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On 3/10/2011 12:20 PM, Kent Hufford wrote:
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<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">We have a 2m repeater on 147.315+ that we have
been told we could add a digi on 144.39.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">Does someone have some words of wisdom, photos,
web pages, diagrams on how to do this?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">Thanks in advance.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">Kent<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 14pt; font-family:
"Calibri","sans-serif"; color: rgb(31,
73, 125);">KQ4KK<o:p></o:p></span></p>
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<font face="Arial"><br>
Presumably the existing voice repeater already has a cavity
duplexer in place to isolate the TX and RX frequencies. <br>
<br>
You will need another set of high-Q cavity filters on the 144.39
radio. Without filtering:<br>
<br>
1) The 144.39 receiver will be massively overloaded &
desensitized every time the voice repeater transmits. <br>
<br>
2) All modern solid-state transmitters that have broadband
untuned output stages, have a tendency to generate low-level
broadband white noise (60-90 dB down from the carrier output) over
many MHz on either side of the TX frequency. Without sharp
bandpass filtering of the 144.39 transmitter, it's TX broadband
noise will desensitize the voice repeater's input every time the
digi transmits. This could show up as either a drop in voice
audio overlaid with a white noise hiss whenever the digi
transmits, or a complete squelch closure if the voice user is not
putting a really strong signal into the repeater receiver.<br>
<br>
<br>
Ideally you would want to place the digipeater's antenna DIRECTLY
above or below the voice repeater's antenna(s) to get the maximum
isolation. I.e. NOT side-by-side on the same yardarm of a tower,
which would MAXIMIZE the coupling between the two antennas.<br>
<br>
</font><font face="Arial">You are lucky that the RX channel of the
voice repeater (147.915 MHz) is about as far away from 144.39 as
it can be, and still be inside the two-meter band </font><font
face="Arial">You might actually be able to share a single antennna
between the voice repeater and the digi IF you use high-quality
pass/reject cavity filters. <br>
<br>
Currently the voice repeater probably has two strings of cavities,
one peaked on the RX frequency at 147.915 and the other at the
transmit frequency at 147.315, joined with a "T" connector at the
antenna feed point. Each string typically also has a notch set
to the pass frequency of the opposite set. With a bit of
tweaking and frequency sweeping with a spectrum analyzer and
tracking generator, you could use a second "T" to connect the
antenna end of the 144.39 cavity string to the same antenna
feedpoint as the voice system. In this case you would tune the
pass-band of the digi's cavities to 144.39 and then tune the notch
settings of it's cavities to 147.315 to minimize desense when the
voice digi transmits. <br>
<br>
This is done all the time in commercial land-mobile installations
and is known as a "transmitter combiner". It is not uncommon to
couple 3 or 4 transmitters into the same antenna at VHF or UHF and
a half-dozen or more in cellular systems at 800/900/1900 MHz.<br>
<br>
<br>
You will make the job of isolating and/or combining much easier if
you use commercial-grade land-mobile gear rather than ham rigs for
this undertaking. Commercial radios tend to have much quieter
and cleaner transmit spectra, while the receivers usually have far
better overload resistance to nearby off-channel transmitters.
<br>
<br>
Due to the currrent mandate to convert commercial and public
safety radio systems to half their current occupied bandwidth in
the next two years (drop dead date for narrowband conversion is
Jan 1st, 2013), and the widespread conversion to P25 digital
systems, there is currently a LOT of perfectly good analog gear
turning up cheap on the secondhand market. <br>
<br>
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