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[aprssig] Re: Duplexer or Diplexer

Stephen H. Smith wa8lmf2 at aol.com
Tue Sep 6 00:29:22 UTC 2005


tyler at allisonhouse.com wrote:

>>kkotch at earthlink.net wrote:
>>
>>    
>>
>>>Scenario:  Dual-band mobile and separate 2m mobile with tnc.
>>>
>>>
>>>
>>>Can a duplexer/diplexer be built (or is one available) that would
>>>allow both radio systems to be hooked up to one dual-band antenna?
>>>The dual-band radios with one antenna lead has a built-in duplexer.
>>>Is that where the problem would occur trying to coordinate another
>>>duplexer?
>>>
>>>Given the above comments about duplexer/diplexer it would be best to run
>>>two antennas. But now I have two antennas in the VHF band that MUST be
>>>within 6 to 10feet of each other given limited space on the top of a car.
>>>
>>>Besides getting them as far apart as possible is there anything else to
>>>protect each of the radios from being fried?
>>>
>>>-Tyler
>>>
>>>
>>>      
>>>
You won't "fry them".  You will only overload and temporarily block the 
receiver of one radio while the other one is transmitting.  In the case 
of the APRS radio transmitting, that means you will get a one second or 
so hole punched in your 2M voice RX whenever the APRS radio beacons.    
[ I have THREE antennas attached to radios capable of transmitting on 2M 
and 450 (Kenwood TM-742, TM-D700 and Yaesu FT-100) within 3 feet of each 
other on the roof of a Passat wagon with no problems other than receiver 
blocking. ]

Minimize the interruptions by shortening the APRS bursts by using Mic-E 
compressed format (which, by the way, is the one and only format the 
D700 and TH-D7 use).  [ The Mic-E format puts out a posit in less than 
ONE-THIRD the time of the normal human-readable "APRS format", and in 
less than ONE-TENTH the length of a raw NMEA beacon! ] 

Send ONLY the position.  Leave the verbose comments off the end of the 
position beacon  Use the separate "STATUS" beacon instead of adding 
comments to the posit.  Set the status beacon to be transmitted only on 
every 5th or 10 posit beacon.

Properly configured, a Mic-E posit can block your other radio(s) for 
less than half a second on each posit beacon. This is about the same 
degree of interruption as running priority scan, on radios that support 
this feature, during a QSO.  

Setting the transmit power of the APRS beacon radio to only 5 watts or 
so will frequently allow you to continue to receive on the voice radio 
if you are working through a local repeater that you are receiving with 
a strong "full smash" signal. 

====  Other alternatives:    ======

o     Spend thousands and thousands of dollars each for high-end 
professional land-mobile radios that have much greater spectral purity 
on transmit and much better receiver overload immunity on receive  than 
"civilian" ham gear.

o     The only TOTAL  solution is to conduct voice operations on another 
band.





Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

"APRS 101"  Explanation of APRS Path Selection & Digipeating
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