MT Express Sample Low Resolution Issue ... - Monitoring Times

Ask Bob
Bob Grove, W8JHD
bobgrove@monitoringtimes.com
Q. Without my having to drag
an expensive ohmmeter up on
the roof, what is another simple
instrument I can use to check the
continuity of the coax for opens
and shorts? (Michael Herman, NY,
NY)
A. Sometimes the easiest answer is the most
elusive. Remember, you either want the line
to have infinite (or extremely high) resistance
between the center conductor and the shield, or
full continuity (short circuit) from end to end of
the center conductor and the shield to indicate
that they are not broken between the two ends
of the line.
You could simply attach a buzzer in series
with a battery; when the circuit is closed by a
short, it will sound, and when it’s an open, it
won’t! You can also do this with a series of two
AA cells, a 470 ohm resistor and an LED for
visual indication.
There are also inexpensive ICs available
from Radio Shack that will give an audible
tone when connected to a speaker or earphone.
Again, you would simply make sure the circuit
is oscillating, then substitute the two probe leads
for the switch.
Q. A major ham radio manufacturer
is about to come out with
an HF rig which has an output of
400 watts – four times as powerful
as most standard ham transmitters.
Will there be much practical
advantage to the additional 300
watts? On the receiving end, how
much real difference is there between
100 watts, 400 watts and
1000 watts? Is it worth shelling
out an extra kilobuck or two for
this rig or an linear amplifier? (Ken
Cohen, NI3F, Bethesda, MD)
A. These are excellent questions. The fact
of the matter is that when you quadruple your
power, the receiving station will see his S-meter
rise one S unit (6 dB). Going from 100 watts to
1000 watts would make a 10 dB improvement
(about 1-1/2 S units). If your signal is right on the
threshold of reception, it will make a noticeable
difference. If the signal is already in the clear,
it will make no difference whatsoever because
your receiver will simply automatically reduce
its sensitivity to accommodate the increased
strength (automatic gain control – AGC).
22 MONITORING TIMES January 2005
As a practical matter, once you have 100
watts or so, concentrate on antenna gain, not
increasing power. That’s why there are so many
100-watt rigs!
Q. I am experiencing “RF in the
shack” with my HF transceiver.
My earth ground is about 15 feet
away. A friend suggested using an
“artificial ground”; what is this?
(Brent Davenport, N7NBC)
A. Nice call, Brent; I hope you don’t work for
CBS!
An artificial ground is nothing more than a
resonant length of wire attached to the chassis
of your transceiver; it acts like a counterpoise
element to avoid SWR-related RF in the shack.
You would need roughly 1/4 wavelength of
insulated wire for each band you operate, and
it can be either run in a random fashion around
the room, or even coiled loosely.
An artificial antenna shouldn’t really be
necessary if you have a resonant antenna system,
or any antenna system with a properly-adjusted
transmatch (tuner).
Another possibility is to run a cable to a
good, moist, earth ground. A heavy-gauge wire,
or the braid of a length of coax should work
fine. A good ground consists of at least one, and
preferably two 8-ft. rods separated by about 10
feet, driven into moist earth.
Yet a third possibility, one which I’ve never
tried, is to connect only the center conductor of
the grounding coax between the ground rod and
the rig grounding terminal, but solder a 0.01 microfarad,
600 volt capacitor between the shield
and center conductor at the top, and another at
the bottom. I’d be interested in hearing from
readers who have tried this.
And finally, at least one manufacturer,
MFJ Enterprises, makes a tuner for an artificial
ground, allowing a random wire to be resonated
on different bands.
But if you are using a properly-tuned transmatch
(antenna tuner), such gimmicks shouldn’t
be necessary.
Q. What do the terms “digital,”
“WFM” and “NFM” mean in scanners?
(Robert Wilkinson, Continental,
OH)
A. In normal scanner parlance, digital refers
both to computer control of the frequencies
(trunking) for spectrum economy, and scrambling
speech for security. While it is unlawful
in the U.S. to listen to, or even possess a scanner
that will decode encrypted communications,
there is one exception: the new APCO Project
25 (P-25) system.
A growing number of civilian and federal
agencies are now using P-25, and although they
can select an encryption level that no outsider
can decode, most of them choose the lowest level
of digitization which can be received by the new
scanners that have P-25 decode capabilities.
At VHF and UHF, there are two dominant
types of modulation (putting sound or data on a
signal): frequency modulation (FM) and amplitude
modulation (AM). Aircraft communications
use AM; terrestrial services use FM (some hams
use other modes).
Because music is sonically so complex, its
signal takes up a wider chunk of spectrum, typically
150-200 kHz, and so is called WFM. Voice
comms are much simpler, with less frequency
range, and can be narrowed down for spectrum
efficiency (NFM).
If you try to listen to a narrow FM signal on
a WFM receiver (FM broadcasting receiver), the
recovered audio will be very weak, and if several
adjacent-channel signals come on simultaneously,
you would hear them all at once, because
the receiver’s selectivity is very broad so it can
process the wider music bandwidths.
On the other hand, if you try to listen to
WFM music or speech on an NFM scanner, it
will be distorted, or you may hear nothing at all
because the modulation occupies a wider amount
of spectrum than the narrow scanner filters can
process.
Q. Is it permissible to listen to an
AM/FM/shortwave portable radio
in a commercial aircraft? And if
not, why not? (Zach Braddock,
email)
A. It is against Federal Aviation Administration
(FAA) Rules and Regulations, but not because
of signal reception. It’s because radiation from
the oscillator in your radio may interfere with
the aircraft’s crucial radionavigation system.
Questions or tips sent to Ask Bob, c/o
MT are printed in this column as space
permits. If you desire a prompt, personal
reply, mail your questions along with a
self-addressed stamped envelope (no
telephone calls, please) in care of MT, or
e-mail to bobgrove@monitoringtimes.
com. (Please include your name and address.)
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