below the external noise level, which limits the maximum usable
sensitivity of HF receivers, and which will be much higher than
in the VHF and UHF spectrum.
Although the receivers now used by most amateurs form part
of complex, factory-built HF transceivers, the operator should
understand the design parameters that determine how well or
how badly they will perform in practice, and appreciate which
design features contribute to basic performance as HF communications
receivers, as opposed to those which may make them
more user-friendly but which do not directly affect the reception
of weak signals. This also applies to dedicated receivers that are
factory-built, such as the one shown in Fig 6.1.
Ideally, an HF receiver should be able to provide good intelligibility
from signals which may easily differ in voltage delivered
from the antenna by up to 10,000 times and occasionally by up
to one million times (120dB) - from less than 1μV from a weak
signal to nearly 1V from a near-neighbour. To tune and listen to
SSB or to a stable CW transmission while using a narrow-band
filter, the receiver needs to have a frequency stability of within a
few hertz over periods of 15 minutes or so, representing a stability
of better than one part in a million. It should be capable of
being tuned with great precision, either continuously or in increments
of at most a few hertz.
A top-quality receiver may be required to receive transmissions
on all frequencies from 1.8MHz to 30MHz (or even
50MHz) to provide 'general coverage' or only on the bands allotted
to amateurs. Such a receiver may be suitable for a number
of different modes of transmission - SSB, CW, AM, NBFM, data
(RTTY/packet) etc - with each mode imposing different requirements
in selectivity, stability and demodulation (decoding).
Such a receiver would inevitably be complex and costly to buy
or build.
On the other hand, a more specialised receiver covering only
a limited number of bands and modes such as CW-only or
CW/SSB-only, and depending for performance rather more on
the skill of the operator, can be relatively simple to build at low
cost.
As with other branches of electronics, the practical implementation
of high-performance communications receivers has
undergone a number of radical changes since their initial development
in the mid-1930s, some resulting from the improved
stability needed for SSB reception and others aimed at reducing
costs by substituting electronic techniques in place of mechanical
precision.
However, it needs to be emphasised that, in most cases,
progress in one direction has tended to result in the introduction
of new problems or the enhancement of others: "What we call
progress is the exchange of one nuisance for another nuisance"
(Havelock Ellis) or "Change is certain; progress is not" (A J P
Taylor). As late as 1981, an Australian amateur was moved to
write: "Solid-state technology affords commercial manufacturers
cheap, large-scale production but for amateur radio receivers
and transceivers of practical simplicity, valves remain incomparably
superior for one-off, home-built projects." The availability of
linear integrated circuits capable of forming the heart of communications
receivers combined with the increasing scarcity
and hence cost of special valve types has tended to reverse this
statement. It is still possible to build reasonably effective HF
receivers, particularly those for limited frequency coverage, on
the kitchen table with the minimum of test equipment.
Furthermore, since many newcomers will eventually acquire a
factory-built transceiver but require a low-cost, stand-alone HF
receiver in the interim period, the need can be met either by
building a relatively simple receiver, or by acquiring, and if necessary
modifying, one of the older valve-type receivers that were
built in very large numbers for military communications during
the second world war, or those marketed for amateur operation
in the years before the virtually universal adoption of the transceiver.
Even where an amateur has no intention of building or servicing
his or her own receiver, it is important that he or she should
have a good understanding of the basic principles and limitations
that govern the performance of all HF communications
receivers.
ป้ายกำกับ: hf radio
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