[KYHAM] KEN Training for January 21, 2002: Interference Issues in Command Post Operations

[Ron Dodson]

Last week, Tony KC4JTV gave us some thoughts on
designing a Mobile Command Post (MCP).  This week, he
offers some ideas and suggestions on functioning in an
MCP and dealing with interference between radio
services/bands.

Thanks Again Tony!!!

73 all, Ron, KA4MAP
===================================

Interference Issues in Command Post Operations
By A. Whobrey, KC4JTV


 On occasion, amateur radio operators are requested to
assist in a command post setting, in a common location
with other responding agencies, such as police, fire
departments and other emergency service organizations.
This is often a somewhat more challenging task than it
would first appear. Besides the obvious check in and
chain of command considerations that have been
previously discussed, care must be taken to avoid
interference between amateur radio operations and other
communications systems in operation at the command
post.

 A command post is one of the more hostile environments
for amateur radio operations, given the usually close
proximity, with regard to both physical location and
frequency, to public safety users. Given the disparity
between public safety's definition of "high power" at
250+ watts and that of commonly used portable ham gear
at a somewhat lower figure, it is a given that you will
see some effect on your amateur operations. One well
known agency uses 110watt mobile transceivers for over
300watts ERP(i), and fixed repeaters that often exceed
that figure. Transmitters in this power range will
cause receiver overload (de-sense) in commonly
available amateur gear at substantial distances.

 Some consideration in choosing which of your available
equipment to deploy will go a long way toward insuring
a successful operation. Don't bring that full tilt dual
bander with all the bells and whistles, if its receiver
overloads every time you drive past the local FM
station. Likewise, if you have an amateur transceiver
that has been modified to transmit outside standard
amateur frequencies, leave it at home, even if you
think you are authorized to use it. This will avoid any
accusation of your operations inadvertently straying
onto unauthorized frequencies. If an interference issue
does arise, it is far preferable to be able to state
that your radio cannot transmit on say, 155.xxxMhz,
than to have to say "I did not transmit on 155.xxxMhz".
Be assured that if you have a genuine need to transmit
on public safety frequencies, most agencies will
eventually provide you with a suitable, type accepted
radio for use on their licensed frequencies.

 When you reach command post proper, you may be
assigned to a specific area. If not, your first task is
to find a suitable location for your station(s). While
it is tempting to "follow the leader", and set your
station right along side others already established,
this approach is not always best. Adequate separation
between your operation and others will yield many
benefits, not the least of which is ease of entry and
exit. It is easier to transport you and your equipment
a few more feet, than to maneuver around others who are
already set up. Such a location will also help you get
as much space as possible between your antenna and
everyone else's. Since a transmitter's near field
strength decreases rapidly with distance, an additional
30 or 40 feet of horizontal separation can often help
your receiver pick out the desired signal from the
virtual "RF storm" created by several non-amateur
transmitters in simultaneous operation. If you are not
able to obtain significant horizontal separation, at
least try to erect your antenna at a different vertical
level than those nearby, to reduce coupling between
your antenna and those used by others. This separation
will also reduce the chance of your own transmitter
causing interference to your friends a few Mhz away.
Another approach is a band pass filter, installed in
your transmission line, which greatly reduces out of
band signals in both directions. These products, while
somewhat expensive, will greatly aid both transmission
and reception under difficult conditions.

 By looking at the antennas used by other agencies on
the scene, you may be able to approximate their
frequency of operation. If the majority of others
appear to be on "high band" (155 Mhz), you will find
less interference on the 440 Mhz frequencies as opposed
to using 2 meters. The opposite is also true, if
everybody else seems to be on UHF, opt for 2 meters. A
155Mhz quarter wave is about 18 inches long; its 450Mhz
counterpart is about 6 inches in length. You probably
already know the frequencies in common use in your own
community. At a large operation there may be a
technician or communications officer that can help you
determine which frequencies to use.

 If the command post is in a remote area, be aware that
you may be on your own to supply a power source for
your operation, as some mobile command posts will not
have generator or battery capacity to spare. At least,
make sure you can supply your own heavy-duty extension
cord if needed. An automobile can supply power for a
limited amount of 12-volt equipment, until a more
suitable power source can be established. If you employ
a portable generator, equip it with an adequate
muffler, to prevent sonic interference issues. A noisy
power plant will greatly degrade the effectiveness of
any receiver in its immediate area.

 If your technical skills are well thought of in your
community, you may accidentally become the de-facto
"radioman" at one time or another, especially when
working with smaller agencies. These smaller
departments often don't have on staff communications
technicians, and may well call on you should one of
their radios fail. If you are comfortable in this role,
you may be able to assist with minor problems, as a
blown fuse or broken antenna affects a commercial radio
in exactly the same manner that it does similar amateur
gear. Conversely, don't be offended if you are not
asked to assist, some larger entities have guidelines
that specify who can service their equipment.

 As each situation has its own characteristics, these
comments are certainly not all inclusive. However, an
awareness of these points will give you some idea of
what to expect, should you be asked to participate in a
command post operation.