©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 4/29/04
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than 80 dB isolation is needed
because the noise must be below the
received signal strength by a margin
great enough so it won't be heard at
all.
The above calculations assume
that everything remains perfectly
tuned. In actual practice, the
antenna system and duplexer are
certain to drift a bit from optimum
tuning; so a little safety margin
should be added. Also note that an
additional dB of isolation will be re-
quired for each dB you increase the
transmitter output if you add a high
power amplifier to the repeater.
Checking Repeater for Desense.
To ensure that the repeater has no
intrinsic desense, welded rf tight com-
partment shields are used, and
feedthrough capacitors and ferrite
beads are in line with all signal lines
which could carry rf between
compartments.
It is fairly easy to check out the
Repeater so that, in your own mind,
you know that any desense problem is
external to the repeater.
With double-shielded coax,
connect the receiver to a good signal
generator, and connect the
transmitter to a shielded dummy load.
You can simulate a received signal
with the signal generator and verify
that there is no cross-talk in the
repeater itself by checking the re-
peater sensitivity with the transmitter
both on and off, using the transmit
inhibit touch tone command. There
should be no noticeable change in
sensitivity with transmitter on or off.
Intermod.
Intermodulation is indicated by the
reception of an unwanted signal on
the repeater receiver, one that clearly
did not originate on the repeater
frequency. Intermod can usually be
identified by the type of conversations
taking place. The intermod signal
sometimes sounds distorted or off
frequency. Another intermod
characteristic is the presence of some
signal which periodically causes the
repeater to latch up. Intermod
interference can be weak or strong,
even to the point of covering up local
wanted signals.
Intermod is caused by two strong
signals mixing in a non-linear circuit
producing a third interfering signal.
The non-linear circuit could be a cor-
roded antenna connector, the trans-
mitter output stage, or the receiver
front end.
The most common form of inter-
mod is a strong signal doubling in the
repeater receiver's mixer and mixing
with a second signal to produce a
product that is on or near the repeater
receive frequency.
This interference is called third-
order intermod. The relationship
between its frequency and that of the
unwanted signals forming it is F
IM
=
2F
1
-F
2
where F
1
and F
2
are the
frequencies of the two signals.
Since the repeater transmitter
usually is the strongest signal, it is
more likely to double in the receiver
mixer and cause a 3rd order intermod
product with another strong in-band
signal. This type of intermod is
characterized by a signal on the
receiver after repeater key-up locks
repeater up and stopping when trans-
mitter is turned off. Note this
intermod never keys repeater itself but
only occurs after someone keys up the
repeater.
Cure for this type of intermod can
be increasing isolation between the
transmitter and receiver. This could
be accomplished by further separation
of transmit and receive antennas,
installing a better duplexer, or
additional filtering in the receive line
with a front-end crystal filter or a
cavity.
Intermod also can be caused by
two strong signals from nearby
transmitters mixing in the repeater
receiver mixer. With this type of
interference, the signal will key up the
repeater. The cure for this is adding a
helical resonator, a bandpass cavity,
or a crystal filter; or by using a
bandpass/bandreject duplexer.
Intermod also can occur in a
transmitter output stage. The output
stage may be that of the repeater
transmitter or a nearby transmitter.
Usually, the frequency doubled is the
transmitter's output frequency, which,
together with an external signal
picked up by the transmitter antenna,
forms a 3rd order intermod product.
The local signals causing the intermod
need to be quite strong due to the lack
of gain in the transmitter's output
stage.
Since this intermod product is
formed outside the receiver and is
within the receive bandpass, it can't
be filtered out in the receive antenna
line. This type of interference can be
eliminated by installing a circulator in
the interfering transmitter's antenna
line. A circulator allows power to pass
in only one direction. This will
prevent signals external to the
transmitter from reaching its output
stage.
Finding the cause of intermod can
be a confusing, frustrating, and
difficult job. The fact that intermod
occurs is not necessarily the fault of
the repeater. Every consideration has
been given in design of the REP-200
Repeater for intermod and desense,
e.g., helical resonators, 8-pole crystal
filter, rf-tight compartment shields,
feedthrough capacitors, etc.
Remember, intermod is the result of
two signals present in some non-
linear device. The cure is simply to
remove one of the interfering signals.
Other sources of information on re-
peaters and repeater problems is
available from several ARRL
publications.
Coax Cables.
High quality coaxial cable is an ab-
solute requirement for proper repeater
operation. For short runs, a good
quality RG-8/u is suitable. Low-loss
hardline is preferred for long runs or
uhf frequencies. All antenna
connections must be thoroughly
waterproofed.
In installations with a duplexer,
double shielded coax, such as RG-
214/u, must be used between the
repeater and the duplexer. Regular
single shielded coax can be used from
duplexer to antenna.
If separate antennas are used for
receive and transmit, the cables
should be separated throughout their
run by 6 to 12 inches. Double
shielded coax or hard line is required
to prevent leakage from the transmit
cable into the receive cable.
Frequency Stability.
Because even at room
temperature, the transmitter
compartment changes temperature
with use, we recommend the crystal
oven option for any crystal controlled
transmitter, especially on uhf or 900
MHz
COR-5 Controller Module.
Having read the Theory of Opera-
tion, you have a good understanding
of how the circuits work. The best
way to troubleshoot is to trace signals
from stage to stage to check the
operation of each circuit, starting with
the function you believe is not
working properly.