Keeping Your HF Signal Where it Belongs
Keeping Your HF Signal
Where it Belongs
Being on the “right” frequency takes more than reading that
radio dial — you need to know why!
Joel R. Hallas, W1ZR
A
mateur Radio operators with a
General class license are authorized
to operate SSB on parts of all amateur phone bands. It’s a great opportunity
to explore worldwide HF communication,
but it carries with it a responsibility to keep
your signal in its authorized segment of each
band.
This article was prompted by a question
from a new General class operator, authorized to operate from 3800 to 4000 kHz on
75 meters. He was told while he was operating LSB at a dial frequency of 3800 kHz that
he was operating out of his authorized band
segment. If he needed to know, perhaps others do as well.
The higher class licensees who (I hope
politely) told him about this were absolutely
right. There are two issues here and we’ll
cover both.
The Transmitted Signal Spectrum
The FCC rules are very clear on this
point. Section 97.307, Emission Standards
states: (b) Emissions resulting from modulation must be confined to the band or
segment available to the control operator.
Emissions outside the necessary bandwidth
must not cause splatter or keyclick interference to operations on adjacent frequencies.
This means that what is important is not the
(suppressed) carrier frequency indicated on
your radio display, but rather the frequency
of your sideband components. If your carrier frequency is at 3800 kHz, your LSB
signal extends below that, typically 4 to
6 kHz, to let’s say 3794 kHz, depending on
the characteristics of your sideband filter as
shown in Figure 1. With that filter you will
need to set your carrier at least as high as
3804 kHz to be operating within the limits
of your license.
The FCC does not specifically say how
far down the slope of your filter you must
have the edge to be legal. In other sections,
however, they require that spurious response
be at least 43 dB below the peak level, so that
From August 2008 QST © ARRL
Figure 1 — Spectrum of typical 3800 kHz
LSB signal. Note the components below
the indicated dial frequency.
might represent good engineering standards.
Many manufacturers specify their filter
bandwidth at the 6 and 60 dB points, so the
bandwidth at the 60 dB level would represent a reasonable level that I would be comfortable with. Since the carrier frequency
is usually set at about the 20 dB point on
the (upper for LSB) slope of the curve, that
should cover both ends with a bit of safety
margin. Check your radio specifications for
the 60 dB bandwidth of the sideband filter
you are transmitting through and that should
be a good number to use.
At the other end of the filter spectrum,
you might find a 2.7 kHz wide filter with
a shape factor of 3:1. This would call for
staying at least 8.1 kHz from the band edge.
Note that for the bands above 40 meters
using USB, one needs to leave same amount
of space above the indicated dial frequency.
This situation is potentially even more
critical, since instead of just interfering with
higher class amateur licensees, you will be
interfering with other services — perhaps
other governments.
What About Display Accuracy?
All the preceding assumes that your
dial calibration is right on. I promised to
discuss two aspects of this question — the
second has to do with how accurate your
frequency readout is. The frequency display
on modern transceivers can often read to
a single Hz, as in 3800.000 kHz. That is
an indication of precision, not accuracy.
The accuracy depends largely on the initial
calibration at a factory in which a technician (hopefully) carefully adjusts a trimmer
capacitor to make your internal reference
clock almost exactly line up with their
(hopefully recently calibrated) factory frequency standard.
That event probably happened at least
10,000 miles from your current location,
was likely followed by an ocean voyage
and perhaps five years of crystal and circuit
aging. Unless you have recalibrated your
internal reference lately, or had it aligned
professionally in a standards laboratory, I
would allow an additional few kHz at any
band or segment edge.
Depending on your receiver architecture,
you may be able to get a guess at how close
your calibration is by listening to WWV
(at 5, 10, 15, 20 or 25 MHz) on both upper
and lower sideband. This works only for a
radio with a frequency synthesizer that covers the whole range, since other types may
have different circuits in play on different
bands. Let the radio get up to operating temperature, perhaps for 30 minutes. Now tune
the radio carefully to a WWV signal that is
strong in your location until the beat note
vanishes and the voice sounds natural. Note
the frequency on the display. Any difference
between, for example, 15,000.000 kHz and
the displayed frequency is an indication of
possible error in your reference oscillator.
I would add at least twice that difference to
the allowance for the sideband components
described above.
Joel R. Hallas, W1ZR, is QST Technical Editor.
He can be reached at [email protected]
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