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Building a Repeater
FAQs
This section will include Frequently
Asked Questions relative to building a ham radio repeater system.
- Can I Use a
Pair of Ham Transceivers
as a Transmitter and Receiver for My Repeater?
The simple answer is "No". The more
comprehensive answer deals with radio frequency (RF) and how a
repeater system will have a receive and transmitter within the same
cabinet or rack with its receiver trying to receive while its
transmitter is transmitting at the same time. The first
problem is that the output power of the transmitter will cause
the receiver to be "deaf" or de-sensed to the incoming
signals it is listening for. A repeater requires very
high isolation of RF signals within its own housing or
cabinet.
Mobile transceivers, by design, are not designed to receive while
its transmitting therefore they are not built to the higher RF
isolation criteria a repeater system.
Did I try it? You bet... this is why I'm going to save you a
whole bunch of hours... may be even days, depending if you were as
persistent as I was. Yes, I got it to work however, it
would not work to my expectations. Bottom line, it worked well
with very strong signals, however, weak signals couldn't get
in. TOP
- How do I know I'm having a
de-sensing problem?
Believe it or not there is a real simple way to determine if
your repeater system is having a de-sensing problem.
First you must know that you will only know there is a
de-sensing problem when receiving a very weak signal. Know
that your transmitter sends its power to the transmitting antenna
while the receiving antenna is receiving the signal from the mobile
unit.
De-sensing really means that your receiver is being
de-sensitized by your transmitter power. Some RF power get
back into your receiver and causes it stop receiving.
Here is the sequence of events that tell you of a de-sensing
problem.
1. Repeater is idle (Receiver Off, Transmitter Off)
2. A weak mobile keys up the repeater (Repeater On, Transmitter On)
3. The repeater transmitter, de-senses the receiver..
4. The receiver no longer receives the mobile and turns the
transmitter off.
5. go back to step 2.
In real time you hear the repeater kerchunking on and off with
only glimpses of speech audio coming through. TOP
- Create your own
weak signal into the repeater to test for de-sensing?
When your a ham radio guy you don't always have access to the
sophisticated commercial gear used for testing radio systems.
Instead we learn to develop our own methods for testing.
In order to test your own repeater system you need to be some
distance away from the repeater with a handheld and one other radio
(handheld or desk top).
Tune both the handheld and radio #2 to the repeater frequency.
Key the handheld and listen to the repeater output with radio #2 to
ensure that you are getting in to the repeater. Adjust the
volume of radio #2 to minimize audio feedback.
While keying the handheld and listening to the repeater output on
radio #2, start to turn the handheld onto its side so that the
signal level to the repeater drops into the noise realm.
If your situation is optimized for this test you should be able to
turn the handheld sideways and drop out of the repeater
completely.
Your repeater is working very well with no de-sensing if:
While you are talking, and tilting the radio your audio in radio #2
drops down to the noise and is barely readable but the transmitter
is still transmitting with no kerchunking.
You have a de-sensing problem if your talking and tilting your
handheld and you notice that your audio is still very strong and
readable but the transmitter starts to kerchunks and won't stay on
steadily as long as you are talking. TOP
- Can there still
be a De-sensing problem even though I have a commercial repeater
and even a duplexer?
Sorry to say the answer is Yes....with RF (Radio
Frequencies) the process of isolation is never precise. Unwanted
RF can cause havoc with any ham gear. TOP
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