Digital Communications by Gerry Crenshaw, WD4BIS
What's all this Digital communications stuff anyway? What equipment
will I need for digital communications modes such as packet? Will
I need additional cables and antennas, or radios?
If you have flipped through the 2 meter and 70 cm bands, you may
have heard short burst of what sound like noise or modem tones.
That's the sound of digital communications over the radio.
Digital communications allow our computers to talk to each other
in a binary language. The messages are broken up into short binary
blocks or packets. A small amount of overhead information (such
as your call sign) is added to the packets, then it is transmitted
over the air as audio tones.
Why use digital communications at all? Isn't Voice good enough?
For many uses these days the answer must be NO. Computers in the
shack are now as much a part of the ham shack as the radio, coax
or power supply. We use them for everything from plotting a satellite's
location to computerized logging of QSO's. Sooner or later we
want to transfer the file to someone else and when that time comes
its easier to transfer that file by some form of digital communication.
It is much better than using sneaker net; where you put the file
on disk, then put on your sneakers and run it over to another
machine. To quote an instructor of mine: "I had the choice
of whether or not to get involved with computers, my children
will not nor will yours." Even the conservative ARRL is on
Packet radio is keeping the ham spirit alive by giving us true
WIRELESS digital communications.
Chances are that you already have the computer and a radio (all
the equipment needed to get started in Packet radio). The only
additional equipment that is needed is the device that breaks
up the data and converts this into audio. This device is known
as TNC or Terminal Node Controller. Prices range from about $100.00
for a simple packet mode only TNC to $500.00 plus for a multimode
TNC. Other than that the only additional software you might need
is a terminal communications package such as ProComm or Crosstalk
for DOS/Windows or Microphone or White Knight for the Macintosh.
One important thing to remember about digital communications,
as the bit rate goes up, so does the occupied bandwidth. For this
reason packet on HF is limited to 300 BPS. Packet on VHF and UHF
where we have a larger spectrum of bandwidth can be 1200 BPS to
Most TNC's have the ability to run both 300 and 1200 BPS but depending
on the manufacturer, running 300 BPS HF Packet may not be recommended.
TNC's that are meant specifically to run HF Packet will have a
tuning indicator on the front of the Chassis to tune in the signal
for optimum transmission.
Because of the large bandwidth used at 9600 BPS packet, the radio
must have a larger receive and transmit IF Band pass for 9600
BPS. If you will look in any amateur radio magazine now you will
probably see ads for radios listed as 9600 BPS Ready meaning that
these radios will work at 9600 BPS packet. Other wise the radio
must be modified for the larger bandwidth required for 9600.
One important thing to remember is that in Packet radio, there
are two sides of the TNC. The Radio side (the part connected to
the radio) and the Terminal side (the part connected to the computer).
The radio side will be limited to 300bps, 1200bps or 9600bps depending
on the TNC you purchase. The terminal side will limited to the
highest speed your communications software can handle (300 to
19.2 kbps). Do not assume that because the radio side is running
at 1200bps you have to run 1200bps on the terminal side.
Interface cables between the radio and TNC usually have to be
custom Manufactured by the user. This is because of the wide variety
of Mike and Speaker connections found on transceivers. Some manufactures
such as MFJ make interface cables that you can buy to interface
there TNC to the more popular radios. The literature that comes
with the TNC will usually have a collection of drawings that shows
how to interconnect the TNC to a wide variety of radios. For this
reason if you are purchasing a previously owned TNC, be sure you
are getting the users manual as well as the hardware.
Packet radio used to be a real insiders game. Tweaking the software
parameters on the TNC was something you had to know how to do
or you needed to be in contact with a BBS operator who would talk
you through the set up. Now that the packet standards have firmed
up and the manufactures are making the TNC's with a very complete
standard set of commands the Packet Radio is pretty much a "Plug
and Play" affair (other than a calibration setup to set the
Transmit and Receive levels).
If you even remotely involved with computers, enjoy the world
of WIRELESS digital communications.
Copyright 1996 Gerald Crenshaw WD4BIS. All rights are reserved.
The Ten Most Common Tower Building Mistakes
By Steve Morris, K7LXC
This appeared in the April 1999 (electronic) edition of "Sunspots",
the newsletter of the Redwood Empire DX Association, Steve Bertsch,
WA6YFD Editor. Copyright Steve Morris. Permission is granted to
reprint this information if credit is given to "Steve Morris,
K7LXC, Champion Radio Products".
After working on over 100 amateur radio tower and antenna systems
over the last 15 years, I have seen many problems and failures
that could have, and should have, been avoided. By not making
these mistakes, you can make your tower and antenna system safer
and more reliable. It'll even let you sleep nights better.
1. Not following the manufacturers specifications - Commercially
manufactured towers have to comply with current standards for
wind loading and structural integrity. Licensed Professional Engineers
(PE's) design the towers and make the calculations to make them
safe. If you don't follow their specs at a minimum, the tower
will not take the stresses and loads that it is subject to. In
other words, it'll probably fail.
2. Overloading - This is the most common reason for amateur tower
failure. The first thing you need to know for planning and building
a tower and antenna system is what the wind speed rating for
your county is. Next, you need the manufacturer's specifications
for that wind speed. Then you must not exceed the wind load rating
based on those factors. This is even more important for crankup
towers. Refer also to number 1.
3. Underestimating wind forces - Wind pressure on a tower and
antenna system can be tremendous. Unless you've been up on a tower
during a windstorm to feel the pressure and the forces, it's difficult
to appreciate how significant they are. Increases in wind pressure
are not linear they are geometric.
4. Not building to the wind speed rating for the county - While
many counties in the US are only rated for 70 MPH winds, many
other counties have ratings much higher, up to 115 MPH. Find out
what the wind speed rating is for your county or your specific
location, and use that as the minimum design parameter for your
tower and antenna system. See also number 1 and 3 above.
5. Using the wrong mast for the job -Medium to large sized HF
beams can put huge stresses on your mast. Know the difference
between pipe and tubing as well as the pressure that your antenna
system will put on the mast (tubing is generally acceptable, pipe
is not). Then you can, make sure that your mast will not bend
or break, in a big windstorm.
6. Not having the guy wires tensioned properly- Proper guy wire
tension is a critical part of a tower's ability to handle wind
stresses. Having the wrong tension can be like driving your car
with over or under-inflated tires: it is potentially dangerous
and is not the proper specification from the manufacturer. Find
out the proper tension for your installation and tension them
7. Not having a proper ground system -A good ground system is
necessary not only for lightning protection but also for minimizing
RFI to adjacent electronic devices.
8. Not doing an annual inspection -Your tower and antennas system
is in a constant state of deterioration. While it may be a slow
process, it is going on continually. The best way to find and
fix small problems before they grow into big problems and potential
calamities is by doing an annual inspection.
9. Not fitting the tower sections on the ground - Tower sections,
new or used. may not fit easily together. It's much easier to
correct alignment problems on the ground than up on the tower.
10. Using the wrong hardware -. Since tower and antenna materials
are in a constant state of deterioration, you should only use
hardware that minimizes corrosion. Galvanized or stainless steel
materials are the only ones that will survive outdoor use reliably.
Using the wrong hardware includes using non-closed-eye eyebolts.
Use only forged or welded eye-bolts since the wrong type can open
up accidentally with disastrous results.
Correct Mobile Antenna Placement
By Wm. Ttigg Tabor, K8NIO. This appeared in the May 1999 issue
of "RADIOgram", the newsletter of the Capital City repeater
Association, Keith Chambers, WQ8H Editor.
The Larsen Company, manufacturer of quality mobile and other antennas,
publishes an antenna location information sheet in their product
catalog. They consider ground plane availability, the antenna
electrical length and the radiation pattern of each design.
Each antenna type (1/4 wave, 1/2 wave, 5/8, etc.) has a very specific
radiating pattern to be considered when selecting a mobile antenna.
Larsen notes that "the signal radiating from a 1/4 wave antenna
is directed more vertically, thus making it ideal in urban environments
where buildings might obstruct the signal. The design of 5dB coIlinear
mobile antenna is designed to direct the signal more towards the
horizon. This type of antenna is ideal for geographically flat
regions where the signal coverage is sparse.
Using a standard automobile, there are five possible general locations
for mounting and antenna: the roof, front fender, rear fender,
trunk and rear (or other) window glass. The center of the roof
is always the best location for a mobile antenna. Using it as
reference, here is a comparison of the other location for different
Roof center (permanent) 0.0dB
Roof center (magnetic mount) -0.2dB
Roof comer (magnetic mount) -0.2dB
Trunk center (permanent) -2.0dB
Trunk center (magnetic mount) -2.8dB
Trunk lip (center) mount -2.8dB
Trunk corner (magnetic mount) -3.4dB Hood corner (magnetic mount) -2.4dB
On-Glass (upper window) -0.5dB
On-Glass (midpart window) -1.2dB
On-Glass (lower window) -3.0dB
Remember: -3.0dB means that half of your antenna gain is lost.
As you can see, the placement of your antenna and the way you
mount it is critical. You can lose all the benefit of that "super"
antenna by putting it in the wrong spot.
GRUMMAN AMATEUR RADIO CLUB MINUTES OF GENERAL MEETING 7/21/99
by Gordon, KB2UB
The meeting was called to order by Pat KE2LJ at 5:07 PM, with
KB2UB as acting secretary.
Treasurer Ted KD2UB gave the financial report.
KB2UB reported no problems. The 145.330 repeater has been offered
for use as a backup communications system in support of the Empire
State Games for the period 21-24 July. This event involve thousands
of athletes converging on Long Island with lots of volunteer hams
giving their time.
Bob W2ILP reported no candidates at last session. Bob also reported
on new FCC Uniform Licensing System which was dictated by Congress
and will be in place in six months. So-called uniform application
system does not adequately accommodate Amateur Radio classes and
there are workarounds by VE Coordinators which were described.
He identified a substitute "L" number which can be obtained
so that applicant's SS number will not be seen on forms. For details,
if you have internet access, see http://www.fcc.gov/wtb/uls
Zack WB2PUE reported on the 2m, 40m, and 20m net activity.
Pat reported that refurbishing work is being done by the Company
in the shack under the Blue Ball to restore the area to good condition.
They will take away some of the junk that has accumulated in the
WAG - No activity:
Pat reported on Field Day. He is gathering and sorting the logs.
Discussions were held on how to improve the event next year although
the overall assessment was that it was a success. See the photos
on our web site at www.qsl.net/wa2lqo. With the smaller number
of active members the Club now has, getting enough bodies for
a round the clock effort is difficult but we were joined with
the Suffolk Radio Club and a few new faces who helped carry it
Special Event Station:
On July 20 the Club operated WA2LQO for twelve hours to commemorate the landing on the moon. Operators were streaming up and down the stairs (and the freight elevator) in Plant 5 all day. Over 200 contacts were made with intense European interest in working The Home of the Lunar Module. Marty NN2C is designing a QSL card and anyone who wants a copy, even if you did not work WA2LQO, should send a request to our PO box. KB2UB reported that the club's TenTec radio needs an alignment and some repair and will be sent to the manufacturer.
The Club voted to approve the membership application of John Eccles,
WB2IHB, as a Sustaining Member.
Twenty Meters: 14.275 at 12:00 PM EST Wednesdays.
Forty Meters: 7.289 at 7:30 AM EST Sundays.
Two Meters: 146.745 at 8:30 PM EST Thursdays.
145.33 at 8:45 PM Thursdays at 9:00 PM EST
VE exams for all classes of amateur licenses are held on the second
Tuesday of each month at 5 :00 PM in the Plant 5 Cafeteria. (See
page 7 for directions to exam site.) The exam fee for 1998 is
$6.35. Thanks to Bob Wexelbaum, W2ILP for this information.
General Meetings of the GARC are held on the third Wednesday of each month, at 5:00 PM in the Plant 5 cafeteria. All who are interested in Amateur Radio are invited to attend. Board meetings are held eight days before the General Meeting and GARC members are invited to attend, but please call Pat Masterson, KE2LJ, at 346-6316 to confirm place and time of meeting.
The map on page 7 shows the new entrance to the plant 5 meeting
site. Attendees should enter at Grumman Road West (A)(Hazel St.)
and drive down to the new entrance at the west side of plant 5,
then go to the same visitors parking lot as before.
This appeared in the February 1999 edition of "Key Klicks'",
the newsletter of the Green Mountain Wireless Society, Deb Clark,
Life's Little Truths
1. I started out with nothing... I still have most of it.
2. Funny, l don't remember being absent minded.
3. All reports are in. Life is now officially unfair.
4. If all is not lost, where is it?
5. It is easier to get older than it is to get wiser.
6. If at first you do succeed, try not to look too astonished.
7. The first rule of holes: If you are in one, stop digging.
8. I tried to get a life once, but they were out of stock.
9. It was all so different before everything changed.
10. Nostalgia isn't what is used to be.
1 I. Old programmers never die. They just terminate and stay resident.
12. Living on Earth is expensive, but it does include a free trip around the sun.
13. If you're living on the edge, make sure you're wearing your seat belt.
14. An unbreakable toy is used for breaking other toys.
15. It's not hard to meet expenses ... they're everywhere.
16. Never knock on Death's door; ring the bell and run (he hates that).
17. Lead me not into temptation I can find the way myself.
18 Jury: Twelve people who determine which client has the better