[aprssig] Putting a digi on a 2m repeater

Stephen H. Smith wa8lmf2 at aol.com
Thu Mar 10 15:50:44 EST 2011

On 3/10/2011 12:20 PM, Kent Hufford wrote:
> We have a 2m repeater on 147.315+ that we have been told we could add a digi 
> on 144.39.
> Does someone have some words of wisdom, photos, web pages, diagrams on how to 
> do this?
> Thanks in advance.
> Kent
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> aprssig at tapr.org
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Presumably the existing voice repeater already has a cavity duplexer in place 
to isolate the TX and RX frequencies.

You will need another set of high-Q cavity filters on the 144.39 radio.   
Without filtering:

1)     The 144.39 receiver will be massively overloaded & desensitized every 
time the voice repeater transmits.

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.

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.

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 
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.

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.

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.

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 

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.


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