KB6NU's Ham Radio Blog

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2016 Extra Class study guide: E8D - Keying defects and overmodulation

Posted: 23 Feb 2016 05:24 PM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email


E8D Keying defects and overmodulation of digital signals; digital codes;
spread spectrum

It is good amateur practice to ensure that the CW and digital signals you
transmit are high quality. Perhaps the biggest problem that youll have when
sending CW signals is key clicks. Key clicks are spurious signals that
cause interference to other stations operating near your frequency. The
generation of key clicks is the primary effect of extremely short rise or
fall time on a CW signal. (E8D04) It follows, then that the most common
method of reducing key clicks is to increaseÂ*keying waveform rise and fall
times. (E8D05) Fortunately, most modern transceivers allow you to set the
rise and fall times of the CW signal, so this is an easy fix.

To ensure high-quality digital signals, such as when transmitting audio
frequency shift signals, such as PSK31, you need to set the audio input
level properly. A common cause of overmodulation of AFSK signals is
excessive transmit audio levels. (E8D07). Strong ALC action indicates
likely overmodulation of an AFSK signal such as PSK or MFSK. (E8D06)

Intermodulation Distortion (IMD) is a parameter that you can measure that
might indicate that excessively high input levels are causing distortion in
an AFSK signal. (E8D08) A good minimum IMD level for an idling PSK signal
is -30 dB. (E8D09)

Digital codes

Although ASCII and Unicode have now become standard codes for sending
textual information, we still use the Baudot code when sending and
receiving RTTY. Some of the differences between the Baudot digital code and
ASCII are that Baudot uses 5 data bits per character, ASCII uses 7 or 8;
Baudot uses 2 characters as letters/figures shift codes, ASCII has no
letters/figures shift code. (E8D10)

Even though it uses more bits per character, ASCII does have some
advantages over Baudot. For example, one advantage of using ASCII code for
data communications is that it is possible to transmit both upper and lower
case text. (E8D11)

In an eight-bit ASCII character, the eighth bit is the partity bit. In
systems that use even parity, the parity bit is set to either a one or a
zero, so that the number of ones in the character is equal to an even
number. In systems that use odd parity, the parity bit is set to either a
one or a zero, so that the number of ones in the character is equal to an
odd number. The advantage of including a parity bit with an ASCII character
stream is that some types of errors can be detected. (E8D12)

Spread spectrum

Amateurs can now use spread-spectrum techniques on all bands above 420 MHz.
The reason these bands are used is because spread-spectrum signals require
more bandwidth than is available on the lower frequency bands.

Spread spectrum transmissions generally change frequency during a
transmission. This is called frequency hopping. The way the spread spectrum
technique of frequency hopping works is that the frequency of the
transmitted signal is changed very rapidly according to a particular
sequence also used by the receiving station. (E8D03) Direct sequence is a
spread spectrum communications technique that uses a high speed binary bit
stream to shift the phase of an RF carrier. (E8D02)

Because transmission and reception occur over a wide band of frequencies,
spread spectrum communications are less susceptable to interference on a
single frequency than are more conventional systems. Received spread
spectrum signals are resistant to interference because signals not using
the spread spectrum algorithm are suppressed in the receiver. (E8D01)

The post 2016 Extra Class study guide: E8D Keying defects and
overmodulation appeared first on KB6NUs Ham Radio Blog.


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FCC Invites Comments on ARRL Petition That Seeks 80/75 Meter Adjustments

Posted: 23 Feb 2016 05:15 PM PST
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SB QST @ ARL $ARLB007

ARLB007 FCC Invites Comments on ARRL Petition That Seeks 80/75
MeterÂ*Adjustments

ZCZC AG07
QST de W1AW

ARRL Bulletin 7 Â*ARLB007

From ARRL Headquarters

Newington CT Â*February 23, 2016

To all radio amateurs
SB QST ARL ARLB007

ARLB007 FCC Invites Comments on ARRL Petition That Seeks 80/75
MeterÂ*Adjustments
The FCC has put the ARRLs January Petition for Rule Making (RMÂ*11759Â*) on
publicÂ*notice and invited interested parties to comment on what the
LeagueÂ*has called minimal but necessary changes to 80 and 75 meters.
TheÂ*ARRL petitioned the FCC to fix a shortfall in available
RTTY/dataÂ*spectrum that the Commission created when it reapportioned 80
andÂ*75 meters 10 years ago.
The Leagues petition asked the FCC to shift the boundary betweenÂ*the 80
meter RTTY/data subband and the 75 meter phone/image subbandÂ*from 3600 kHz
to 3650 kHz. The proposed change received strongÂ*support from ARRL members,
and the ARRL Board of Directors adoptedÂ*it as policy at its July 2015
meeting. At that time the Board alsoÂ*agreed to seek RTTY and data
privileges for Novice and TechnicianÂ*licensees within their current 15
meter CW subband, and to do theÂ*same on 80 meters, depending on the outcome
of the 80/75 meterÂ*subband revision.
The petition asks the FCC to make the following changes to the PartÂ*97
Amateur Radio Service rules, with respect to 80/75 meters:

Modify the RTTY/data subband, so that it extends from 3500 kHz toÂ*3650 kHz.
Modify the phone/image subband, so that it extends from 3650 kHzÂ*to 4000
kHz.
Make 3600-3650 kHz available for General and Advanced ClassÂ*licensees, as
was the case prior to 2006.
Make 3600-3650 kHz available to Novice and Technician licenseesÂ*for
telegraphy consistent with existing rules permitting NovicesÂ*and
Technicians to operate CW in the 80, 40, and 15 meter GeneralÂ*and Advanced
RTTY/data subbands.
Modify the rules governing automatically controlled digitalÂ*stations
(ACDS), to shift the ACDS segment from 3585-3600 kHz toÂ*3600-3615 kHz,
consistent with the IARU Region 1 and 2 band plans.

According to the ARRL, the FCC R&O in Docket 04-140 released in
2006Â*departed substantially and without justification from the
rulesÂ*proposed in the FCCs so-called Omnibus Notice of Proposed RuleÂ*Making
(NPRM), with respect to 75 and 80 meters. Among otherÂ*actions, the
resulting changes expanded voice privileges onÂ*additional frequencies in
various bands, including 75 meters. TheÂ*FCC shifted the phone/image subband
from 3750-4000 kHz to 3600-4000Â*kHz, trimming the 80 meter RTTY/data
subband from 3500-3750 kHz toÂ*3500-3600 kHz and substantially changing the
entire dynamic of thisÂ*band, the League said.
Although the Omnibus R&O had indicated that incumbent licenseesÂ*would not
lose any operating privileges, some clearly did, the ARRLÂ*has pointed out.
The most substantial adverse effect of theÂ*unexpected and vast expansion of
the 75 meter phone/image subband,Â*the League said, was the elimination of
access to 3620-3635 kHz byÂ*ACDS.
The Omnibus R&O rule changes limited 80 meters to 3500-3600 kHz, andÂ*no
longer authorized RTTY and data emissions above 3600 kHz. ThatÂ*the Omnibus
R&O did not modify Part 97.221 of the rules to provideÂ*for ACDS was clearly
an oversight by the Commission.
After the FCC denied a subsequent ARRL Petition for Reconsideration,Â*the
Commission replaced the inadvertently deleted 3620-3635 kHz ACDSÂ*segment
with 3585-3600 kHz.
Far from fixing the problem created by the error in the OmnibusÂ*R&O, the
moving of the inadvertently deleted digital subbandÂ*downward in frequency
below 3600 kHz made the situation in the 80Â*meter RTTY/data subband even
worse than it was, the ARRL said. TheÂ*result has been a shortfall in
available RTTY/data spectrum at 80Â*meters.
ARRL has analyzed the regulatory limitations as part of aÂ*comprehensive
effort to make more efficient the use of those HFÂ*allocations, especially
with respect to encouraging furtherÂ*experimentation and proficiency in
narrowband digital communicationsÂ*technologies, the League said in
concluding its Petition. TheÂ*recommendations for modified band plans
developed by ARRLÂ*necessitate the few, but important regulatory changes
proposed.
NNNN

/EX

The post FCC Invites Comments on ARRL Petition That Seeks 80/75 Meter
Adjustments appeared first on KB6NUs Ham Radio Blog.