[aprssig] Re: Duplexer or Diplexer

Stephen H. Smith wa8lmf2 at aol.com
Mon Sep 5 23:33:35 CDT 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 
>>High-power commercial radios will often block hammy rigs across multiple
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

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