Subject: [NEXTEL-1] Re: Radio Interoperability Boxes

From: [email-masked]
Date: Thu, 15 Apr 2004 13:15:16 -0400

-------- Original Message --------
From: [email-masked]
Subject: [APCO Exchange:] Re: Radio Interoperability Boxes

  MESSAGE:  (#2730) Re: Radio Interoperability Boxes 
            <http://www.apco911.org/theexchange/index.cgi?rev=2730> 
  AUTHOR:   Dwight Purtle 
  DATE:     Thursday, 15 April 2004, at 10:47 a.m. 

  Reply To: (#2729) Re: Radio Interoperability Boxes 
  Author:   Mary Messamore 
  Date:     Thursday, 15 April 2004, at 9:14 a.m. 

Mary, you're right about the box that was displayed at Kansas APCO. It
only 
links one VHF, one UHF and one 800 MHz channel at a time. The ACU 1000
can 
have up to twelve radios connected together (depending on configuration)
and 
even more if you buy the second expansion chassis. Those twelve radios
can be 
patched into several groups operating separately. Yes, the ACU 1000 is
much 
more versatile - but it's also much more expensive. 

The biggest problem with patching multiple frequencies in the same radio
band 
is interference between the radios themselves. Suppose you have two VHF
radios 
patched together using a gateway box. Radio A picks up a signal. The
gateway 
sends the audio from Radio A to Radio B. Radio B transmits the audio. So
far 
so good. But the strong signal from Radio B can overload the receiver in
Radio 
A, causing it to be severely desensed (hearing-impaired). Even though
the 
signal from Radio B is not on the same frequency Radio A is trying to
receive, 
it is within the range of frequencies Radio A is designed to receive.
The 
strong off-frequency signal affects Radio A's ability to hear weaker 
on-frequency signals. The patch works poorly or not at all. 

How do we solve this problem? Three ways. First, try not to patch
frequencies 
in the same band if you can avoid it. It's much better to have users in
the 
same frequency band switch to a common channel than to try to
cross-patch the 
frequencies. Second, limit the output power of the radios you are
patching. If 
five watts will work, don't use fifty watts. Third, separate the
antennas of 
the two radios involved. Vertical separation is more effective than
horizontal 
separation. JPS has a section about this in the two-day ACU 1000
training 
course they offer. 

I'm hearing about some new devices on the market designed to address
this 
desense problem. They use two antennas - one to transmit, one to
receive. 
Multiple radios can connect to the device and use its antennas without 
interfering with each other. The radios are free to change channels
without 
any readjustment of the device. But these devices are priced around
$10,000 
and cover only a single radio band. 

So while it's possible to cross-connect two channels in the same
frequency 
band, you're a lot better off if you don't have to do it. 

This got really long - hope it made sense. 

Dwight Purtle, ENP 
Johnson County KS Emergency Communications

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