Transmitter.

TM 11-490-5

interference is present in the receiver output would be to

receiver output is now interference free, the receiver

first check to determine whether the assigned RF is

was at fault. If the signal is still not normal the receiver

interference free prior to conducting any further isolation

antenna system should be checked. If available, use

tests.

another receive antenna system on circuit to minimize

circuit downtime during testing.

(d ) Multiplex equipment. In simple terms,

3. The above procedure is closely related to

multiplex equipment combines multiple input signals

that described in paragraph 4-3b (1) (a ), "Determine

(normally discrete audio channels) into a composite

origin of the interference." Both are examples of similar

signal that is used to modulate a transmitter without any

isolating techniques. There are other variants.

interchannel interference and reverses the process at

the receiver by converting the composite received signal

(b) Transmitter.

Using the orderwire,

into multiple channel output signals. To isolate an

operational personnel at the distant transmitter station

interference problem to the multiplex equipment, the

should be requested to check whether transmitter

output and input signals are checked to determine

operation is normal. Generally, transmitter checks are

whether they are normal. If the input is normal and the

limited to assuring that modulation is normal and at the

output has interference, the problem is in the multiplex

proper level and that all monitoring indicators and

equipment. If the system uses double multiplexing

instruments are reading normally (for example, the drive

(group and supergroup), composite input/output signals

level is correct, the transmitter is on the correct antenna,

of each of the multiplex elements should be checked in

and VSWR readings are normal). At some stations,

a similar fashion to that described above. Generally,

spectrum analyzers are available, permitting visual

grounding problems or direct external interference

observation of the output signal. A spare receiver or

affecting a multiplex equipment will interfere with all

broadband portable radio with proper input attenuation

output channels of the specific multiplex element. If

to prevent receiver overload or damage can provide an

only one output channel has interference, the most

indication of proper transmitter output, to include the

probable cause is a component malfunction within the

transmitting antenna system. If the transmitter output is

equipment or a misadjusted drive level. Individual

normal, the interference problem is associated with the

channel drive levels are normally very critical on the

antenna systems or the transfer media.

transmit multiplex. Check to be sure these drive levels

(c ) Antenna.

are adjusted to specified values.

In most

1. Generally, transmitting antenna

communication systems some interference (noise and

problems will be indicated by changes in the readings of

tines) is usually present. In a well-designed system, it

the output and loading at the transmitter. Interference

would be usual for interfering noise to be lower in level

problems associated with receiver antennas generally

than the system's intrinsic noise and thus could be

will cause fluctuations in the receiver signal level.

ignored. However, if noise levels increase above

However, interference in the incoming RF signal can

predetermined levels, action is necessary to determine

cause similar indications. Interference problems could

whether a maintenance problem or an interference

arise from nonlinearities in multichannel HF antenna

problem exists. Up to a point, isolation procedures for

isolation or multicoupler devices.

both types of problems are identical and along the lines

2. If antenna problems are suspected,

described above. Further information which will assist

another antenna can be substituted, or repair personnel

in isolating the problem is as follows:

can be requested to make a voltage standing wave ratio

1. Type of noise.

-(VSWR) or noise power ratio (NPR) test on the antenna

(a) Whether the noise is wideband or

system.

narrowband.

3. If the distant transmitter output and

(b) The frequency range of the

the antenna systems are normal, the interference is

interference.

entering through the RF medium. A spare receiver or

battery operated broadband portable radio with some

whether the tone appears in more than one channel.

type of directional antenna is probably the most rapid

Also, whether the tone is a modulated frequency of a

means of isolating the source, direction, and type of

tone in the channel.

interference. The experience and capability of the

operating personnel are most important in the

(d) Whether the interference is a signal

recognition of the type of interference and its probable

interference such as music, voice, on-off keying.

source.

If appropriate monitoring equipment is

(e) Whether the noise seems to be

available, the logical procedure when

powerline related.

2. Levels.

(a) Whether the receive signal power

levels are normal.

4-12