[aprssig] Re: diplexer/duplexers for APRS

Stephen H. Smith wa8lmf2 at aol.com
Tue Sep 6 21:12:53 CDT 2005

kkotch at earthlink.net wrote:

>radio 1 is a dual-band, radio 2 is a 2m packet radio all connected to 1
>dual-band antenna.  Both sides of the dual-band and the packet radio can
>receive at the same time.  It's only when one of the three transmits,
>(actually it would be when one of the two radios transmits) that the others
>need to have the rf blocked.  When the packet radio transmits, the signal is
>blocked from backfeeding through the coax to the other radio.  
[ I assume the "dual-band" radio means 2M/450; i.e. VHF/UHF ]

Huh??  All connected to one antenna?   If you are actually connecting 
two radios to one antenna with a T connector or something similar,  
NOTHING is  preventing one radio from "backfeeding" into the other and 
you are almost certainly going to blow out the front end of the other 

>When the
>dual-band transmits the internal duplexer there protects one side and the
>external duplexer circuit protects the packet radio.
Only if the two radios are on DIFFERENT BANDS, not just different 
frequencies.  That is, only if the dualband is transmitting on UHF.   
You CANNOT isolate two radios on the SAME band with a simple diplexer.  

The external diplexer is a band-sensitive device (just like the internal 
one) and since presumably the separate packet radio is using the 
VHF-only port of the diplexer, the dual-bander must be connected to the 
UHF-only port of the diplexer.  As a result, you will only be able to 
transmit and receive on the UHF half of the dualbander since the 
external diplexer WILL NOT pass 2M on the UHF port.  [You might transmit 
on VHF and be heard 50-100 feet away,  but not much farther due to the 
40-50 db loss that the diplexer's UHF passband will inflict on a VHF 
signal. ]

>If it can be done in any dual-band, dual-receive radio, why can't it be done
>outside the radio?
>Keith, KF4BXT
Because you are trying to use two frequencies on the SAME BAND.  As I 
explained in my previous post, these diplexer  devices are simple 
hi-pass/lo-pass filters with the "UHF" side passing everything above 300 
MHz or so, and the "VHF" side passing everything below 300 MHz.     
Simple diplexers can only combine or isolate DIFFERENT BANDS.  They 
CANNOT isolate different frequencies on the SAME band.     The only 
reason the diplexer inside the dual radio works is because the two haves 
of this radio ARE ON DIFFERENT BANDS.  

>-----Original Message-----
>From: aprssig-bounces at lists.tapr.org [mailto:aprssig-bounces at lists.tapr.org]
>On Behalf Of Jim Lux
>Sent: Tuesday, September 06, 2005 8:18 PM
>To: aprssig at lists.tapr.org
>Subject: [aprssig] diplexer/duplexers for APRS
>It occurs to me that since APRS is a single frequency kind of operation, 
>you might be able to get a SAW filter made that just passes 25kHz around 
>144.39, and another that rejects the 25kHz around that same frequency 
>(actually, it might be the same filter).
SAW filters are low power devices suitable only for receiving.  You 
can't place them in the external antenna line where they would be 
exposed to transmit power.   Inserting these into the D700 would require 
extensive internal hacking of the D700 to insert these devices in low 
level receive-only front end circuitry inside the radio.   In turn, this 
would be a major undertaking since the D700 doesn't have the traditional 
metal cans interconnected by mini-coax with RCA connectors for it's 
receiver frontend, mixer, IF block, etc.   EVERYTHING RF, IF, audio and 
digital, except for the TX PA, are on a single PC board. This means 
hacking PC traces with an Xacto knife and tacking on coax jumpers 
running to external filters. 

For some pics of the inside of the D700, visit this page on my website:

Scroll about 3/4 of the way down the page and look for the topic
  *"Interior Views of Kenwood TM-D700 146/440 Dual-Bander with built-in TNC"

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