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[aprssig] Re: Duplexer or Diplexer

Stephen H. Smith wa8lmf2 at aol.com
Tue Sep 6 04:33:35 UTC 2005


tvsjr at sprynet.com wrote:

>>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. ]
>>    
>>
>
>Don't be so sure. I think I'm OK to comment here... I've got 15 mounts on
>the roof of my Expedition, and just installed 20 today on the roof of my new
>truck (actually, in the topper). I have fried an amateur radio (D700) and
>the preamp in an Optoelectronics Scout connected to an external antenna with
>a high-power (110-watt) Motorola VHF Astro Spectra.
>
>  
>
20 antennas on one vehicle roof?  How close together were they?   Just 
inches?  You might as well as had the radio transmission lines connected 
together with coax T connectors for all the isolation  you had!  

With minimum separation of  a couple of feet or more, I've never  had 
any problems. 

>No reason to spend thousands and thousands of dollars. Since the analog
>Spectra has been EOLed, you can pick them up sub-$200 (Astro P25 radios are
>quite a bit higher, but that's another story.) The Spectra is arguably the
>best commercial radio ever made.
>  
>

I would agree. I've used Spectras as a front-end for my radio coverage 
mapping system for years.  I tap out the 109 Mhz first IF and feed them 
into my IFR-1500 service monitor.  The Specta working as a preselector 
and pre-amp allows the IFR analyzer to see down to -120 dBm or so  while 
not getting overloaded by out-of-band RF.  (Like most service monitors, 
the IFR receiver front end consists of a double-balanced mixer with no 
selectivity in front of it.  For listening to a radio's TX hardwired 
into it via coax, it's no problem.  Off-air monitoring with an external 
antenna causes the IFR RX to overload on mixes of everything from  "DC 
to light") 
     For passive selectivity, I have a bunch of single cavities for 
various bands (VHF, UHF, 800/900) that I peak up on the test freq with 
the IFR's tracking generator; then use as preselectors for the RX.  When 
I need serious sensitivity as well, I break out the Spectras.


>>o     The only TOTAL  solution is to conduct voice operations on another 
>>band.
>>High-power commercial radios will often block hammy rigs across multiple
>>bands.
>>
>>    
>>
Here I meant using alternate bands on the same dual band ham transceiver 
(i.e. UHF) so that the usual simple hipass/lopass diplexer could provide 
the isolation.  





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
  http://webs.lanset.com/wa8lmf/DigiPaths

Updated APRS Symbol Chart
  http://webs.lanset.com/wa8lmf/miscinfo/APRS_Symbol_Chart.pdf   

New/Updated "Rev G" APRS     http://webs.lanset.com/wa8lmf/aprs
Symbols Set for UI-View,
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