Hal Communications ST-6000, Instruction Manual

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6-1
APPENDIX A
1. RF-INDUCED PROBLEMS
HAL Communications equipment is designed to operate in close proximity to radio frequency
transmitting and receiving equipment. Particular attention has been paid to the shielding of circui-
try through the use of all-metal enclosures and good common grounds. However, under certain
conditions in an RF-saturated environment, HAL equipment may be susceptible to RF-induced in-
terference. This may manifest itself in any of a number of ways, such as partial or complete lack of
response to operator commands, or erratic behavior of a video display.
The first thing that should be checked if RF problems are suspected is the ground system. The
transmitter should be properly grounded for RF (in addition to electrical ground) and all other sta-
tion equipment grounds should be connected to the transmitter chassis. The RF ground should
consist of a short length of heavy copper wire or braid terminated at a good earth ground (ground
rod or copper cold-water pipe). If a water-system ground is used, be sure that the pipes are 100
percent metal from the point of connection to the water mains – plastic plumbing will break the
ground path. If the distance between your transmitter and ground rod or water-main ground is
more than a quarter wavelength at the highest operating frequency, make the ground wire a half
wavelength, or a multiple of a half wavelength long. If you plan to operate on 10 and 15 meters
you may need to run a separate ground wire for each band if the distance requires the use of half-
wavelength wires. For example, if the distance from the ground point exceeds about 8 to 10 feet,
a 10-meter half-wave ground wire (16 feet long) and a 15-meter half-wave ground wire (22 to 23
feet long) would be used. Consult any of the amateur handbooks or antenna books for a more in-
depth discussion of grounding techniques.
The best way to confirm that a problem is being caused by RF induction is to temporarily elimi-
nate the source. This may be done in stages, starting with a partial reduction in exciter drive, and
ending with transmitter shut-off, Since RF energy can be induced in the demodulator or video ter-
minal circuitry through several different paths, connecting the transmitter to a dummy load may
not eliminate all RF related problems, although this is an excellent first step in verifying RF prob-
lems.
Radiation of RF energy from linear amplifiers, antenna tuners, coaxial switches, monitor scopes,
and interconnecting coax-cable jumpers is also possible. In fact, it is this type or radiation that is
most likely to be coupled into nearby I/O and power cables going to HAL equipment. To locate the
point or points of radiation, experiment with different cable arrangements to see if the RF-induced
problem can be eliminated by reducing coupling between any of the HAL cables and nearby co-
axial lines carrying RF power. Fig. 1A contains several cable arrangements, both bad and good,
showing how to keep RF coupling to a minimum. The drawing in Fig. 1B shows the use of high-mu
(950 or 2000) ferrite toriods or rods to choke the flow of RF on audio and control lines.
If cable rearrangement doesn't yield positive results, then begin eliminating pieces of equip-
ment and sections of coaxial cable until the transmitter is connected directly into a shielded dum-
my load, As each piece of equipment is removed from the transmission line, check to see if the RF-
related problems have diminished or disappeared. If the RF problem Persists with the exciter con-
nected directly to a dummy load reduce the drive level to see if that eliminates the problem.
If operation into a dummy load does not significantly reduce the RF-related problems, discon-
nect all I/O cables from the affected piece of HAL equipment. Test operation of the unit while it is
connected only to AC power. At the same time, enable the transmitter so that it sends a CW signal
into a dummy load. If RF problems are still present, then RF energy is probably being introduced
to the HAL equipment circuitry through the power cord by means of the common AC power line.
This is usually indicative of poor AC-line filtering in the radio transmitter power supply section. Fig.